P21 Calculator
Also known as: CNTF-derived peptide, Peptide 021
What is P21?
P21 is an eleven-amino-acid synthetic peptide derived from a region of ciliary neurotrophic factor (CNTF) that was identified as the active fragment responsible for promoting neurogenesis in adult rodent brain. Unlike full-length CNTF, the P21 fragment was designed to retain neurogenic activity while crossing the blood-brain barrier and avoiding the hypothalamic side-effect profile that ended clinical development of recombinant CNTF in obesity trials. Preclinical studies in rodent models report increases in hippocampal neurogenesis, dendritic spine density, and performance on cognition and memory tasks following peripheral administration. P21 is studied in models of Alzheimer's disease, Down syndrome, and age-related cognitive decline. Published human trial data remains very limited.
| 5 mg | lyophilized powder |
| 0.1 mg | ≈ 4 U-100 units (at 5mg / 2mL) |
| 0.2 mg | ≈ 8 U-100 units (at 5mg / 2mL) |
How it's typically prepared
Lyophilized P21 is reconstituted with bacteriostatic water and refrigerated at 2–8 °C. The peptide is small and water-soluble; standard research-vendor presentations are 5 mg vials reconstituted in 2 mL of BAC water for a 2.5 mg/mL working concentration. Reference doses in published rodent work translate to sub-milligram subcutaneous amounts in humans on a body-weight-adjusted basis, which is why a U-100 insulin syringe is the appropriate draw tool.
P21 Dosage in Preclinical Research
Reference doses for P21 in published rodent studies cluster in the 0.05–0.5 mg/kg range administered subcutaneously or intraperitoneally, with most cognitive-behavioral studies using a single-digit-milligram-per-kilogram course over several weeks. In a research-peptide context where humans are sometimes dosed by analogy, that translates to roughly 0.1–0.2 mg per dose for an adult, which is the range shown on this calculator preset. None of these figures are clinical doses — there is no FDA-approved P21 product and the published human data is sparse.
At the standard 5 mg / 2 mL reconstitution (2.5 mg/mL concentration), a 0.1 mg dose draws to 4 units on a U-100 insulin syringe and a 0.2 mg dose draws to 8 units. Because the milligram amount is so small, draw accuracy matters more than for larger-volume peptides — half-unit graduated U-100 syringes are the appropriate tool, and the dose chart below shows the specific unit conversions.
Cycle length in preclinical work is typically 7–28 days of daily or every-other-day dosing. Longer human protocols seen in research-peptide forums extrapolate from these preclinical schedules and are not validated by published trials. Whether peripheral administration produces clinically meaningful CNS exposure in humans is, again, not established.
P21 Dosage Chart
U-100 insulin syringe units for common P21 doses at the standard 5 mg / 2 mL reconstitution (2.5 mg/mL).
| 0.05 mg | 2 units · 0.02 mL |
| 0.1 mg | 4 units · 0.04 mL |
| 0.2 mg | 8 units · 0.08 mL |
| 0.5 mg | 20 units · 0.20 mL |
Frequently asked questions
- What is P21 derived from?
- P21 is an eleven-amino-acid fragment of ciliary neurotrophic factor (CNTF). The CNTF gene encodes a 200-residue cytokine that promotes survival of motor neurons and adult neurogenesis. Researchers identified a small active region of CNTF and synthesized it as a standalone peptide, called P21 or peptide 021, to retain the neurogenic activity without the metabolic and inflammatory effects that limited full-length CNTF as a drug candidate.
- What does P21 do in animal models?
- Published rodent studies report increased hippocampal neurogenesis, increased dendritic spine density, improvements on Morris water maze and novel-object recognition tasks, and rescue of cognitive deficits in transgenic Alzheimer's models. Effects have been reported following intraperitoneal, subcutaneous, and intranasal administration in mice and rats.
- Is there human clinical data on P21?
- As of 2026, published human trial data on P21 is very limited. Most of the available evidence is preclinical, in rodent models. P21 is not an FDA-approved drug and is not in widespread clinical development under that name.
- How is P21 different from Cerebrolysin or Semax?
- All three are studied for cognition and neuroprotection but they are mechanistically distinct. Cerebrolysin is a porcine-brain-derived peptide preparation used clinically in some countries for stroke and dementia. Semax is a synthetic ACTH(4-10) heptapeptide analogue. P21 is a CNTF fragment selected specifically to drive adult neurogenesis. The peptides do not target the same receptor or pathway.
- Why is P21 dosed in such small amounts?
- Because P21 is a small synthetic peptide with high specific activity in preclinical models, reference doses are typically in the 0.1–0.2 mg range — well under one milligram. That puts a 0.1 mg dose at roughly 4 units on a U-100 insulin syringe at the standard 2.5 mg/mL reconstitution, which is why the calculator presets a U-100 syringe for this peptide.
- Does P21 cross the blood-brain barrier?
- Preclinical work suggests that the P21 peptide retains CNTF-derived neurogenic activity following peripheral administration, which the authors interpret as evidence of meaningful CNS exposure. Whether peripheral dosing produces therapeutically relevant CNS concentrations in humans has not been established.
Other Cognitive peptides
PeptideDose is an educational reference. It is not medical advice and does not replace consultation with a licensed healthcare provider. Doses shown in presets are derived from published protocols and product labels — they are not personal recommendations.