P21 vs Dihexa: evidence, cost, and legality

Which is better, P21 or Dihexa?

Both are graded animal only and both come from single research groups, so neither is a supported choice for human use. P21 wins on the quality of the animal record: normal mice showed better memory and dentate gyrus neurogenesis, and 12 months of oral dosing in 3xTg-AD mice reduced tau pathology and soluble amyloid beta, with no integrity flags. Dihexa's HGF/c-Met mechanism paper was retracted and its design paper is under an expression of concern. On regulation they are equivalent: neither can lawfully be compounded. [1] [2] [3] [4] [5]

How do P21 and Dihexa compare?

P21 and Dihexa compared by dimension
Dimension	P21Favored	Dihexa
What it is [1] [4]	Adamantane modified tetrapeptide derived from the active region of ciliary neurotrophic factor (CNTF).	Synthetic oligopeptide derived from angiotensin IV.
Proposed mechanism [1] [3] [6]	CNTF mimetic that inhibits LIF signaling and raises BDNF, promoting neurogenesis and synaptic plasticity in mice.	Reported to potentiate hepatocyte growth factor and c-Met signaling; the supporting paper was retracted in 2025.
Animal evidence [1] [2] [4]	Better learning, memory, and neurogenesis in normal mice; 12 months of oral dosing in 3xTg-AD mice reduced tau hyperphosphorylation and soluble amyloid beta and rescued cognition.	Reversed scopolamine induced water maze deficits and improved aged rat performance in one laboratory's studies.
Data integrity [3] [4] [6]	No retractions or expressions of concern identified; single research group.	Mechanism paper retracted (April 2025); design paper under a 2021 expression of concern; single research group.
Theoretical safety concern [6] [7]	A growth factor mimetic could in theory influence tumor growth; not studied.	Potentiating c-Met, an established oncogene, could promote tumor growth; no toxicology published.
Human evidence [4] [6]	None. No trial registration, pharmacokinetic study, or safety report.	None. No human study of any kind.
FDA and compounding status [5] [8]	Never nominated for the 503A or 503B bulks lists and not on the Category 2 page; not compoundable.	Nominated and placed in Category 2, then withdrawn; not on the bulks list and not recommended by PCAC; not compoundable.
WADA status [9]	Prohibited at all times under S0 (non-approved substances).	Prohibited at all times under S0 (non-approved substances).

Evidence grade and regulatory status

Pulled from each peptide’s own record, so it stays in step with the peptide pages.

P21 and Dihexa: grade, status, and cost
Attribute	P21	Dihexa
Class	Synthetic adamantane-modified tetrapeptide derived from the active region of ciliary neurotrophic factor (CNTF)	Synthetic angiotensin IV derived oligopeptide (hexanoyl capped Tyr-Ile with an aminohexanoic amide tail), hepatocyte growth factor/c-Met potentiator
What it is	Lab-made peptide mimicking part of CNTF, a protein that keeps nerve cells alive; mouse and rat memory and Alzheimer data; no human studies of any kind.	Oral lab-made relative of the brain peptide angiotensin IV: rat memory data from one lab, a retracted mechanism paper, no human trials, no FDA approval.
Evidence	Evidence: Animal-only evidence	Evidence: Animal-only evidence
FDA status	Regulatory: Unscheduled	Regulatory: Removed from Category 2 (Apr 2026)
Compounding	P21 has never been nominated to or reviewed for the FDA 503A or 503B bulks lists and does not appear in any category on the FDA Category 2 page (current as of April 22, 2026). It is not a component of any approved drug, so a 503A pharmacy has no lawful basis to compound it. It is not an FDA approved drug.	Dihexa has never been reviewed or approved by FDA for any use. It was named on the FDA 503A Category 2 list (substances with significant safety risks) in 2023. The FDA Category 2 page current as of April 22, 2026 now lists dihexa acetate under bulk drug substances nominated but withdrawn, so it is no longer in Category 2, but it is not on the 503A bulks list and was not among the peptides the Pharmacy Compounding Advisory Committee recommended on July 23 to 24, 2026. With no active nomination and no listing, a 503A pharmacy has no lawful basis to compound it. It is not eligible for 503B outsourcing and no licensed compounding pathway has been identified.
WADA	WADA: WADA prohibited	WADA: WADA prohibited
Routes	Oral in diet (mouse studies), Peripheral injection (mouse studies), Sold as oral capsules or injection by research chemical sellers (not lawful for human use)	Oral (rat studies; sold as capsules or powder), Intraperitoneal injection (rat studies), Transdermal and subcutaneous products exist without absorption data
Typical cost	Not available through licensed channels. Research chemical sellers list it, but those products are not lawful for human use and are not verified for identity or purity.	Not available through licensed channels. Products sold as research chemicals are not lawful for human use and are not verified for identity or purity.
Last verified	Sep 22, 2026	Sep 22, 2026

Has P21 been tested directly against Dihexa?

No published trial has compared P21 and Dihexa directly. The dimensions above come from separate studies and labels, and cross-trial comparisons are less reliable than a direct trial.

What do P21 and Dihexa cost?

Typical cost of P21 and Dihexa
Dimension	P21	Dihexa
Typical cost	Not available through licensed channels. Research chemical sellers list it, but those products are not lawful for human use and are not verified for identity or purity.	Not available through licensed channels. Products sold as research chemicals are not lawful for human use and are not verified for identity or purity.
Price detail	Price index in progress	Price index in progress

Where a price index exists, the range is the cash price per month across branded, compounded, and telehealth channels, without insurance. The cost pages give each channel separately with its sources. Otherwise the typical cost from the peptide record is shown. Legal access depends on your state: see peptide legality by state.

Frequently asked questions

Is P21 or dihexa better for memory?

Neither has been tested in humans. In mice, P21 improved memory and neurogenesis and reduced Alzheimer type pathology over 12 months; dihexa improved rat water maze performance in studies whose mechanism paper was later retracted. Neither result can be assumed to apply to people. [1] [2] [3] [4]

Can either be prescribed or compounded?

No. P21 has never been nominated for the bulks lists, and dihexa's nomination was withdrawn without a bulks list recommendation, so neither has a lawful basis for 503A or 503B compounding. Research chemical products are not lawful for human use. [5] [8]

Is there any human safety data?

No. Neither has a published human study, and neither has a published formal animal toxicology study. Both have theoretical concerns tied to growth factor signaling. [6] [7]

Are they banned in sport?

Yes. Both are prohibited at all times under S0 of the WADA Prohibited List as non-approved substances. [9]

Conditions studied

Cognitive decline and nootropic use

Sources

Numbered citations above point to these primary sources. PubMed entries link to the indexed abstract.

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APA style: PeptideAgent. (2026, September 22). P21 vs Dihexa: evidence, cost, and legality. https://peptideagent.ai/compare/p21-vs-dihexa
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