Semax and Selank are two different seven-amino-acid peptides that share only their last three residues, Pro-Gly-Pro. Semax begins Met-Glu-His-Phe; Selank begins Thr-Lys-Pro-Arg. Their human research covers different conditions — stroke rehabilitation for Semax, anxiety for Selank — and neither is FDA approved or on the 503A bulks list.
Side by side
What the shared tail does and does not mean
The Pro-Gly-Pro ending is why the two are so often sold and discussed as a pair. It is a structural detail, not a shared mechanism: the first four residues differ entirely, and those are the part each peptide was built around. Two molecules with the same last three amino acids are not interchangeable, and evidence about one says nothing about the other.
The chemistry also matters before any comparison of doses. The FDA's Semax evaluation treats Semax free base (813.93 g/mol) and Semax acetate (874.0 g/mol) as different bulk drug substances, and noted that nominators gave inconsistent identities for the two. A bottle that says only “Semax” may not say which one it holds.
What the human studies actually tested
The clearest Semax regimen is from a 2018 study of 110 adults recovering from ischemic stroke: 6,000 mcg per day in two 10-day courses separated by 20 days. The clearest Selank figure is 2,700 mcg per day intranasally in a 20-person generalized-anxiety report, with a separate 60-patient trial comparing intranasal Selank against phenazepam.
Those numbers describe different people, conditions and endpoints. Setting them next to each other as if one peptide were “stronger” misreads both: neither study was designed to establish a general dose, and there is no head-to-head comparison at all. The detail on each is in Semax nasal spray: dosage claims vs evidence and Selank dosage and withdrawal.
Regulatory status in the US
Neither is a component of an FDA-approved drug. The FDA's 2026 review concluded the evidence weighed against adding Semax free base or acetate to the 503A bulks list, citing gaps in characterization, effectiveness and safety. Selank acetate is listed among withdrawn nominations on the FDA's compounding-risk page, which flags possible immunogenicity risk for some routes. A withdrawn nomination is not a clearance; it means Selank never reached the list.
Both are usually sold as nasal sprays — which adds its own unknown
For either peptide, the amount per spray is the bottle concentration multiplied by the pump's delivered volume, and both have to be verified. The FDA treated the pump as a critical component in its Semax review, listing delivery, content uniformity and droplet size among the quality attributes no proposal had controlled. The same arithmetic and the same caveat apply to Selank.