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P21

Tier 3 · Reported use
Also known as P021 peptide · Ac-DGGLAG-NH2 · CNTF-derived peptide

Strongest evidence is Tier 2: preclinical (animal, in-vitro, and mechanistic) data, chiefly rodent Alzheimer's/aging/Down-syndrome models plus in-vitro stability and pharmacokinetic work. There are no completed or registered human clinical trials, no regulatory approvals, and no human safety or efficacy data. Nearly all efficacy findings originate from a single laboratory (Khalid Iqbal group) without independent replication, and human dosing/benefit reports are anecdotal community/vendor sources (Tier 3). Note also that many secondary sources conflate this CNTF-derived peptide with the unrelated cell-cycle protein p21 (CDKN1A/WAF1/Cip1); those claims were excluded.

Half-life
Not recorded
Routes
Oral (used in rodent studies; reported orally bioavailable) · Subcutaneous injection (anecdotal/community) · Intranasal (anecdotal/community)
Goals
Cognitive enhancement / memory · Neuroprotection · Neurodegenerative disease research (Alzheimer's, Down syndrome, TBI) · Healthy aging / age-related cognitive decline
Cost / mg
Not recorded

How it works

P21 is a tiny, chemically stabilized fragment of ciliary neurotrophic factor (CNTF) designed to mimic CNTF's brain-protective effects while being small enough to cross into the brain and stable enough to survive oral or injected delivery. In animal studies it boosts the brain's own production of BDNF (a key growth factor), encourages the birth and maturation of new hippocampal neurons, strengthens synaptic connections, and reduces the abnormal 'tangling' (hyperphosphorylation) of the tau protein linked to Alzheimer's disease. Unlike full CNTF, it does not appear to suppress appetite or cause weight loss in animals. All of these findings are preclinical or anecdotal — there is no human trial data.

Overview

Overview

P21 (formal designation P021; aliases Ac-DGGLAG-NH2, CNTF-derived peptide) is a small, chemically modified peptide derived from the biologically active region of ciliary neurotrophic factor (CNTF). It was engineered to mimic CNTF's neurotrophic properties while overcoming the limitations of the full-length protein — namely poor blood-brain-barrier (BBB) penetration, a very short half-life, and the need for parenteral administration.

Full-length CNTF is roughly 23,000 daltons, far too large to cross the BBB when given peripherally; P21 is approximately forty times smaller. The peptide corresponds to CNTF residues 148-151 (one source instead cites 147-150) and carries an N-terminal acetylation plus a C-terminal modification (amidation and/or an adamantylated glycine). The adamantane group — a highly lipophilic tricyclic alkane — increases lipid solubility to aid BBB penetration, while the terminal caps resist exopeptidase/carboxypeptidase degradation, improving metabolic stability and oral bioavailability.

Proposed Mechanism

In preclinical work, P21 stimulates adult hippocampal neurogenesis, primarily by:

  • Upregulating BDNF transcription and secretion from hippocampal neurons (with elevated phospho-CREB)
  • Competitively inhibiting LIF signaling (acting as a partial LIF antagonist), relieving suppression of neural progenitor proliferation

BDNF acts through the TrkB receptor to activate PI3K/Akt, promoting neuronal survival, synaptic plasticity, and long-term potentiation. Akt in turn phosphorylates and inactivates GSK-3beta, the primary kinase driving tau hyperphosphorylation, thereby reducing tau pathology. P21 has also been proposed to engage the tripartite CNTF receptor complex (CNTFR-alpha/LIFR/gp130) and activate JAK/STAT (STAT3) and possibly RAS/MAPK.

Preclinical Findings

Across rodent studies, P21 has been reported to:

  • Increase newly born mature neurons (BrdU/NeuN double-positive cells) in the dentate gyrus by 30-50%
  • Enhance dendritic arborization, spine density, and synaptic markers (synaptophysin, PSD-95, MAP-2)
  • Prevent and rescue cognitive deficits, reduce tau pathology, and promote neurogenesis in transgenic Alzheimer's models
  • Improve hippocampus-dependent, spatial (Morris water maze), and recognition memory
  • Reduce amyloid-beta and phospho-tau and decrease mortality in 3xTg-AD mice
  • Reverse age-related memory decline in aged rats and lower elevated tau in CSF of aged rats

Models tested include 3xTg-AD, APP/PS1, and Tg2576 Alzheimer's mice, Ts65Dn Down syndrome mice, normal aging rodents, and traumatic brain injury models; one report describes prevention of age-related macular degeneration pathology in rodents.

Development & Regulatory Status

P21 was developed by the Khalid Iqbal laboratory at the New York State Institute for Basic Research in Developmental Disabilities. It is not FDA approved, is not on the 503A bulks list, and remains a research chemical under 'research use only' labeling. No human clinical trials have been published or registered on ClinicalTrials.gov; the corporate developer (Phanes Biotech) targeted a Phase 1 IND for 2025-2026, but no IND had been filed or trial announced as of early-to-mid 2026.

Important Caveats

  • Structural disagreement: Sources conflict on P21's sequence, length, and molecular weight (variously a 4-mer ~470-592 Da, a 5-6-mer Ac-DGGLAG-NH2 ~578 Da, an 8-mer, 11-mer, or a 9-mer at ~985 Da). This reflects inconsistent secondary reporting rather than an agreed structure.
  • Single-lab data: Essentially all efficacy findings come from one laboratory with no independent replication.
  • Name confusion: Many online sources describe an entirely unrelated molecule — the cyclin-dependent kinase inhibitor protein p21 (CDKN1A/WAF1/Cip1) — which has nothing to do with this CNTF-derived peptide.
  • Contested premise: Adult human hippocampal neurogenesis, central to P21's proposed benefit, is itself debated in neuroscience, qualifying how rodent findings might translate.

What the research shows

116 findings extracted from the 17 sources cited below, strongest evidence first within each group. Every one links to the source it came from.

What human studies found

Based on 7 animal findings and 2 theoretical findings.

  • animalChronic treatment with P021 restores cognitive function, increases neurogenesis and synaptic markers, and reduces Aβ and tau in 3xTg-AD mice1

  • animalP021 decreases mortality in a mouse model of Alzheimer's disease1

  • animalLoss of p21 increased trabecular bone formation2

  • animalLoss of E2f1 increased cortical bone formation2

  • animalLoss of E2f1 led to poorer bone repair overall2

  • animalSirt1 protects cardiac function post-myocardial infarction8

  • animalOverexpression of p21 results in G1 arrest and has been shown to suppress effectively tumor growth in vitro and in vivo10

  • theoreticalNo studies have tested P021 in humans yet1

Show 1 more finding

  • theoreticalIn 1994, p21 was found as a tumor suppressor in brain, lung and colon cancer by targeting p53 and was associated with tumorigenesis and metastasis13

How it works

Based on 24 animal findings, 35 in vitro findings, 3 expert opinion findings and 30 theoretical findings.

  • animalP021 increases neurogenesis, BDNF, and synaptic markers in mouse model of Alzheimer's disease1

  • animalP021 reduces phosphorylated tau and soluble Aβ in mouse model of Alzheimer's disease1

  • animalLoss of p21 leads to increased bone formation post-injury2

  • animalE2f1 is downstream of p21 and as a transcription factor can act directly on gene expression2

  • animalWhen E2f1 was absent, there was a dramatic decrease of the number of osteoblasts, osteoclasts, and chondrocytes at the site of injury compared to p21 and C57BL/6 mice2

  • animalE2f1 regulates the cell populations required for bone repair and has a distinct role in bone formation/repair compared to p212

  • animalResidual p21 levels in the S phase control DNA replication speed and origin firing to preserve genomic stability7

  • animalp21 inhibits cardiomyocyte proliferation and cardiac regeneration8

Show 84 more findings

  • animalDeacetylation of p21 by Sirt1 deacetylase may reduce p21 abundance and remove p21-induced cell cycle arrest8

  • animalAcetylation of p21 induces cardiomyocyte proliferation arrest8

  • animalBlocking the acetylation of p21 increases cardiomyocyte proliferation8

  • animalP21 can be acetylated by Sirt18

  • animalSirt1 activates p21 ubiquitination through deacetylation8

  • animalOverexpression of Sirt1 induces EdU-, pH3-, and Aurora B-positive cardiomyocytes in neonatal and adult mice8

  • animalDepletion of Sirt1 reduces cardiomyocyte proliferation8

  • animalSirt1 reduces cardiac remodeling post-myocardial infarction8

  • animalSirt1 inhibits cardiomyocyte apoptosis post-myocardial infarction8

  • animalSirt1 attenuates cardiomyocyte hypertrophy post-myocardial infarction8

  • animalp21 can induce G1 arrest and block entry into S phase by inactivating Cdks or by inhibiting activity of proliferating cell nuclear antigen (PCNA)10

  • animalIn normal cells, p21 exists in quaternary complexes with cyclin, Cdk, and PCNA10

  • animalTranscription of the p21 gene is activated by p53-dependent and -independent mechanisms10

  • animalp21 function has been demonstrated to be necessary for p53-mediated G1 arrest following irradiation of p21-deficient mouse embryonic fibroblasts10

  • animalThe function of p21 under normal circumstances appears to be redundant10

  • animalp21 is expressed in terminally differentiating cells of a variety of tissues in a p53-independent manner10

  • in vitrolincRNA-p21 is proposed to act in cis to promote p53-dependent expression of the neighboring cell cycle gene, Cdkn1a/p213

  • in vitrofull-length transcription, splicing, and accumulation of lincRNA-p21 are dispensable for the chromatin organization of the locus and for cis-regulation3

  • in vitroproduction of lincRNA-p21 through conserved regions in exon 1 of lincRNA-p21 promotes cis-activation3

  • in vitroactivation of nascent transcription from the lncRNA locus, but not the generation or accumulation of a mature lncRNA transcript, is necessary to enact local gene expression control3

  • in vitrop53/p21 interaction influences the transcriptional activity of p534

  • in vitroExpression of p53 promoter-based reporter gene is dependent on p21 levels4

  • in vitrop53 promoter activity requires p53/p21 interaction4

  • in vitroPuma expression is dependent on p53 regardless of γ-irradiation4

  • in vitrop53 mediates expression of Mdm2 and Gadd45a only in irradiated cells4

  • in vitrop53/p21 interaction is required for p53-dependent expression of Puma and not for Mdm2 and Gadd45a in non-irradiated cells4

  • in vitrop21 is required for binding of p53 to promoters of Puma, Mdm2, and Gadd45a4

  • in vitrop53/p21 complex is involved in regulating p53-dependent gene expression4

  • in vitrop21 places cells under immunosurveillance to establish a biological timer mechanism that controls cell fate6

  • in vitrop21 activates retinoblastoma protein (Rb)–dependent transcription at select gene promoters to generate a complex bioactive secretome, termed p21-activated secretory phenotype (PASP)6

  • in vitroPASP includes the chemokine CXCL14, which attracts macrophages6

  • in vitroMacrophages disengage if cells normalize p21 within 4 days6

  • in vitroIf p21 induction persists, macrophages polarize toward an M1 phenotype and lymphocytes mount a cytotoxic T cell response to eliminate target cells, including preneoplastic cells6

  • in vitrop21 concurrently induces proliferative arrest and immunosurveillance of cells under duress6

  • in vitrop21 is a small unstructured protein that binds and inactivates cyclin-dependent kinases (CDKs)7

  • in vitrop21 levels increase following the activation of the p53 tumor suppressor7

  • in vitroCDK inhibition by p21 triggers cell-cycle arrest in the G1 and G2 phases of the cell cycle7

  • in vitroS-phase function of p21 depends fully on its ability to displace partners from chromatin-bound proliferating cell nuclear antigen (PCNA)7

  • in vitroPCNA prevents p21 upregulation in the S phase, even in the context of robust p21 induction by irradiation7

  • in vitroThe cyclin-dependent kinase inhibitor p21WAF1/CIP1 is a major player in cell cycle control and it is mainly regulated at the transcriptional level9

  • in vitroInduction of p21 predominantly leads to cell cycle arrest9

  • in vitroRepression of p21 may have a variety of outcomes depending on the context9

  • in vitroThe major mode of p21 transcriptional repression by negative regulators is the interference with positive transcription factors without direct binding to the p21 promoter9

  • in vitroNegative factors may either inhibit binding of positive regulators to the promoter or hinder their transcriptional activity9

  • in vitroThe ability of p21 to inhibit proliferation may contribute to its tumor suppressor function9

  • in vitroA number of oncogenes repress p21 to promote cell growth and tumorigenesis9

  • in vitrop21 is an inhibitor of apoptosis9

  • in vitrop21 repression may have an anticancer effect9

  • in vitroc-Myc and chemical p21 inhibitors, which repress p21, sensitize tumor cells to apoptosis by anticancer drugs9

  • in vitroCombination treatment with bortezomib and ELP-bound p21 Cip1/Waf1 protein leads to increased cell cycle arrest11

  • in vitroCombination treatment with bortezomib and ELP-bound p21 Cip1/Waf1 protein leads to apoptosis11

  • expert opinionp21 was initially identified as a widespread inhibitor of cyclin-dependent kinases, transcriptionally modulated by p53 and a marker of cellular senescence5

  • expert opinionp21 acts as a tumour suppressor mainly to restrain cell cycle progression, thereby resulting in growth suppression5

  • expert opinionp21 regulates radiation responses via participating in multiple cellular processes, including cell cycle arrest, apoptosis, DNA repair, senescence and autophagy5

  • theoreticalP021 is a tetra-peptide derived from the biologically active region of human ciliary neurotrophic factor (CNTF; amino acid residues 148–151)1

  • theoreticalP021 competitively inhibits leukemia inhibitory factor (LIF) signaling to promote formation of neural progenitor cells1

  • theoreticalP021 increases expression of neurotrophic factor BDNF, promoting neurogenesis1

  • theoreticalp21, encoded by gene, also named p21, was first identified as a cyclin-dependent kinase regulator that suppresses cell cycle G1/S phase and retinoblastoma protein phosphorylation13

  • theoreticalp21 acts as the downstream target gene of p53, and its expression is induced by wild-type p53 and it is not associated with mutant p5313

  • theoreticalp21 has been characterized as a vital regulator that involves multiple cell functions, including G1/S cell cycle progression, cell growth, DNA damage, and cell stemness13

  • theoreticalp21 plays a significant role in tumor development through p53-dependent and p53-independent pathways13

  • theoreticalexpression of p21 is closely related to the resting state or terminal differentiation of cells13

  • theoreticalp21 is associated with cancer stem cells and acts as a biomarker for such cells13

  • theoreticalp21 functions as a cell cycle inhibitor and anti-proliferative effector in normal cells14

  • theoreticalp21 is dysregulated in some cancers14

  • theoreticalp21 plays a role in cell cycle arrest under p53 transcription factor activity14

  • theoreticalp21 has tumor-suppressor function in cancer14

  • theoreticalp21 has a role in phenotypic plasticity14

  • theoreticalp21 has oncogenic/anti-apoptotic function depending on p21 subcellular localization and p53 status14

  • theoreticalPrecision gene editing can be used to manipulate p21 and proteins that interact with it for therapeutic approaches in cancer14

  • theoreticalCyclin-dependent kinase (CDK) inhibitor p21 (also known as p21(WAF1/Cip1)) promotes cell cycle arrest in response to a variety of stimuli15

  • theoreticalThe inhibitory effect of P21 on cell cycle progression correlates with its nuclear localization15

  • theoreticalP21 can be induced by both p53-dependent and p53-independent mechanisms15

  • theoreticalP21 has functions in transcriptional regulation, modulation or inhibition of apoptosis15

  • theoreticalP21 functions are largely dependent on direct p21/protein interactions and on p21 subcellular localizations15

  • theoreticalP21 can play a role in DNA repair by interacting with proliferating cell nuclear antigen (PCNA)15

  • theoreticalp21(Cip1) is a multifunctional protein and a key player in regulating different cellular processes16

  • theoreticalThe transcription of p21 is regulated by p53-dependent and -independent pathways16

  • theoreticalThe expression of p21 is increased in response to various cellular stresses to arrest the cell cycle and ensure genomic stability16

  • theoreticalp21 binds to and inhibits the activity of Cdk1/cyclin B116

  • theoreticalp21 is important for a fine-tuned mitotic progression16

  • theoreticalLoss of p21 prolongs the duration of mitosis and results in severe mitotic defects like chromosome segregation and cytokinesis failures promoting consequently genomic instability16

  • theoreticalp21 is dramatically stabilized in mitotic tumor cells upon treatment with mitotic agents like paclitaxel or mitotic kinase inhibitors16

  • theoreticalIncreased p21 is mainly localized in the cytoplasm and associates with cell survival indicating a crucial role of p21 in susceptibility to mitotic agents in tumor cells16

How the body handles it

Based on 1 theoretical finding.

  • theoreticalP021 has adamantylated glycine added at C-terminal to increase lipophilicity and blood-brain permeability while decreasing degradation by exopeptidases1

Safety and side effects

Based on 3 animal findings and 1 theoretical finding.

  • animalP021 does not have anorectic effects in rodents1

  • animalP021 may increase body weight in rodents1

  • animalP021 treatment for up to 18 months in rodents does not result in weight loss, tumors, or signs of pain1

  • theoreticalSafety in humans has not been assessed1

What people use it for

Based on 2 animal findings, 4 expert opinion findings and 4 theoretical findings.

  • animalP021 appears to prevent pathologies of age-related macular degeneration in rodents1

  • animalMice deficient in p21 exhibit no apparent phenotype10

  • expert opinionp21 influences the effect of cancer radiotherapy via involving in multiple signaling pathways5

  • expert opinionp21 is a promising therapeutic target for cancer radiotherapy5

  • expert opinionCDK-independent functions of p21 may have potential for the improvement of cancer treatments7

  • expert opinionp21 may have potential gene therapy applications10

  • theoreticalp21/E2f1 could be potential druggable targets that could be leveraged in clinical therapies to improve bone healing in pathologies such as osteoporosis2

  • theoreticalSirt1-induced p21 deacetylation could be a novel therapeutic strategy for myocardial infarction8

Show 2 more findings

  • theoreticalp21 is implicated in response to many cancer treatments and p21 promotes the effect of oncolytic virotherapy13

  • theoreticalp21 has been shown to be a tumor suppressor and an oncogene as well16

Points of contention

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

Other

Many cited sources describe a completely different molecule that shares the name 'p21'.

Sources Thermally targeted p21 peptide enhances bortezomib ... - PMC through Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy. (and parts of P21 - Exploring Peptides) discuss the cyclin-dependent kinase inhibitor protein p21 (also called p21WAF1/Cip1/CDKN1A), a tumor-suppressor/cell-cycle regulator that is entirely unrelated to the CNTF-derived research peptide P021. P21 - Exploring Peptides also incorrectly states P21 is derived from cerebrolysin and acts by blocking CDK5/p25. None of these claims apply to the CNTF-derived peptide and were excluded from the peptide claims above.

Contested

Reported half-life values conflict widely.

Plasma half-life is reported as over 3 hours in mice (1 Last updated on May 12, 2025 Cognitive Vitality Reports ...), greater than 6 hours in rodents (P21 — Molecular Specifications & Research Monograph | Peptide Biologix), 4-6 hours (P21 (P021) — Dosage, Half-Life & Research | Peptide Reference), and 'typically less than one hour' (P21 - Exploring Peptides). These cannot all be reconciled, and low-tier sources contribute to the spread.

Limited evidence

No human data exists; efficacy and safety are entirely preclinical or anecdotal.

There are no completed or registered human clinical trials, no human pharmacokinetic/dose-response/efficacy data, and no human safety data. Human dosing, side-effect, and benefit reports come only from low-to-mid-tier community/vendor sources (P21 (P021) — Dosage, Half-Life & Research | Peptide Reference, P21 Peptide Research, P21 (Nootropic Peptide Candidate) — Dosing, Side Effects, FDA Status & Research | PeptIQ | PeptIQ, P21 - The International Peptide Society).

Contested

The core premise (adult human hippocampal neurogenesis) is itself debated.

P21 Peptide: Research, Evidence, Dosing & Safety | Peptidings notes that adult human hippocampal neurogenesis—central to P21's proposed benefit—remains a contested concept in neuroscience, which qualifies how directly rodent neurogenesis findings would translate to humans.

Using it with other compounds

Check a stack
  • SelankSame downstream effect

    No documented conflict

    Selank supports BDNF (and NGF) signaling alongside its anxiolytic/nootropic actions, overlapping with the BDNF/TrkB pathway P21 activates. Coming from different upstream mechanisms and converging on BDNF-mediated plasticity, they are plausibly complementary for cognition and mood support.

    Tier 4Theoretical — not established

    Shares BDNF signaling

  • SemaxSame downstream effect

    May be complementary

    P21 raises BDNF that then activates TrkB; Semax also upregulates BDNF/NGF and signals through TrkB and CREB. Both end up driving the same BDNF/TrkB neurotrophic and neurogenic output through different starting points, so they are potentially additive for memory and neuroprotection rather than redundant. Monitor for over-stimulation, since both also touch CREB and plasticity pathways.

    Tier 4Theoretical — not established

    Shares neurogenesis · BDNF signaling

  • PinealonComplementary

    Worth caution

    Both report enhanced dendritic arborization, synaptic plasticity and neurogenesis. P21 drives BDNF/TrkB and PI3K/Akt–GSK-3β signaling, while Pinealon is proposed to reach similar endpoints via gene-expression/ERK modulation. The distinct mechanisms pointing at the same plasticity outcome make them complementary in principle, with only preclinical/anecdotal support for combining.

    Tier 4Theoretical — not established

    Shares neurogenesis

  • HumaninSame mechanism

    Worth caution

    This is a real overlap worth understanding: humanin's reported neuroprotection depends on the CNTFR-alpha/WSX-1/gp130 cytokine receptor complex acting through JAK2/STAT3 and PI3K/Akt, and P21 is a CNTF-fragment that engages the same CNTF/LIFR/gp130 receptor family and STAT3/Akt signaling. Because they converge on the same gp130/JAK-STAT machinery, combining them is more likely to be redundant than additive at that receptor, though their broader effects differ.

    Tier 4Theoretical — not established

    Shares mTOR PI3K

  • CerebrolysinComplementary

    May be complementary

    Cerebrolysin stimulates BDNF and signals through TrkB and PI3K/Akt survival pathways, while P21 (a CNTF fragment) drives BDNF expression that also feeds TrkB and neurogenesis. They reach the same BDNF/TrkB neuroplasticity endpoint from different starting points, so they can reinforce each other's neurotrophic and pro-neurogenic effects. Evidence for the specific combination is anecdotal.

    Tier 4Theoretical — not established

    Shares neurogenesis · mTOR PI3K · BDNF signaling

  • NoopeptSame downstream effect

    May be complementary

    Both peptides ultimately drive the BDNF/TrkB neurotrophin cascade and promote synaptic plasticity and memory — P21 as a CNTF-derived fragment that upregulates BDNF, Noopept via its metabolite acting on TrkB and AMPA. They reach the same downstream output (enhanced BDNF signaling and neuroprotection) from different upstream starting points, which is generally synergistic but worth noting as overlapping rather than independent.

    Tier 4Theoretical — not established

    Shares BDNF signaling

  • DavunetideSame downstream effect

    May be complementary

    Both peptides converge on reducing tau hyperphosphorylation, but by different routes: davunetide stabilizes microtubules and dampens GSK-3 activity directly, while P21 raises BDNF and inactivates GSK-3beta via the Akt pathway. They arrive at the same anti-tau, pro-synaptic endpoint from different starting points, which is why they are complementary rather than redundant.

    Tier 4Theoretical — not established

    Shares mTOR PI3K

  • CortexinComplementary

    May be complementary

    P21 is a CNTF fragment that strongly upregulates BDNF and hippocampal neurogenesis while reducing tau phosphorylation, whereas Cortexin provides broad neurotrophic and anti-excitotoxic protection. Different upstream mechanisms both feeding neuroplasticity and neuronal survival, so the two are complementary.

    Tier 4Theoretical — not established

    Shares neurogenesis · BDNF signaling

Safety and side effects

Safety Overview

There are no completed human clinical trials of P21, no regulatory approvals, no validated human dose, and no human safety data. All toxicology and tolerability information is preclinical or anecdotal.

Preclinical (Animal) Safety

  • In rodents, P21 does not have anorectic effects and may actually increase body weight.
  • Treatment for up to 18 months produced no weight loss, no tumors, and no signs of pain.
  • Unlike full-length CNTF, P21 reportedly lacks appetite-suppressing side effects — attributed to its not activating STAT3 signaling in the hypothalamus — and appears to circumvent CNTF adverse effects such as weight loss and immunogenicity.

Contrast with Full-Length CNTF

Clinical trials using full-length CNTF in ALS patients showed no benefit and produced serious side effects including severe weight loss, anorexia, cramps, and muscle pain. P21 was specifically engineered to avoid these liabilities, but this advantage has only been demonstrated in animals.

Anecdotal (Community) Reports

These come from low-to-mid-tier community/vendor sources and should be treated cautiously:

  • Reported side effects: headache, nasal irritation (intranasal route), mild fatigue, mild anxiety/restlessness, sleep disturbance, vivid dreams, rare mood changes, and hair thinning with heavy dosing.
  • Users report no psychostimulant effects, dependency, or crash.
  • One source reports subcutaneous P21 as safe at recommended dosages, with possible redness and pain at the injection site.

Reported Combination Cautions (Vendor Source)

One vendor source suggests P21 is compatible with NAD+ and BPC-157, advises caution when combining with stimulants (modafinil, Adderall) and racetams, and recommends monitoring when combined with MAOIs/SSRIs(vendor claim). These are not clinically validated recommendations.

Bottom line: Human safety is entirely unestablished. Any use is experimental, and reported human tolerability rests on anecdote alone.

Reconstitution and handling

Reconstitution & 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.

Note: No human dose has been clinically validated. The guidance below reflects reported preclinical dosing and anecdotal community/vendor practices — not established protocols.

For reconstitution, expert/vendor sources advise:

  • Use bacteriostatic water — saline may cause precipitation.
  • Use the reconstituted solution within 28 days.
  • Avoid vigorous shaking; swirl gently.

Administration Routes

  • Oral — used in rodent studies; P21 is reported to be orally bioavailable and metabolically stable (>95% stability in simulated gastric fluid, plasma stability >3 hours).
  • Subcutaneous injection — anecdotal/community route.
  • Intranasal — anecdotal/community route.

Reported Dosing

Preclinical (animal):

  • Mouse studies have used 60 nM per gram of feed.

Anecdotal (community — wide variation, not validated):

  • 1-2 mg intranasal or subcutaneous daily
  • 100-300 mcg/day subcutaneous
  • 200-400 mcg cited as a median dose
  • Typically cycled (e.g., 4-8 weeks)

Anecdotal users describe subjective onset of effects (memory consolidation, learning speed, cognitive clarity, mood/focus, sleep quality) over 1-4 weeks.

Pharmacokinetic Context

Pharmacokinetic studies have been published in mice and cynomolgus monkeys, with plasma half-life reportedly extended from minutes (full-length CNTF ~2.9 min) to hours, though reported values conflict widely across sources (see half-life notes). P21's molecular weight (~578 Da) sits at the upper boundary of the typical BBB peptide exclusion threshold (400-600 Da), with the adamantane modification reported to enable CNS penetration.

Sources

Ordered by evidence quality — the strongest first.

  1. 1 Last updated on May 12, 2025 Cognitive Vitality Reports ...(opens in a new tab)
    Tier 2Web · alzdiscovery.org
  2. Absence of E2f1 Negates Pro-osteogenic Impacts of p21 Absence.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2024
  3. Functional elements of the cis-regulatory lincRNA-p21.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2022
  4. Involvement of the p53/p21 complex in p53-dependent gene expression.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2022
  5. Multiple functions of p21 in cancer radiotherapy.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2021
  6. p21 produces a bioactive secretome that places stressed cells under immunosurveillance.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2021
  7. CDK-Independent and PCNA-Dependent Functions of p21 in DNA Replication.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2020
  8. Sirt1-inducible deacetylation of p21 promotes cardiomyocyte proliferation.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2019
  9. Lost in transcription: p21 repression, mechanisms, and consequences.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 2005
  10. p21--negative regulator of the cell cycle.(opens in a new tab)
    Tier 2PubMed · pubmed.ncbi.nlm.nih.gov · 1996
  11. Thermally targeted p21 peptide enhances bortezomib ... - PMC(opens in a new tab)
    Tier 4Web · pmc.ncbi.nlm.nih.gov
  12. An inducible-Cre mouse model to monitor and manipulate-highly-expressing senescent cells.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2021
  13. Multifaceted p21 in carcinogenesis, stemness of tumor and tumor therapy.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2020
  14. p21 in Cancer Research.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2019
  15. Multiple functions of p21 in cell cycle, apoptosis and transcriptional regulation after DNA damage.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2016
  16. Less understood issues: p21(Cip1) in mitosis and its therapeutic potential.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2015
  17. LincRNA-p21 suppresses target mRNA translation.(opens in a new tab)
    Tier 4PubMed · pubmed.ncbi.nlm.nih.gov · 2012

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