Semax and Selank Compared

For laboratory research use only. Not for human or veterinary use.

Semax and Selank are nearly always sold side by side and nearly always described in the same vague sentence. That is a shame, because what they genuinely have in common is specific, structural and checkable on any certificate. Both are seven-residue peptides. Both came out of the same Moscow institute. And both are a short natural fragment with the identical three residues added to the end. This guide sets out what each one is, where its first four residues come from, and what the shared tail is there for.

The same three residues on the end#

Write the two sequences one above the other and the pattern is immediate:

• Semax — Met-Glu-His-Phe-Pro-Gly-Pro
• Selank — Thr-Lys-Pro-Arg-Pro-Gly-Pro

In each case the first four residues are a fragment that already occurs in biology, and the last three are the same synthetic addition: proline, glycine, proline. The two molecules are unrelated in origin and were designed for different research questions. The tail is the family resemblance.

Where the first four come from#

Semax begins with Met-Glu-His-Phe, which is residues 4 to 7 of adrenocorticotropic hormone. It is usually described as an analogue of the ACTH(4-10) fragment, because ACTH(4-10) is the piece that the earlier literature worked on; the synthetic molecule keeps the first four of those residues and replaces the rest.

Selank begins with Thr-Lys-Pro-Arg, which is tuftsin — a tetrapeptide cut from the heavy chain of immunoglobulin G, and a molecule with its own literature going back to the 1970s.

So one starts from a pituitary hormone fragment and the other from an antibody fragment. Anything written about them that treats the pair as variations on a theme has skipped the part that matters.

What the tail is for#

Both parent tetrapeptides are cleaved quickly by peptidases. The published rationale for adding Pro-Gly-Pro is that proline residues sit awkwardly in the active sites of many of those enzymes, so a proline-flanked tail slows the molecule's breakdown and the fragment survives long enough to be studied.

That is the reasoning as stated. How much of the observed difference in duration it actually accounts for is not something the literature settles cleanly, and a guide that tells you otherwise is overselling a design note as a finding.

Side by side#

The sulfur is worth noticing. Semax carries a methionine, Selank does not, and methionine is the residue most likely to pick up an oxygen during careless storage. That is the one handling difference between the two that has a chemical reason behind it rather than a habit.

What the research literature actually covers#

Both peptides have been studied in neuropeptide research — Semax largely in work on neurotrophic factor expression in animal models, Selank largely in work on GABAergic signalling and on the immune activity inherited from tuftsin.

The honest caveat is about the evidence base rather than the molecules. Most of the published work on both is Russian-language, much of it predates modern reporting standards, and the English-language peer-reviewed literature is thin and heavily preclinical. That is a statement about how much has been published and where, not a verdict on the compounds. Anyone citing a confident human conclusion for either one is going well past what has been shown.

What to check on the certificate#

These are short sequences, which has two consequences for documentation.

First, the mass is unambiguous. At 813.93 and 751.87 g/mol they are small enough that mass spectrometry separates them from each other, and from a wrong sequence, without any difficulty. A certificate that reports an identity check should show a mass that matches to the decimal place. If it only reports a purity percentage, it has told you the sample is clean without telling you what the sample is.

Second, check the fill. A purity figure describes the material that is present; it says nothing about how much of it there is. The two numbers answer different questions and a strong chromatographic result does not rule out a short fill.

Our certificates of analysis are being re-checked, and we are not publishing them or making testing claims until that is finished. When you are given a certificate for either, our guide on verifying a certificate walks through checking it with the laboratory, and it takes about a minute.

Handling and storage#

Nothing here is specific to this pair except the methionine point above, but it is worth stating rather than assuming.

• Both arrive as a white lyophilised powder and dissolve readily in bacteriostatic or sterile water. See which solvent to use.
• Lyophilised, both are stable refrigerated and keep longest in a freezer. Once in solution, refrigerate and use within the working window rather than the shelf life on the box.
• Protect from light, and avoid repeated freeze–thaw cycles — each one costs a little material.
• For Semax in particular, minimise how long a reconstituted vial spends warm or exposed to air. Methionine oxidation is the most likely route to a sample that no longer matches its own certificate.

The storage guide and the reconstitution guide cover the general procedure.

So which one#

This is the question the pair invites and it does not have a research answer. They descend from different parent molecules, they have been studied against different questions, and the literature does not contain the head-to-head comparison that would be needed to rank them. They are sold together because they were developed together and arrive from the same place — not because they are interchangeable.

If the structural difference matters to your work, it is in the table above. If it does not, nothing on this page or any other will make the choice for you.

Sources#

Status elsewhere: Both peptides are used clinically in Russia, where they were developed.

Each entry links to its PubMed record, where the title and abstract can be read. The descriptions are ours: they say what each paper examined, not what it is claimed to show. Records checked on PubMed on 28 October 2026.

1. Asmarin IP, Nezavibat’ko VN, Miasoedov NF, et al. (1997). Zh Vyssh Nerv Deiat Im I P Pavlova 47(2):420–30. (in Russian) Overview by the Moscow group that designed Semax, covering fifteen years of work that took it from the laboratory into clinical use. PubMed 9173745
2. Dolotov OV, Karpenko EA, Inozemtseva LS, et al. (2006). Brain Res 1117(1):54–60. Rat study measuring BDNF and trkB levels in the hippocampus after a single exposure to Semax. PubMed 16996037
3. Gusev EI, Martynov MY, Kostenko EV, et al. (2018). Zh Nevrol Psikhiatr Im S S Korsakova 118(3 Vyp 2):61–68. (in Russian) Clinical study in 110 patients in rehabilitation, comparing blood BDNF levels and recovery scores with and without Semax. PubMed 29798983
4. Najjar VA, Nishioka K (1970). Nature 228(5272):672–3. The first description of tuftsin, the four-residue immunoglobulin G fragment that Selank is built on. PubMed 4097539
5. Volkova A, Shadrina M, Kolomin T, et al. (2016). Front Pharmacol 7:31. Rat study measuring the expression of 84 neurotransmission genes, including GABA receptor subunits, after Selank or GABA. PubMed 26924987
6. Vyunova TV, Andreeva L, Shevchenko K, et al. (2018). Protein Pept Lett 25(10):914–923. Radioligand study of how Selank changes GABA binding in brain membrane preparations. PubMed 30255741
7. Zozulia AA, Neznamov GG, Siuniakov TS, et al. (2008). Zh Nevrol Psikhiatr Im S S Korsakova 108(4):38–48. (in Russian) Clinical comparison of Selank with a benzodiazepine in 62 patients. PubMed 18454096

Both compounds, in our catalogue#

• Semax — ACTH(4–10) analogue, the one carrying a methionine
• Selank — tuftsin analogue, no sulfur in the sequence

Both are stocked in 5mg and 10mg. Ask about the documentation for the batch we would ship before you order.

Pioneer Research Peptides — UK supplier of research peptides for laboratory use. Ask about batch documentation before you order.

For laboratory research use only — not for human consumption. 18+.