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Follistatin-344

Tier 2 · Preclinical
Also known as FS-344 · Follistatin 344 · FST-344

The strongest human data come from two small open-label AAV1-FS344 gene therapy trials at Nationwide Children's Hospital (Becker muscular dystrophy n=6; sporadic inclusion body myositis n=6 vs matched controls), reported by Mendell et al. (2015/2017). These represent Tier 1 human clinical evidence but are tiny, open-label, and deliver sustained AAV-driven follistatin expression — NOT injectable recombinant peptide. No randomized, placebo-controlled trials of injectable Follistatin-344 peptide exist, and there are no published human Phase 1 or clinical pharmacokinetic data for the injectable form. Extensive supporting Tier 2 animal/in-vitro data (mice, macaques, dogs, transgenic pigs) and Tier 3 anecdotal/vendor dosing and safety reports round out the profile.

Half-life
~2 h
Routes
Subcutaneous injection · Intramuscular injection · Intravenous (research) · AAV gene therapy (intramuscular vector delivery)
Goals
Muscle growth / hypertrophy · Strength and athletic performance · Muscle-wasting/dystrophy research (Becker MD, sIBM, cachexia, sarcopenia) · Tissue repair and recovery (theorized) · Anti-fibrotic / anti-inflammatory research
Cost / mg
Not recorded

How it works

Follistatin is a natural glycoprotein that acts as a 'blocker' of muscle-limiting signals. It grabs onto and neutralizes myostatin (GDF-8) and activin A — proteins that normally put the brakes on muscle growth and promote muscle wasting. By mopping these up before they can dock onto their receptors on muscle cells, follistatin removes that brake, which in animal and gene-therapy studies allows muscles to grow larger and stronger. It also originally came to light for its ability to reduce release of follicle-stimulating hormone (FSH) from the pituitary. Importantly, the dramatic muscle-growth results reported in the research come mostly from genetic overexpression or gene therapy that keeps follistatin present continuously — not from injecting the short-lived peptide.

Overview

Overview

Follistatin-344 (FS-344, FST-344) is a research-grade form of the endogenous glycoprotein follistatin, one of the most potent natural antagonists of the TGF-beta superfamily. It was first isolated from porcine ovarian follicular fluid in 1987 as a substance that inhibits the release of follicle-stimulating hormone (FSH) from pituitary cells, and in 1990 (Nakamura et al., Science) it was identified as the endogenous high-affinity binding protein for activin.

Molecular identity

The FS gene sits on chromosome 5q11.2 and is alternatively spliced. Follistatin-344 is the 344-amino-acid precursor/isoform. After removal of a 29-amino-acid signal peptide, most sources describe it yielding the mature circulating 315-amino-acid isoform (FST-315); one source instead describes proteolytic trimming to the cell-surface-bound FS-288. These isoforms differ functionally: FS-288 has an exposed heparin-binding sequence (residues 75-86) enabling high-affinity binding to cell-surface heparan sulfate proteoglycans and paracrine action, whereas FS-315 carries an acidic C-terminal tail that masks the heparin-binding motif so it circulates more freely. FS-344 is described as having lower cell-surface binding and a more freely circulating profile, making it the isoform most studied for systemic delivery research. Reported molecular weight is ~37.8-38 kDa (36-38 kDa FST-315; 38-44 kDa glycosylated FST-344). CAS 106463-69-6; PubChem CID 378611.

How it works

Follistatin binds and neutralizes myostatin (GDF-8), activin A, activin B, and several BMPs. Myostatin normally binds activin type II receptors (ActRIIA/ActRIIB), triggering SMAD2/3 phosphorylation that negatively regulates muscle mass. By sequestering myostatin and activin extracellularly, follistatin blocks engagement of ActRIIA/B and ALK4/5 and shuts down the SMAD2/3 cascade that restricts hypertrophy and drives atrophy. Crystallography shows two follistatin molecules encircling a single activin A dimer, burying ~one-third of its surface. Binding affinities are very high — activin A ~45 pM; myostatin reported variously as ~915 pM, ~0.5-1.0 nM, and ~584 pM. Downstream, mTOR/Akt signaling and satellite-cell activation are theorized to drive hypertrophy and hyperplasia. Notably, follistatin's effect persists in a myostatin-null background and an IGF-1R dominant-negative abolishes ~63% of follistatin-induced hypertrophy, indicating the muscle-building effect is broader than myostatin inhibition alone. Follistatin is expressed across many tissues (skeletal muscle, ovary, pituitary, liver, skin, kidney, heart, endothelium, satellite cells) and is upregulated in muscle by resistance exercise and mechanical loading.

Preclinical evidence

Animal data on genetic overexpression are striking: follistatin transgenic mice developed muscle mass increases of 194-327% (exceeding myostatin-knockout gains); muscle-specific Fst overexpression at least doubled skeletal muscle mass. Systemic follistatin in normal mice produced dose-dependent 10-40% muscle gains with improved grip strength. In mdx (Duchenne) mice, engineered follistatin increased muscle mass, grip and tetanic force, and reduced damage, inflammation, and fibrosis versus anti-myostatin antibody alone. In aged mice it improved muscle size, strength, and neuromuscular junction structure. A single AAV-delivered FS-344 dose produced sustained >20% muscle gains lasting over 2 years in normal and dystrophic mice, and AAV1-FS344 increased muscle size/strength in cynomolgus macaques with no adverse immune response and no effect on reproductive hormones. Transgenic pigs expressing human FS-344 showed increased lean meat (72.95% vs 69.18%), reduced Smad2 phosphorylation, increased Akt phosphorylation, no cardiac hypertrophy, and no reproductive abnormalities. Key papers: Whittemore et al. 2003, Lee et al. PNAS 2008, Kota et al. Sci Transl Med 2009. Muscle gains occur over weeks to months and plateau.

Human evidence

Human data come exclusively from AAV1-FS344 gene therapy (AAV1.CMV.FS344, intramuscular) in Phase 1/2a trials at Nationwide Children's Hospital: Becker muscular dystrophy (n=6) and sporadic inclusion body myositis (n=6 vs 8 matched untreated), reported by Mendell et al. (2015, 2017 Mol Ther). The Becker trial was described as the first-ever gene therapy trial to demonstrate functional improvement in any muscular dystrophy, with 6-minute walk gains of +58-125 m and improvement in 4 of 6 patients; the sIBM trial showed +56 vs -25.8 m/year versus controls. Patients showed increased muscle size/strength, reduced fibrosis, no dose-limiting toxicity, no significant organ changes, and no detectable reproductive/endocrine dysfunction over 2-year follow-up. Benefits were described as modest but durable, without serious adverse events.

Critical caveat: these are tiny, open-label gene-therapy trials producing sustained follistatin expression — fundamentally different from injecting the short-lived peptide. No large randomized, placebo-controlled human trials of injectable Follistatin-344 exist, and no injectable recombinant FS-344 peptide human trials exist. Retail/research-grade peptide is not expected to reproduce the systemic myostatin antagonism achieved by AAV delivery.

Investigated and marketed uses

Follistatin-based therapies have been investigated for sporadic inclusion body myositis, cancer cachexia, fibrosis, hair loss, metabolic inflammation, and sarcopenia. FS-344 is marketed in sports medicine for tissue repair and athletic performance as an unapproved peptide sold direct to patients. Some low-tier sources suggest wound-healing, GI-lining, and anti-inflammatory joint benefits (single source). No injectable follistatin product is FDA-approved; it remains a research compound with no approved clinical indications.

What the research shows

248 findings extracted from the 25 sources cited below, strongest evidence first within each group. Every one links to the source it came from.

What human studies found

Based on 4 human trial findings, 9 human study findings, 24 animal findings and 8 expert opinion findings.

  • human trialPhase 1/2 Becker muscular dystrophy gene therapy trial showed measurable improvement in 6-minute walk distance17

  • human trialMendell et al., Mol Ther, 2017 reported Phase 1/2 AAV-follistatin gene therapy human trial in Becker muscular dystrophy patients with 6-minute walk distance improvement at higher dose levels in small trial of 6 patients17

  • human trialA Phase 1/2a gene-therapy trial in Becker muscular dystrophy patients showed functional improvement in a small cohort (4 of 6 patients showed improved six-minute walk distance)18

  • human trialFirst-ever gene therapy clinical trial to demonstrate functional improvement in any form of muscular dystrophy, using AAV1.CMV.FS344 in patients with Becker muscular dystrophy (BMD)20

  • human studyPhase 1/2a gene therapy trial in Becker muscular dystrophy patients showed improved walking distance and reduced fibrosis10

  • human studyDirect intramuscular injection of AAV1.CMV.FS344 improved 6-minute walk test distance by 58 and 125 meters in the first two patients in Becker muscular dystrophy, with no serious adverse events15

  • human studyAAV1-FS344 is an adeno-associated virus packaging FS-344 cDNA injected intramuscularly tested in Phase 1/2a in Becker muscular dystrophy (n=6) and sporadic inclusion-body myositis (n=6)16

  • human studyThe Mendell AAV gene-therapy work produced modest but durable functional benefit in two small open-label trials16

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  • human studyFollistatin-344 has been studied via AAV-mediated gene therapy in clinical settings19

  • human studyMost human evidence comes from a gene therapy trial using AAV1-FS344 in Becker muscular dystrophy and inclusion body myositis patients, which demonstrated increased muscle volume and strength over 2 years without serious adverse events19

  • human studyTwo small open-label AAV1-FS344 gene-therapy trials showed clinical benefit (Mendell 2015 BMD n=6; Mendell 2017 sIBM n=6 vs 8 matched untreated)21

  • human studyGene therapy produced 6MWT gains of +58-125 m in BMD and +56 vs -25.8 m/year in sIBM21

  • human studyPatients who received AAV1-FS344 gene transfer showed increased muscle size in treated muscles compared to untreated controls, improvements in strength and functional tests, no dose-limiting toxicity, and no significant organ changes23

  • animalMice genetically engineered to overexpress Fst specifically in muscle had at least twice the amount of skeletal muscle mass as controls5

  • animalTransgenic pigs expressing Fst in muscle tissue exhibited increased proportion of skeletal muscle and reduced proportion of body fat similar to myostatin-null cattle5

  • animalLean percentage of lean meat was significantly higher in F1 generation of TG pigs than in WT pigs5

  • animalAnimal studies showed doubled muscle mass in mice and significant gains in primates via gene therapy10

  • animalAnimal studies show dramatic and permanent muscle hypertrophy with single gene therapy doses11

  • animalTransgenic pig studies demonstrating increased skeletal muscle mass utilized tissue-specific overexpression rather than exogenous administration12

  • animalTransgenic expression of human follistatin-344 in pigs resulted in significant increases in skeletal muscle mass compared to control animals13

  • animalEvery animal model tested has shown dramatic muscle mass increases15

  • animalA single AAV-delivered dose of follistatin-344 produced sustained muscle mass increases exceeding 20% in both normal and dystrophic mice, with effects lasting over 2 years15

  • animalAAV1-delivered FS344 increased muscle size and strength in cynomolgus macaques, with no adverse immune response and no effect on reproductive hormones15

  • animalTransgenic pigs expressing human follistatin-344 showed significantly increased skeletal muscle mass (72.95% vs 69.18% lean meat percentage) due to myofiber hypertrophy15

  • animalLee et al., PNAS, 2008 showed follistatin transgenic mice had substantially increased muscle mass through myostatin antagonism17

  • animalKota et al., Sci Transl Med, 2009 demonstrated AAV-delivered follistatin gene therapy in cynomolgus macaques produced sustained muscle hypertrophy17

  • animalWhittemore et al., 2003 established follistatin as a potent inducer of muscle growth in mouse models17

  • animalAnimal models demonstrate dramatic muscle hypertrophy via myostatin and activin inhibition18

  • animalPreclinical data in mice, primates, and dogs consistently shows marked increases in muscle mass (2-3x) with myostatin inhibition19

  • animalFollistatin transgenic mice developed muscle mass increases of 194-327% -- exceeding even the gains seen in myostatin knockout animals20

  • animalUnapproved peptides like FS-344 demonstrate favorable tissue repair and metabolic outcomes in animal models22

  • animalIn normal mice, systemic follistatin produced clear dose-dependent increases in muscle mass and fiber size across multiple muscle groups23

  • animalGrip strength and other functional measures improved in mice treated with follistatin23

  • animalIn mice with Duchenne muscular dystrophy (mdx model), engineered follistatin treatments increased muscle mass, improved grip strength and tetanic force, reduced muscle damage and inflammatory infiltration, and lowered fibrosis compared with anti-myostatin antibodies alone23

  • animalIn aged mice, follistatin overexpression led to larger muscles, stronger grip strength, improved neuromuscular junction structure, and better overall muscle function despite advanced age23

  • animalMuscle mass increases of 10-40% above baseline are typical in mice with systemic follistatin exposure23

  • animalMuscle gains occur over weeks to months, not days, and plateau after a time23

  • expert opinionResearch interest centers on its potential to increase skeletal muscle mass through myostatin antagonism, though human clinical trials remain limited12

  • expert opinionThe strongest published efficacy evidence is for follistatin gene therapy (AAV-delivered FS-344) in mouse, primate, and Phase 1/2 human trials in muscular dystrophy17

  • expert opinionHuman data is extremely limited and no injectable follistatin protocol has been validated in clinical trials19

  • expert opinionNo injectable recombinant follistatin-344 peptide human trials exist19

  • expert opinionGene therapy trials for muscular dystrophy show promise, but injectable peptide forms remain unapproved and understudied in humans21

  • expert opinionFollistatin 344 is a potent myostatin blocker capable of driving muscle hypertrophy in animals and early human gene-therapy trials23

  • expert opinionNo large randomized human trials of injectable Follistatin 344 for fitness exist23

  • expert opinionNo randomized, placebo-controlled trials in healthy adult humans using injectable Follistatin 344 peptides exist23

How it works

Based on 21 animal findings, 33 in vitro findings, 54 expert opinion findings and 3 theoretical findings.

  • animalFollistatin is an essential regulator for muscle development5

  • animalFollistatin findings in mice are similar to results obtained in myostatin-knockout mice5

  • animalMyofiber hypertrophy was observed in the longissimus dorsi of transgenic pigs, possibly contributing to increased skeletal muscle mass5

  • animalWestern blot analysis showed significantly reduced level of Smad2 phosphorylation in skeletal muscle tissue of transgenic pigs5

  • animalWestern blot analysis showed increased level of Akt phosphorylation in skeletal muscle tissue of transgenic pigs5

  • animalFollistatin (FS), a monomeric glycoprotein which specifically binds activin, is expressed in many tissues8

  • animalFS mRNA 344 levels in the kidney showed a profound increase from the day of birth to adulthood8

  • animalFS mRNA 344 levels in the muscle peaked during the infantile period and then declined8

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  • animalBrain cortex, heart and thymus showed tissue specific expression in the steady-state mRNA level of FS during postnatal development8

  • animalNone of the tissues showed a measurable change in the ratio of the mRNA for FS 344 and FS 3178

  • animalFS mRNA 344 levels in male and female kidney were not different8

  • animalThe ontogeny of steady state FS mRNA varies in a tissue specific manner during postnatal development of the rat and may be involved in modulating the outcome of activin8

  • animalFollistatin (FS) or FSH-suppressing protein is a polypeptide which exists in multiple forms and has inhibin-like activity9

  • animalFollistatin mRNA was detected in cerebral cortex, pituitary, adrenal, thymus, pancreas, gut, kidney, heart, uterus, skeletal muscle and lung in addition to ovary and testis9

  • animalTreatment of female rats with combined Pregnant Mare Serum Gonadotropin (PMSG) and human Chorionic Gonadotropin (hCG) increased expression of FS mRNA in the ovary9

  • animalThe mRNA for the precursor of the larger known form of FS (FS 344) is much more abundant than the mRNA for the smaller molecule (less than 5%)9

  • animalStimulation of FS transcription by PMSG and hCG in the ovary does not change the relationship between FS 344 and smaller form mRNA abundance9

  • animalAlternative splicing of the rat FS gene occurs9

  • animalFollistatin inhibits both myostatin and activin A to regulate muscle size, suggesting its muscle-building effect is broader than myostatin inhibition alone15

  • animalFollistatin effect demonstrated in myostatin-null background21

  • animalIGF-1R dominant-negative abolishes approximately 63% of hypertrophy21

  • in vitroAll nine products that contained follistatin had His-tagged FS344 and a high degree of its oligomers4

  • in vitroFollistatin is the specific binding protein of activin6

  • in vitroFollistatin has broad tissue distribution and is found in serum6

  • in vitroOvary has the highest level of FS expression but ovariectomy does not cause permanent reduction in serum FS level6

  • in vitroEndothelial cells from blood vessels express FS messenger RNA and protein6

  • in vitroFS steady state levels in aortic endothelial cells decreased with time in culture6

  • in vitroFS mRNA levels in aortic endothelial cells increased approximately 20-fold within 4 hours during incubation with 100 nM phorbol 12-myristate, 13-acetate6

  • in vitro0.5 nmol/ml forskolin had no significant effect on FS mRNA levels in aortic endothelial cells6

  • in vitro0.1 microM ocadaic acid caused significant increase in FS mRNA levels6

  • in vitroFS mRNA levels in aortic endothelial cells were not significantly affected by various concentrations of porcine FSH, epidermal growth factor, or retinoic acid6

  • in vitroTreatment with 0.01-10 micrograms/ml bacterial lipopolysaccharides caused dose-dependent increase up to 10-fold in FS mRNA steady state level6

  • in vitro1-1000 nM RU 28362 synthetic glucocorticoid inhibited FS mRNA steady state levels in dose-dependent manner6

  • in vitro100 nM RU 28362 completely blocked induction of FS mRNA with 1 microgram/ml LPS6

  • in vitroStimulatory effects of LPS were visible after 4 h of treatment but not after 24 or 48 h6

  • in vitroFS proteins were secreted into culture medium upon stimulation with LPS6

  • in vitroTreatments did not have obvious effect on ratio of two different forms of FS mRNA (FS 344:FS 317)6

  • in vitroFS mRNA is expressed in uncultured plexus choroideus epithel and meninges6

  • in vitroFS protein is found in human cerebrospinal fluid6

  • in vitroEndothelial cells produce and secrete FS protein and are a likely source of FS in serum6

  • in vitroTreatment of cultured rat renal mesangial cells with epidermal growth factor (10 nM) caused an 8- to 9-fold increase in follistatin mRNA level after 4 h7

  • in vitroBasic fibroblast growth factor (0.28 or 0.56 nM) showed no consistent change in follistatin mRNA levels after treatment for between 4 and 48 h7

  • in vitroSomatostatin (3.7-73 nM) showed no consistent change in follistatin mRNA levels after treatment for between 4 and 48 h7

  • in vitroFollistatin itself (0.29 nM) showed no consistent change in follistatin mRNA levels after treatment for between 4 and 48 h7

  • in vitroRetinoic acid significantly increased follistatin steady state mRNA level at 3 nM, and at 1000 nM stimulated follistatin mRNA up to 5-fold within 4 h7

  • in vitroProstaglandin E2 (30 microM) for 4 h caused a 10-fold increase in follistatin mRNA level7

  • in vitroPhorbol 12-myristate, 13-acetate (100 nM) increased follistatin mRNA level 3-fold within 4 h7

  • in vitroForskolin (25 microM) increased follistatin mRNA level 4-fold within 4 h7

  • in vitroRU 28362 synthetic glucocorticoid (100 nM) caused a 5- to 6-fold increase in follistatin mRNA level7

  • in vitroProgesterone (1000 nM) increased follistatin mRNA level up to 3.5-fold above control7

  • in vitroNone of the tested hormones had an obvious effect on the ratio of the two different forms of follistatin mRNA (FS 344:FS 317)7

  • in vitroCrystal structure analysis at 2.0 angstrom resolution revealed that two follistatin molecules completely encircle a single activin A dimer, burying approximately one-third of activin's surface residues20

  • in vitroFollistatin binds activin A with extremely high affinity (Kd ~45 pM)21

  • in vitroCrystal structure shows receptor-site occlusion21

  • expert opinionFollistatin is a myostatin-inhibiting protein4

  • expert opinionFollistatin is a naturally occurring glycoprotein that binds and neutralizes myostatin, the hormone that limits muscle growth10

  • expert opinionBlocking myostatin signaling allows muscles to grow beyond their normal genetic limits10

  • expert opinionFollistatin 344 isoform has lower affinity for heparin sulfate proteoglycans compared to the 317 isoform, which means it circulates more freely10

  • expert opinionThe 344 isoform has less impact on reproductive hormones like FSH10

  • expert opinionFollistatin 344 is a 344-amino acid glycoprotein that functions as an activin-binding protein, specifically targeting myostatin and activin-A to modulate TGF-β signaling pathways12

  • expert opinionThe peptide binds to myostatin with high affinity (Kd = 915 pM), effectively neutralizing its muscle growth-inhibitory effects through Smad2/3 pathway inhibition12

  • expert opinionThe peptide exists naturally in multiple splice variants, with the 344-amino acid form representing the full-length protein containing three follistatin domains and a C-terminal acidic tail12

  • expert opinionFollistatin 344 functions as a potent antagonist of myostatin and activin-A, two key regulators within the transforming growth factor-beta (TGF-β) superfamily13

  • expert opinionAdditional 27 amino acid C-terminal extension that enhances tissue binding affinity compared to follistatin 315 variant13

  • expert opinionHigh-affinity binding to myostatin (Kd ≈ 0.5-1.0 nM)13

  • expert opinionFollistatin-344 has a naturally occurring regulatory protein with a well-characterized role in myostatin inhibition and activin signaling14

  • expert opinionFollistatin was first identified in the late 1980s as a protein capable of suppressing follicle-stimulating hormone (FSH) secretion14

  • expert opinionFollistatin suppression of FSH was mediated through its high-affinity binding to activin, a member of the transforming growth factor-beta (TGF-β) superfamily14

  • expert opinionThe 344-amino acid isoform, follistatin-344, includes a heparan sulfate–binding domain that governs its local tissue retention14

  • expert opinionFollistatin-344 is physiologically distinct from the shorter FS-288 isoform, which circulates more freely14

  • expert opinionFollistatin is expressed across skeletal muscle, ovaries, pituitary gland, liver, and skin14

  • expert opinionFollistatin expression in skeletal muscle is upregulated in response to resistance exercise and mechanical loading14

  • expert opinionFollistatin is present in satellite cells—the progenitor cells responsible for muscle regeneration14

  • expert opinionFollistatin neutralizes activin and related ligands to fine-tune the magnitude of TGF-β pathway activity14

  • expert opinionMyostatin (GDF-8) is a member of the TGF-β superfamily that functions as a potent negative regulator of skeletal muscle mass14

  • expert opinionMyostatin acts by binding to activin type II receptors (ActRIIA and ActRIIB), triggering downstream SMAD2/3 phosphorylation14

  • expert opinionNatural mutations resulting in myostatin deficiency have been documented across multiple species—including cattle, dogs, and humans—producing a consistent phenotype of dramatically increased muscle mass and reduced adiposity14

  • expert opinionFollistatin neutralizes myostatin, the protein that actively limits how much muscle your body can build15

  • expert opinionFollistatin is an endogenous glycoprotein that binds and neutralizes members of the TGF-beta superfamily, particularly myostatin (GDF-8) and activin A15

  • expert opinionFollistatin physically wraps around myostatin in a high-affinity interaction, preventing myostatin from reaching its receptor (ActRIIB) and triggering the Smad2/3 signaling cascade that suppresses muscle growth15

  • expert opinionFollistatin-344 is the name of a gene-transcript isoform, not a single defined drug product16

  • expert opinionFS-344 mRNA is a 344-residue precursor transcript that after signal-peptide cleavage and proteolytic trimming gives rise to FS-288 mature protein16

  • expert opinionFS-288 is 288 amino acids with cell-surface and intracellular localization and short plasma half-life16

  • expert opinionFS-315 is 315 amino acids with circulating systemic form and longer acidic C-terminal tail16

  • expert opinionFollistatin sequesters myostatin and activin A, two members of the TGF-beta superfamily that restrain muscle growth16

  • expert opinionFS-288 has heparan-sulfate binding and paracrine action16

  • expert opinionFollistatin 344 is the 344-amino-acid isoform of human follistatin, an endogenous antagonist of myostatin (GDF-8) and activin17

  • expert opinionRetail research-grade follistatin peptide does not produce the same systemic myostatin antagonism that AAV-delivered follistatin produces17

  • expert opinionFollistatin is an endogenous glycoprotein secreted by various tissues including ovary, pituitary, and skeletal muscle17

  • expert opinionFollistatin 288 (FS-288) has higher cell-surface binding affinity and localizes to cell membranes17

  • expert opinionFollistatin 344 (FS-344) has lower cell-surface binding and more freely circulating profile17

  • expert opinionFS-344 is the isoform most-studied in research applications because its circulating profile is more compatible with systemic delivery17

  • expert opinionFollistatin is an endogenous glycoprotein that binds and neutralizes myostatin, activin, and several bone morphogenetic proteins, suppressing their downstream signaling18

  • expert opinionFS-288 and FS-315 differ in their relative affinities for activin, myostatin, and bone morphogenetic proteins (BMPs)18

  • expert opinionFollistatin-344 is a naturally occurring glycoprotein that potently inhibits myostatin and activin A19

  • expert opinionFollistatin-344 acts as a high-affinity binding protein and functional antagonist of myostatin (GDF-8) and activin A by binding these ligands extracellularly and preventing them from engaging their type I/II serine-threonine kinase receptors (ActRIIA/B, ALK4/5)19

  • expert opinionBlocking myostatin and activin A prevents the downstream SMAD2/3 signaling cascade that restricts muscle fiber hypertrophy and promotes atrophy19

  • expert opinionFS-344 is the full-length precursor: cleavage of its 29-residue signal peptide yields the mature circulating isoform FS-31519

  • expert opinionThe acidic 27-residue C-terminal tail present in FS-315 reduces heparan-sulfate binding, so FS-315 circulates more freely while FS-288 binds cell surfaces more tightly19

  • expert opinionFollistatin also suppresses activin-driven FSH secretion from the pituitary, with significant reproductive endocrine consequences19

  • expert opinionFollistatin-344 is a naturally occurring single-chain glycoprotein first isolated from porcine ovarian follicular fluid in 1987 as a substance capable of inhibiting the release of follicle-stimulating hormone (FSH) from pituitary cells20

  • expert opinionIdentified in 1990 as the endogenous high-affinity binding protein for activin, a member of the transforming growth factor-beta (TGF-beta) superfamily20

  • expert opinionThe 344-amino-acid precursor form after removal of the 29-amino-acid signal peptide yields the mature 315-amino-acid circulating isoform (FST-315)20

  • expert opinionFollistatin functions as one of the most potent endogenous antagonists of the TGF-beta superfamily, binding and neutralizing myostatin (GDF-8), activin A, activin B, and several bone morphogenetic proteins (BMPs)20

  • expert opinionFST-288 is cell-surface bound and retains an exposed heparin-binding sequence, enabling high-affinity binding to cell-surface heparan sulfate proteoglycans20

  • expert opinionFST-315 is the predominant systemic isoform with an acidic C-terminal tail that masks the heparin-binding motif, reducing cell-surface association20

  • expert opinionFollistatin is an endogenous glycoprotein that inhibits myostatin and activin signaling, potentially allowing muscle growth beyond genetic limits21

  • expert opinionFollistatin binds myostatin and activin with high affinity, preventing these proteins from activating their receptors23

  • theoreticalFollistatin 344 is a naturally occurring glycoprotein that binds and neutralizes TGF-β superfamily members, most notably myostatin (GDF-8) and activin A11

  • theoreticalHigh-affinity binding and sequestration of myostatin (GDF-8), activin A, and activin B; relieves myostatin-mediated inhibition of muscle satellite cell activation; allows unrestricted muscle fiber hypertrophy and hyperplasia11

  • theoreticalThe disinhibition of mTOR and satellite cell activation drives both hypertrophy of existing fibers and hyperplasia via myoblast proliferation19

Dosing

Based on 1 human study finding, 1 animal finding, 17 expert opinion findings and 3 anecdotal findings.

  • human studyAll patients injected complete 1 mg vials of follistatin-344 subcutaneously in the abdomen1

  • animalTransgenic pig studies demonstrating significant muscle mass increases used equivalent human doses of approximately 250 mcg administered every other day13

  • expert opinionNo clinically validated injectable dose exists for follistatin 34410

  • expert opinionNo established safe human dose — research setting only11

  • expert opinionStarting dose 100 mcg once daily subcutaneous injection11

  • expert opinionStandard dose 250 mcg once daily subcutaneous injection11

  • expert opinionNo FDA-approved dosing guidelines exist for follistatin 344, as the peptide lacks regulatory approval for human therapeutic use12

  • expert opinionFollistatin 344 requires subcutaneous injection using 29-31 gauge insulin syringes with 0.5-1.0 mL capacity12

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  • expert opinionThe peptide arrives as lyophilized powder requiring reconstitution with bacteriostatic water at concentrations of 1-5 mg/mL depending on vial size and dosing requirements12

  • expert opinionSubcutaneous injection depth should be 4-6mm at a 45-90 degree angle depending on subcutaneous fat thickness12

  • expert opinionSubcutaneous administration represents the gold standard for research applications13

  • expert opinion200 mcg administered three times weekly (Monday, Wednesday, Friday) being the most frequently reported protocol in published case series13

  • expert opinion100 mcg starting dose allows for tolerance assessment13

  • expert opinion300 mcg upper range approaches the threshold where adverse effects become more prevalent in research subjects13

  • expert opinionIntramuscular injection protocols typically employ slightly higher doses (150-400 mcg)13

  • expert opinionResearch investigating muscle hypertrophy typically employs 200-300 mcg subcutaneously three times weekly for 6-8 week cycles13

  • expert opinionClinical research facilities commonly initiate protocols at 150 mcg twice weekly, escalating to 200 mcg three times weekly based on biomarker response and tolerance assessment13

  • expert opinionCommunity peptide protocols use 100 mcg/day subcutaneous, but the degree of myostatin inhibition achieved through brief subcutaneous exposure is unknown15

  • expert opinionNo data on optimal dosing for recreational or fitness-oriented use23

  • anecdotalPeptide vendors commonly suggest 100 mcg/day for 10-30 day cycles, but this is not supported by published clinical trials10

  • anecdotalCommunity protocols use subcutaneous route at 50–200 mcg daily for 10-30 days, then cycle off, or 50-100 mcg/day for 10 days to 100-200 mcg 2-3x weekly19

  • anecdotalCommunity pulse dosing protocol uses intramuscular route at 50–100 mcg on training days only, injected 30 minutes pre-workout into target muscle group19

How the body handles it

Based on 13 expert opinion findings and 2 theoretical findings.

  • expert opinionInjectable peptide form has very limited clinical data and a short half-life of roughly 90 minutes10

  • expert opinionHuman pharmacokinetic and safety data are extremely limited11

  • expert opinionPlasma half-life of 3-4 hours following subcutaneous administration, with peak concentrations achieved within 1-2 hours post-injection13

  • expert opinionSubcutaneous injection achieves 85-90% bioavailability13

  • expert opinionIntramuscular injection reaches 75-80% bioavailability13

  • expert opinionOral administration results in less than 5% bioavailability due to extensive first-pass metabolism and proteolytic degradation in the gastrointestinal tract13

  • expert opinionIntravenous administration achieves 100% bioavailability but requires medical supervision13

  • expert opinionGene therapy produces weeks to months of continuous follistatin expression, while subcutaneous injection has a half-life measured in hours15

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  • expert opinionFS-315 has minutes to ~1 hour half-life16

  • expert opinionFS-288 has minutes half-life in plasma16

  • expert opinionInjectable peptide-grade FS-344 has very different pharmacokinetics from gene therapy; serum half-life is short (minutes to hours), unlike the sustained expression from AAV delivery17

  • expert opinionFS-288 binds heparan sulfate proteoglycans on cell surfaces and in the extracellular matrix, keeping it locally tethered, while FS-315 circulates freely in the bloodstream18

  • expert opinionNative follistatin has very short half-life (~4 min initial, ~130 min terminal) limiting systemic effects21

  • theoreticalThe molecular weight of follistatin 344 is approximately 37.8 kDa12

  • theoreticalPlasma half-life estimated at 2-3 hours based on protein degradation kinetics12

Safety and side effects

Based on 7 human study findings, 2 animal findings, 2 in vitro findings, 12 expert opinion findings, 10 anecdotal findings and 1 theoretical finding.

  • human studyFollistatin-344 injection can be considered as a risk factor for central serous chorioretinopathy (CSCR)1

  • human studyIn patients with a history of only one follistatin-344 injection, subretinal fluid completely disappeared after an average of 2.3 ± 0.7 months and symptoms regressed1

  • human studyRecurrent CSCR developed in patients with a history of multiple follistatin-344 injections1

  • human studyA 2020 case series documented vision impairment in bodybuilders using roughly 1 mg doses10

  • human studyFollistatin-344 injectable use carries documented ocular risks including retinal detachment19

  • human studyA 2020 case series documented 11 male bodybuilders who developed central serous chorioretinopathy (CSCR, a retinal condition) following subcutaneous follistatin-344 injections19

  • human studyNo clinically detectable reproductive or endocrine dysfunction occurred during follow-up in gene therapy trial patients23

  • animalNo cardiac muscle hypertrophy was observed in transgenic pigs expressing muscle-specific Fst5

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  • animalNo reproductive abnormality was observed in transgenic pigs expressing muscle-specific Fst5

  • in vitroOnly 9 of 17 tested black market follistatin 344 products actually contained follistatin4

  • in vitroSome black market products labeled as follistatin contained other growth promoting peptides (e.g. MGF, GHRP-2) instead4

  • expert opinionWADA banned follistatin-related substances in 201910

  • expert opinionGenerally well-tolerated in research settings11

  • expert opinionWADA prohibits follistatin 344 under Section S4 (Hormone and Metabolic Modulators) of the 2024 Prohibited List, making it illegal for competitive athletes12

  • expert opinionGrey-market Follistatin-344 research vial has no published human Phase 1, no published clinical pharmacokinetics, and no FDA approval16

  • expert opinionGrey-market Follistatin-344 is prohibited by WADA at all times16

  • expert opinionAnti-doping detection literature describes wide vial-content variability in grey-market follistatin vials16

  • expert opinionNo completed human efficacy data exists for injected follistatin fragments as of April 202618

  • expert opinionFollistatin-344 is WADA prohibited19

  • expert opinionInjectable peptide forms are not FDA approved and remain understudied in humans21

  • expert opinionRigorous human safety data for unapproved peptides including FS-344 are scarce22

  • expert opinionThere is potential for serious harm to patients from unapproved peptides like FS-34422

  • expert opinionNo long-term (>2 year) safety data from human use of Follistatin 344 outside gene-therapy trials23

  • anecdotalMild nausea with higher doses11

  • anecdotalInjection site redness or discomfort11

  • anecdotalRare: Dizziness or lightheadedness11

  • anecdotalPotential effect on blood pressure (monitor if hypertensive)11

  • anecdotalBlack market analysis reveals significant quality control issues, with only 9 of 17 tested products containing detectable follistatin protein12

  • anecdotalContamination with growth-promoting substances occurred in products lacking the target peptide12

  • anecdotalCase reports of central serous chorioretinopathy occurred with doses exceeding 500 mcg administered 3-4 times weekly over 8-12 week periods12

  • anecdotalThe 11 bodybuilding athletes who developed CSCR were using subcutaneous injections of 500-1000+ mcg doses for muscle mass enhancement12

  • anecdotalRecovery of CSCR typically occurs within 3-6 months following discontinuation12

  • anecdotalBlack market FS-344/FS-315 products have significant quality issues (WADA found only 9/17 products contained follistatin)21

  • theoreticalFollistatin is prohibited according to chapter S4 of the WADA 2019 List of Prohibited Substances and Methods4

What people use it for

Based on 1 human trial finding, 1 human study finding, 3 animal findings, 8 expert opinion findings and 1 theoretical finding.

  • human trialFollistatin gene therapy (AAV1-FS344) has been investigated in Phase 1/2a clinical trials at Nationwide Children's Hospital for Becker muscular dystrophy and sporadic inclusion body myositis18

  • human studyHigh-dose subcutaneous follistatin-344 injections are used by bodybuilding athletes as a performance and image enhancing drug to increase muscle mass1

  • animalDemonstrated acceleration of wound healing and soft tissue repair in research11

  • animalResearch shows beneficial effects on intestinal lining and GI function11

  • animalAssociated with reduced joint inflammation and improved mobility11

  • expert opinionNo approved pharmaceutical formulations of follistatin are currently available4

  • expert opinionFollistatin 344 remains classified as a research compound with no approved clinical indications12

  • expert opinionAdministration 2-3 hours post-exercise to potentially enhance the anabolic window13

Show the remaining 6

  • expert opinionNo injectable follistatin product is FDA-approved as of April 202618

  • expert opinionGene therapy approach using adeno-associated virus serotype 1 (AAV1) to deliver the FS344 transgene has progressed through preclinical studies in mice and nonhuman primates into human clinical trials20

  • expert opinionFollistatin-based therapies have been investigated in sporadic inclusion body myositis (sIBM), cancer cachexia, fibrosis, hair loss, metabolic inflammation, and sarcopenia20

  • expert opinionFS-344 (follistatin-344) is an unapproved peptide marketed direct to patients22

  • expert opinionFS-344 is utilized in sports medicine for tissue repair and athletic performance22

  • theoreticalMay support faster recovery from muscle strains and exercise stress11

Other findings

Based on 1 in vitro finding and 6 expert opinion findings.

  • in vitroDetection method based on immunomagnetic purification followed by SDS-PAGE and Western blotting can differentiate FS344 from endogenous follistatin due to His-tags4

  • expert opinionUse bacteriostatic (BAC) water only. Avoid saline — may cause precipitation11

  • expert opinionRefrigerate and use within 28 days after reconstitution11

  • expert opinionStorage requires refrigeration at 36-46°F (2-8°C) for both lyophilized and reconstituted forms12

  • expert opinionReconstituted solutions maintain stability for 14-28 days when stored properly with bacteriostatic water containing 0.9% benzyl alcohol12

  • expert opinionThe peptide contains 344 amino acids with a molecular weight of approximately 37.8 kDa13

  • expert opinionFollistatin was identified as an activin-binding protein in a 1990 paper in Science by Nakamura, Takio, Eto, Shibai, Titani, and Sugino18

Points of contention

Where the evidence is unsettled, thin, or says less than the popular claim — worth knowing before you draw conclusions.

Limited evidence

Human efficacy evidence is limited to tiny gene-therapy trials, not injectable peptide

The strongest human data come from two small open-label AAV1-FS344 gene therapy trials (Becker muscular dystrophy n=6; sporadic inclusion body myositis n=6), totaling fewer than 50 human subjects. No randomized, placebo-controlled trials of injectable Follistatin-344 peptide exist, and there are no published human Phase 1 or clinical pharmacokinetic data for the injectable peptide. Gene therapy produces sustained expression, unlike the short half-life of injected peptide.

Contested

Sources disagree substantially on injectable follistatin half-life

Reported plasma half-life estimates for injectable follistatin peptide range widely: native follistatin ~4 min initial/~130 min terminal (Follistatin: 38 Studies Reviewed (2026) | PepCodex); minutes to ~1 hour for FS-315 (Follistatin-344 Peptide Guide 2026: Myostatin Inhibitor); roughly 90 minutes (Follistatin 344 — Independent peptide research & reviews); 2-3 hours (Follistatin 344 Dosage for Bodybuilding: Protocols and); and 3-4 hours (Follistatin 344 Dosage Guide: How Much Should You Take? (2026)). Several sources describe half-life only vaguely as 'minutes to hours.'

Contested

Sources disagree on the identity of the mature product of FS-344

Most sources state the 344-residue precursor, after signal-peptide cleavage, yields the mature 315-amino-acid circulating isoform FS-315 (Follistatin-344 (FST-344): Research Evidence & Safety Profile | PeptideInsight, Follistatin 344: Myostatin Pathway Research and the Gene Therapy Evidence · Peptides:Enhanced, Follistatin-344: Myostatin Inhibitor Research Guide). However, one source states FS-344 mRNA after signal-peptide cleavage and proteolytic trimming gives rise to the FS-288 mature protein (Follistatin-344 Peptide Guide 2026: Myostatin Inhibitor). Sources also vary on which isoform circulates more freely versus binds cell surfaces.

Contested

Reported myostatin binding affinity (Kd) varies by more than an order of magnitude

Myostatin binding affinity is reported as Kd = 915 pM (Follistatin 344 Dosage for Bodybuilding: Protocols and), Kd ≈ 0.5-1.0 nM (Follistatin 344 Dosage Guide: How Much Should You Take? (2026)), and Kd ~5.84 x 10^-10 M i.e. ~584 pM (Follistatin-344: Remove the Brake on Muscle (2026)). Activin A binding is separately reported at Kd ~45 pM (Follistatin: 38 Studies Reviewed (2026) | PepCodex).

Limited evidence

Dosing protocols are anecdotal or vendor-derived, not clinically validated

All specific injectable dosing guidance (e.g., 50-200 mcg, 100 mcg/day, 200-300 mcg 3x weekly, bioavailability percentages, injection specifics) comes from community protocols, vendor suggestions, or expert-opinion blog content, explicitly noted as not supported by published clinical trials. The degree of myostatin inhibition achieved through brief subcutaneous exposure is unknown.

Limited evidence

Ocular harm evidence is from a single case series of high doses

The central serous chorioretinopathy (CSCR) safety signal derives from a single 2020 case series of 11 male bodybuilders using high doses (typically full 1 mg vials, or 500-1000+ mcg 3-4x weekly), far above community-suggested doses. This is observational and cannot establish incidence rates.

Limited evidence

Extreme muscle-growth figures come from transgenic/gene-therapy animal models, not peptide injection

Dramatic muscle mass increases (194-327% in transgenic mice, 2-3x doubling, >20% with single AAV dose lasting 2 years, transgenic pig gains) were produced by genetic overexpression or AAV gene therapy — sustained expression models — not by exogenous injectable peptide administration, which sources note has very different pharmacokinetics.

Single source

Some pharmacokinetic and mechanism details rest on a single low-tier source

Specific bioavailability figures (85-90% SC, 75-80% IM, <5% oral) appear only in Follistatin 344 Dosage Guide: How Much Should You Take? (2026), and theoretical anti-inflammatory/wound-healing/GI use cases appear only in Follistatin 344 (FST344 / Myostatin Inhibitor) — Dosing, Side Effects, FDA Status & Research | PeptIQ | PeptIQ. Both are tier-3 web sources labeled expert_opinion or theoretical.

Using it with other compounds

Check a stack
  • TB-500Complementary

    No documented conflict

    TB-500 promotes tissue repair, angiogenesis and matrix remodeling, while Follistatin reduces muscle fibrosis and inflammatory infiltration and drives hypertrophy. Different mechanisms both favoring healthier muscle/soft-tissue recovery, so they can be viewed as complementary repair agents.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms support the proposed complementary relationship on the claimed shared dimensions. (1) Anti-inflammatory: Follistatin-344 shows 'Reduced muscle fibrosis and inflammatory infiltration' and 'Reported tissue repair / anti-inflammatory effects'; TB-500 explicitly targets 'Anti-inflammatory (reduced TNF-α, IL-1β, IL-6)' and 'NF-κB (suppression, anti-inflammatory)'. Both are tagged anti_inflammatory. (2) mTOR_PI3K: Follistatin-344 mechanisms include 'mTOR pathway (disinhibited)' and 'Akt phosphorylation (increased)' with mTOR_PI3K tag; TB-500 pathways include 'PI3K/Akt survival pathway' and 'mTOR' with mTOR_PI3K tag. The explanation correctly identifies that they operate via different primary mechanisms (Follistatin via TGF-β antagonism/myostatin blockade; TB-500 via actin sequestration and integrin signaling) but converge on shared outcomes: anti-inflammatory effects and mTOR/PI3K pathway activation. Both support muscle/tissue recovery through distinct but complementary pathways, justifying the 'complementary' relationship type.

    Shares anti inflammatory · mTOR PI3K

  • TesamorelinComplementary

    May be complementary

    Tesamorelin raises GH/IGF-1 and preferentially strips visceral fat, while Follistatin adds muscle mass and reduces adiposity by blocking myostatin. Their combined effect leans toward improved body composition (more lean mass, less fat) through distinct pathways.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms establish IGF1_signaling as a shared dimension: Follistatin explicitly lists IGF-1R signaling as a partial mediator of hypertrophy (approved tag: IGF1_signaling), and Tesamorelin's mechanism directly increases endogenous IGF-1 via GH release and hepatic JAK2/STAT5 induction (approved tag: IGF1_signaling). The proposed complementary relationship is justified—they operate through distinct primary mechanisms (ligand sequestration vs. GHRH receptor activation) but converge on IGF-1 signaling to promote anabolic effects. The explanation accurately reflects that Follistatin drives muscle hypertrophy and reduces adiposity through TGF-beta antagonism while Tesamorelin increases GH/IGF-1 and preferentially targets visceral fat through cAMP/GH axis activation, making them mechanistically complementary for body composition improvement.

    Shares IGF1 signaling

  • SermorelinComplementary

    May be complementary

    Sermorelin boosts the body's own GH/IGF-1 output to support lean mass, whereas Follistatin drives hypertrophy by neutralizing myostatin. Different upstream mechanisms converge on the same goal of building muscle, so they complement rather than duplicate each other.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly support the proposed complementary relationship. Follistatin-344 directly targets myostatin/activin signaling to disinhibit muscle growth and explicitly lists IGF-1R signaling and protein_synthesis as approved tags. Sermorelin stimulates endogenous GH release, which increases IGF-1 and protein_synthesis (both approved tags). The mechanisms describe distinct upstream pathways (ligand sequestration via Follistatin vs. GHRH receptor activation via Sermorelin) that converge on shared downstream dimensions (IGF1_signaling and protein_synthesis). Both improve body composition and lean mass through different routes, which is the hallmark of complementary action. The explanation accurately reflects the mechanistic material provided.

    Shares protein synthesis · IGF1 signaling

  • MOTS-cSame downstream effect

    Worth caution

    MOTS-c is reported to regulate myostatin through a CK2-PTEN-mTORC2-AKT-FOXO1 axis, converging on the same muscle-limiting signal that Follistatin neutralizes directly. Both also add metabolic/anti-inflammatory benefits, so they may reinforce each other on body composition, though this convergence is only weakly characterized.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    The proposed relationship claims both peptides converge on myostatin regulation as a shared downstream mechanism. However, the mechanism descriptions show fundamentally different modes of action: Follistatin directly targets and sequesters myostatin/GDF-8 as a ligand trap, while MOTS-c's connection to myostatin is indirect and occurs through a complex CK2-PTEN-mTORC2-AKT-FOXO1 axis. The MOTS-c description itself notes this pathway 'regulates myostatin' but does not establish that this is a primary or well-characterized mechanism—it appears as one element among many (NRF2, AMPK, GLUT4, STAT3, IL-10). Additionally, while both peptides have 'anti_inflammatory' tags, the mechanisms underlying this effect differ substantially: Follistatin works through TGF-beta superfamily antagonism and SMAD2/3 inhibition, while MOTS-c works through Keap1-Nrf2 antioxidant pathways and AMPK activation. The claim of 'same_downstream' convergence on myostatin is not clearly supported by the provided mechanisms, which show distinct upstream pathways with only speculative indirect overlap on myostatin regulation.

    Shares anti inflammatory

  • HexarelinComplementary

    May be complementary

    Hexarelin raises IGF-1 and, notably, suppresses TGF-beta driven fibrosis — overlapping with Follistatin's own anti-fibrotic, muscle-sparing action. One works through GH/IGF-1, the other through myostatin/activin blockade, so they converge on healthier, larger muscle through different routes.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly support the three claimed shared dimensions. (1) IGF1_signaling: Follistatin explicitly lists 'IGF-1R signaling (partial mediator of hypertrophy)' and Hexarelin 'Increases IGF-1 with prolonged administration' — both documented. (2) mTOR_PI3K: Follistatin shows 'mTOR pathway (disinhibited)' and 'Akt phosphorylation (increased)'; Hexarelin shows 'PI3K/Akt' pathway activation — both present. (3) anti_inflammatory: Follistatin lists 'anti_inflammatory' tag and 'Reduced muscle fibrosis and inflammatory infiltration'; Hexarelin lists 'anti_inflammatory' tag and 'NLRP3 inflammasome / IL-18 / NF-κB suppression' plus 'TGF-β suppression (anti-fibrotic)'. The explanation correctly identifies that they converge on muscle health via different primary mechanisms (GH/IGF-1 axis vs. myostatin/activin blockade), with overlapping downstream effects on fibrosis reduction and anti-inflammatory signaling. The complementary relationship is justified by mechanistic convergence on shared pathways despite distinct primary targets.

    Shares IGF1 signaling · mTOR PI3K · anti inflammatory

  • MK-677Complementary

    May be complementary

    MK-677 elevates GH/IGF-1 and improves nitrogen balance, supporting protein synthesis from the anabolic-hormone side, while Follistatin releases the myostatin brake. The two attack muscle growth from separate angles, making them additive rather than redundant.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly establish the claimed shared dimensions. Follistatin explicitly targets IGF-1R signaling as a 'partial mediator of hypertrophy' and is tagged with IGF1_signaling and protein_synthesis. MK-677 directly elevates IGF-1 levels and IGFBP-3, with approved tags for both IGF1_signaling and protein_synthesis, plus reversal of protein catabolism via improved nitrogen balance. The proposed complementary relationship is justified: Follistatin removes inhibitory signals (myostatin/activin blockade, SMAD2/3 inhibition, mTOR disinhibition), while MK-677 provides positive anabolic drive via the GH/IGF-1 axis. These represent mechanistically distinct pathways (ligand sequestration + receptor antagonism vs. GHS-R1a agonism → GH secretion) converging on the same downstream effectors (IGF-1 signaling, protein synthesis, mTOR activation). The mechanisms support the claim that they are additive rather than redundant.

    Shares protein synthesis · IGF1 signaling

  • KlothoSame downstream effect

    No documented conflict

    Klotho suppresses TGF-beta/SMAD2/3 signaling to curb fibrosis, and Follistatin also acts as a TGF-beta superfamily antagonist that inhibits the SMAD2/3 cascade and reduces muscle fibrosis. They arrive at the same anti-fibrotic downstream endpoint from different upstream targets.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly converge on anti-inflammatory and IGF1_signaling endpoints through distinct upstream pathways. Follistatin antagonizes TGF-beta superfamily members (myostatin, activin, BMPs) leading to SMAD2/3 inhibition and reduced fibrosis. Klotho suppresses TGF-β/Smad2/3 signaling directly and via multiple pathways (FGF23, Wnt, NF-κB, NLRP3). Both show anti-inflammatory effects and IGF-1R signaling involvement. The proposed relationship accurately describes parallel but mechanistically distinct routes to shared downstream outcomes (anti-fibrotic, anti-inflammatory, IGF-1 signaling enhancement), which is the essence of 'same_downstream' classification.

    Shares anti inflammatory · IGF1 signaling

  • CJC-1295Complementary

    Worth caution

    Follistatin-344 builds muscle by blocking myostatin/activin (disinhibiting mTOR/Akt), an entirely different mechanism from CJC-1295's GH/IGF-1 elevation. Both converge on protein synthesis and lean mass, so they can be complementary for body-composition goals via independent pathways.

    Tier 4Theoretical — not established

    What the research doesn't fully establish

    Both peptides' mechanisms clearly support the claimed complementary relationship and shared dimensions. CJC-1295 increases plasma GH and IGF-I through GHRH-R signaling and cAMP-PKA pathways, with IGF-1_signaling and protein_synthesis explicitly tagged. Follistatin-344 blocks myostatin/activin, disinhibiting mTOR/Akt and IGF-1R signaling, also with IGF1_signaling and protein_synthesis tagged. The mechanisms are mechanistically distinct (GH axis elevation vs. TGF-beta antagonism) yet both converge on protein synthesis and lean mass via independent pathways—the definition of complementarity. The explanation accurately reflects the provided mechanism material without contradiction.

    Shares protein synthesis · IGF1 signaling

Safety and side effects

Safety and side effects

Regulatory and evidence status. No injectable follistatin product is FDA-approved, and there are no approved pharmaceutical formulations. Injectable peptide forms remain understudied in humans: no published human Phase 1, no published clinical pharmacokinetics, extremely limited human safety data, and no long-term (>2 year) safety data outside the gene-therapy trials. Rigorous safety data for unapproved peptides including FS-344 are scarce, with potential for serious harm.

Documented ocular risk

The most concrete human safety signal is ocular: retinal detachment and central serous chorioretinopathy (CSCR). A 2020 case series documented 11 male bodybuilders (mean age 36.8) who developed CSCR after subcutaneous FS-344 injections of complete 1 mg vials into the abdomen (10 unilateral, 1 bilateral). Doses involved exceeded 500 mcg (500-1000+ mcg), given 3-4 times weekly over 8-12 weeks. In patients with only a single injection, subretinal fluid resolved after an average of 2.3 ± 0.7 months (typical recovery 3-6 months) and symptoms regressed; recurrent CSCR developed in those with multiple injections. Caveat: this is a single observational case series at high doses (far above community-suggested amounts) and cannot establish incidence rates.

Gene-therapy trial safety

In the AAV1-FS344 trials, there was no dose-limiting toxicity, no significant organ changes, no serious adverse events, and no clinically detectable reproductive or endocrine dysfunction over 2-year follow-up. Because follistatin can suppress activin-driven FSH secretion, reproductive endocrine consequences remain a theoretical concern, though the 344 isoform is reported to have less impact on FSH.

Anecdotal side effects

In research settings FS-344 is reported as generally well-tolerated, with anecdotal effects including mild nausea at higher doses, injection-site redness or discomfort, rare dizziness/lightheadedness, and potential effects on blood pressure (single low-tier source).

Product quality and contamination

Grey/black-market products have serious quality-control problems: WADA testing found only 9 of 17 products actually contained follistatin; some were instead other growth-promoting peptides (e.g., MGF, GHRP-2) or contaminated with growth-promoting substances. All nine that did contain follistatin had His-tagged FS344 with a high degree of oligomers. A detection method (immunomagnetic purification + SDS-PAGE + Western blot) can distinguish His-tagged FS344 from endogenous follistatin (detection limits ~0.1 ng/mL in urine, 5 ng/mL in serum).

Anti-doping status

Follistatin-344 is prohibited by WADA at all times under Section S4 (Hormone and Metabolic Modulators), with follistatin-related substances banned in 2019.

Reconstitution and handling

Reconstitution and preparation

No dose has been established for this compound. No regulatory label exists for it, so the figures below are what sources report — not guidance.

Important: No clinically validated or FDA-approved injectable dose exists for Follistatin-344, and there is no data on optimal dosing for recreational or fitness use. All specifics below are drawn from community protocols, vendor suggestions, and expert-opinion/blog content — they are not supported by published clinical trials. The degree of myostatin inhibition achievable through brief subcutaneous peptide exposure is unknown.

Reconstitution

Guidance from low-tier sources recommends reconstituting with bacteriostatic water (avoiding saline, which may cause precipitation), to a concentration of roughly 1-5 mg/mL. Subcutaneous injection is described using 29-31 gauge insulin syringes at 4-6 mm depth and a 45-90° angle. Reconstituted product is stored refrigerated at 2-8°C, with reported stability of 14-28 days after reconstitution.

Reported (anecdotal/vendor) dosing protocols

  • Subcutaneous: 50-200 mcg daily for 10-30 days then cycle off; or 100 mcg/day for 10-30 day cycles (common vendor suggestion 100 mcg/day).
  • Pulse/pre-workout: intramuscular 50-100 mcg on training days only, injected ~30 minutes pre-workout into the target muscle group.
  • One source: research SC dosing starting at 100 mcg once daily, standard 250 mcg once daily.
  • Another source: SC 200 mcg three times weekly (Mon/Wed/Fri) as most commonly reported, 100 mcg starting dose for tolerance, 300 mcg upper range where adverse effects become more prevalent; IM 150-400 mcg; typical hypertrophy research 200-300 mcg SC 3x weekly for 6-8 week cycles.
  • Timing: one source suggests administration 2-3 hours post-exercise to potentially enhance the anabolic window.

Reported bioavailability (single low-tier source, src-10)

Subcutaneous 85-90%, intramuscular 75-80%, intravenous 100%, oral <5% (due to first-pass metabolism and GI proteolytic degradation), with peak concentrations 1-2 hours post-injection.

Context vs. gene therapy

All of the above concerns injectable peptide, which has a very short serum half-life (estimates range from minutes to a few hours across sources). This differs fundamentally from AAV gene therapy, which produces sustained follistatin expression over weeks to months and is responsible for the dramatic muscle-growth figures seen in animal and the small human trials. A transgenic pig study demonstrating large muscle gains modeled an equivalent human dose of ~250 mcg every other day — but achieved via tissue-specific overexpression, not exogenous injection. Ocular (CSCR) risk in the human case series was associated with high doses (full 1 mg vials, 500-1000+ mcg, 3-4x weekly).

Sources

Ordered by evidence quality — the strongest first.

  1. Central serous chorioretinopathy associated with high-dose follistatin-344: a retrospective case series.(opens in a new tab)
    Tier 1PubMed · pubmed.ncbi.nlm.nih.gov · 2020
  2. Long-term enhancement of skeletal muscle mass and strength by single gene administration of myostatin inhibitors(opens in a new tab)
    Tier 2Web · pmc.ncbi.nlm.nih.gov
  3. Detection of black market follistatin 344.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2020
  4. Detection of black market follistatin 344.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2019
  5. The transgenic expression of human follistatin-344 increases skeletal muscle mass in pigs.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2017
  6. Production of follistatin in porcine endothelial cells: differential regulation by bacterial compounds and the synthetic glucocorticoid RU 28362.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 1996
  7. Follistatin steady state messenger ribonucleic acid levels in confluent cultures of a rat renal mesangial cell line are regulated by multiple factors.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 1992
  8. Rat follistatin: ontogeny of steady-state mRNA levels in different tissues predicts organ-specific functions.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 1991
  9. Rat follistatin: gonadal and extragonadal expression and evidence for alternative splicing.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 1990
  10. Follistatin 344 — Independent peptide research & reviews(opens in a new tab)
    Tier 3Web · peptidecompared.com
  11. Follistatin 344 (FST344 / Myostatin Inhibitor) — Dosing, Side Effects, FDA Status & Research | PeptIQ | PeptIQ(opens in a new tab)
    Tier 3Web · peptiq.io
  12. Follistatin 344 Dosage for Bodybuilding: Protocols and(opens in a new tab)
    Tier 3Web · mypeptidematch.com
  13. Follistatin 344 Dosage Guide: How Much Should You Take? (2026)(opens in a new tab)
    Tier 3Web · mypeptidematch.com
  14. Follistatin-344: Myostatin Regulation & Muscle Signaling(opens in a new tab)
    Tier 3Web · newtropin.com
  15. Follistatin-344: Remove the Brake on Muscle (2026)(opens in a new tab)
    Tier 3Web · thepeptidecatalog.com
  16. Follistatin-344 Peptide Guide 2026: Myostatin Inhibitor(opens in a new tab)
    Tier 3Web · thepeptidetoolkit.com · 2026
  17. Follistatin 344: Myostatin Pathway Research and the Gene Therapy Evidence · Peptides:Enhanced(opens in a new tab)
    Tier 3Web · peptides-enhanced.com · 2026
  18. Follistatin: A Myostatin-Inhibiting Glycoprotein(opens in a new tab)
    Tier 3Web · superpower.com · 2026
  19. Follistatin-344: Myostatin Inhibitor Research Guide(opens in a new tab)
    Tier 3Web · peptahub.com · 2026
  20. Follistatin-344 (FST-344): Research Evidence & Safety Profile | PeptideInsight(opens in a new tab)
    Tier 3Web · peptideinsight.com · 2026
  21. Follistatin: 38 Studies Reviewed (2026) | PepCodex(opens in a new tab)
    Tier 3Web · pepcodex.com · 2026
  22. Safety and Efficacy of Approved and Unapproved Peptide Therapies for Musculoskeletal Injuries and Athletic Performance.(opens in a new tab)
    Tier 3PubMed · pubmed.ncbi.nlm.nih.gov · 2026
  23. Follistatin 344 Research | Peptides.wiki(opens in a new tab)
    Tier 3Web · peptides.wiki · 2025
  24. Pepzilla — Peptide Research Calculator & Tracker for Semaglutide, Tirzepatide, BPC-157 Research(opens in a new tab)
    Tier 4Web · pepzilla.app
  25. Sub-200  fs, 344  MHz mode-locked Tm-doped fiber laser.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2020