Semax vs Selank — Two Peptides, Two Different Questions

Summary

Semax and Selank come from the same laboratory tradition, arrived through the same design trick, and are almost always listed side by side by suppliers. That is where the similarity ends.

Semax is an ACTH(4–7) fragment extended with Pro-Gly-Pro. Its research literature is dominated by gene expression — BDNF and TrkB in particular — and by ischaemia models. Selank is a tuftsin analogue extended with the same Pro-Gly-Pro. Its literature is dominated by anxiety endpoints and by immune peptide biology.

Different parent fragments, different receptor territory, different experimental questions. A researcher choosing between them is not choosing between two versions of one thing. This article sets out what separates them, what genuinely connects them, and how much weight the evidence on each side can actually carry.

The One Thing They Genuinely Share

Both peptides end in the same three residues: Pro-Gly-Pro.

That is not a coincidence, and it is the single most useful fact for understanding either one. Short peptide fragments are cleared quickly by peptidases. Attaching a proline-rich tail to the C-terminus slows that clearance, because proline-containing bonds resist many of the enzymes that would otherwise chew through the chain. Both compounds were built by taking a known bioactive fragment and bolting the same stabilising tripeptide onto the end of it.

• Semax = ACTH(4–7), which is Met-Glu-His-Phe, plus Pro-Gly-Pro
• Selank = tuftsin, which is Thr-Lys-Pro-Arg, plus Pro-Gly-Pro

Both are therefore members of the wider glyproline family, and both came out of work at the Institute of Molecular Genetics of the Russian Academy of Sciences. That shared design history explains why they get grouped together commercially. It does not make them interchangeable, any more than two houses with the same foundations are the same house.

What Semax Is

Semax is a synthetic heptapeptide, sequence Met-Glu-His-Phe-Pro-Gly-Pro, molecular formula C₃₇H₅₁N₉O₁₀S, molecular weight 813.93 g/mol, PubChem CID 9811102.

The parent fragment, ACTH(4–7), is a piece of adrenocorticotropic hormone. Critically, Semax is not studied as a hormone analogue in the usual sense: the fragment was selected precisely because it appears to retain the behavioural and neurotrophic effects observed with ACTH fragments while dropping the corticotropic activity of the full hormone.

The bulk of the mechanistic literature concerns neurotrophin gene expression. Shadrina and colleagues reported rapid induction of neurotrophin mRNAs in rat glial cell cultures.² Dolotov and colleagues reported regulation of BDNF and TrkB expression in the rat hippocampus, and separately reported increased BDNF protein in the rat basal forebrain.^3,4 Medvedeva and colleagues ran a genome-wide transcriptional analysis in a rat model of focal cerebral ischaemia and reported effects on immune and vascular gene sets.⁵

That is a reasonably coherent body of work, and it is worth noting what it is: almost entirely animal and cell-culture research on gene and protein expression. It describes things measured in a laboratory. It is not a demonstration of cognitive effect in humans, and it should not be presented as one.

What Selank Is

Selank is a synthetic heptapeptide, sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro, molecular formula C₃₃H₅₇N₁₁O₉, molecular weight 751.89 g/mol, PubChem CID 11765600.

The parent fragment here is tuftsin (Thr-Lys-Pro-Arg), a naturally occurring tetrapeptide derived from the heavy chain of immunoglobulin G. Tuftsin is an immune peptide: its established biology is about phagocyte activation, not about the brain. Selank inherits that lineage, which is why the Selank literature runs along two tracks at once — anxiolytic endpoints on one side, immune modulation on the other.

The anxiolytic track is where the commercial interest sits, and it is also where the evidence needs the most careful handling. Zozulya and colleagues published a trial in generalised anxiety disorder and neurasthenia.⁶ A 2021 review in Pharmacotherapy examined Selank alongside phenibut and placed it explicitly in the category of poorly studied products.⁷ That is a fair characterisation. Most of the clinical work is Russian-language, small, and has not been replicated outside that setting.

Side by Side

The Studies That Ran Both Together

Most comparison articles on this pair are assembled from two separate literatures and a guess. There is a better source: a small number of published studies put both compounds into the same experiment.

Human neuroimaging

The most directly relevant is Panikratova and colleagues, published in Doklady Biological Sciences in 2020.¹ Fifty-two healthy human participants received Selank, Semax or placebo, with resting-state functional MRI at baseline, five minutes and twenty minutes after administration. The analysis focused on functional connectivity involving the amygdala and the dorsolateral prefrontal cortex.

The authors reported that both compounds affected connectivity between the right amygdala and the right temporal cortex, and described a mixture of general effects shared by both compounds and specific effects distinguishing them. That is an unusually useful result for anyone trying to understand the pair: it is direct evidence that they are neither identical nor entirely unrelated in their downstream signature.

It is also a single study, in healthy volunteers, measuring connectivity rather than outcome, at very short timepoints. It establishes that a difference exists. It does not characterise what that difference is good for.

Shared enzyme target

Earlier work reported that both Semax and Selank inhibit enkephalin-degrading enzymes in human serum.⁸ This is the clearest shared mechanism between them: both appear to slow the breakdown of endogenous enkephalins, which has been proposed as a contributor to the effects seen in behavioural models. If you are looking for a single reason the two compounds are discussed together on anything other than marketing grounds, this is it.

Other side-by-side work

Both compounds also appear together in the wider glyproline literature, including comparative work on the anticoagulant effects of proline-containing regulatory oligopeptides, and in rat behavioural studies that included both. These are useful for confirming family resemblance, and largely irrelevant to choosing between them for a specific research question.

The GABA Story, Told Accurately

Selank is routinely described as a GABA modulator. The primary source most often cited for that claim says something more restrained than the claim itself.

Filatova and colleagues used human neuroblastoma IMR-32 cells expressing functional GABA_A receptors and measured mRNA expression across 84 genes involved in GABAergic neurotransmission.⁹ The reported finding was that Selank on its own produced no changes in the mRNA levels of the genes studied. Only when applied together with GABA did it suppress the gene expression changes that GABA alone induced.

That is an interesting result, and it is consistent with Selank acting on the GABA receptor interaction rather than driving GABAergic gene expression directly. It is not the same as showing Selank is a GABAergic drug. The distinction matters if you are designing an experiment around it, and it disappears entirely in most secondhand summaries.

Handling Differences That Matter at the Bench

The sequences diverge in one way that has direct practical consequences.

Semax carries a methionine

Methionine oxidises readily, and it does so particularly in DMSO. For Semax, that means DMSO should be avoided as a solvent wherever an alternative exists. In practice this is rarely a constraint, because Semax dissolves readily in aqueous diluent — but it becomes relevant the moment a protocol reaches for an organic co-solvent out of habit.

Selank carries none of the fragile residues

Thr-Lys-Pro-Arg-Pro-Gly-Pro contains no methionine, no cysteine and no tryptophan. That makes it one of the more forgiving short peptides in the catalogue from an oxidation standpoint. It is still a short peptide in solution and still degrades; it simply has fewer specific weak points.

Both are proline-rich and highly water soluble

Neither compound should need a co-solvent for routine reconstitution in bacteriostatic water. If either fails to go into solution cleanly, the vial is the more likely explanation than the method. Working concentrations follow the same arithmetic as everything else — mass divided by volume — and both are covered in our reconstitution calculator. The reconstitution guide covers the adsorption losses that matter more than either sequence difference at low concentrations.

Choosing Between Them

The honest framing is that the choice is made by the research question, not by a ranking.

• Neurotrophin and gene expression work points to Semax. That is where its literature is deepest and where its published mechanisms are best characterised.
• Anxiety-related behavioural models, or immune peptide biology point to Selank. Its tuftsin lineage means the immune side is not a footnote; it is half the compound’s history.
• Enkephalin metabolism is the one area where either is defensible, since the shared inhibition of enkephalin-degrading enzymes is documented for both.

What does not hold up is the common framing of Selank as “the calm one” and Semax as “the focus one”, presented as though a researcher picks by desired mood. That framing describes a consumer product. Neither compound is one, neither holds a UK marketing authorisation, and neither is supplied for human use.

Both are available from Pureline with their batch certificates published in advance: Semax 5mg and 10mg and Selank 10mg. The full evidence reviews live on the Semax compound profile and the Selank compound profile.

Common Questions

Is Semax or Selank stronger?

The question does not have a meaningful answer, because the two are measured against different endpoints in different experimental systems. There is no shared potency scale on which they can be ranked, and no study has established equivalent doses for comparison.

Can Semax and Selank be studied together?

The published literature that includes both compounds has generally run them as separate arms rather than in combination. Combination protocols are common in informal discussion and largely absent from the peer-reviewed record, so a researcher designing one is working without a published precedent to cite.

Do they work through the same receptor?

No established shared receptor has been identified. What has been documented is a shared downstream effect on enkephalin-degrading enzymes, which is a different kind of overlap. The precise receptor mechanisms for both compounds remain incompletely characterised.

Why do they both end in Pro-Gly-Pro?

Because both were designed the same way: a known bioactive fragment extended with a proline-rich tail to slow enzymatic degradation. It is a design convention from the same research group, not evidence of shared biology.

Which has better evidence in humans?

Selank has more clinical publications, but they are small, mostly Russian-language, and an independent 2021 review placed it among poorly studied products. Semax has less clinical work but features in a controlled human neuroimaging study alongside Selank. Neither has an evidence base that would meet the standard for a licensed medicine in the UK.

Are they legal to buy in the UK?

Both are supplied strictly as research chemicals for in vitro laboratory use. Neither holds a UK marketing authorisation, neither is a licensed medicine here, and neither is sold for human or veterinary use. The UK peptide law guide sets out how research-use materials sit within the regulations.

References

1. Panikratova YaR, Lebedeva IS, Sokolov OYu, Rumshiskaya AD, Kupriyanov DA, Kost NV, Myasoedov NF. Functional Connectomic Approach to Studying Selank and Semax Effects. Doklady Biological Sciences. 2020;490:9–11. DOI: 10.1134/S001249662001007X
2. Shadrina MI, Dolotov OV, Grivennikov IA, et al. Rapid induction of neurotrophin mRNAs in rat glial cell cultures by Semax, an adrenocorticotropic hormone analogue. Neurosci Lett. 2001;308(2):115–118. PubMed 11457573
3. Dolotov OV, Karpenko EA, Inozemtseva LS, et al. Semax, an analog of ACTH(4-10) with cognitive effects, regulates BDNF and trkB expression in the rat hippocampus. Brain Res. 2006;1117(1):54–60.
4. Dolotov OV, Seredenina TS, Levitskaya NG, et al. Semax, an analogue of adrenocorticotropin(4-10), binds specifically and increases levels of brain-derived neurotrophic factor protein in rat basal forebrain. J Neurochem. 2006.
5. Medvedeva EV, Dmitrieva VG, Povarova OV, et al. The peptide Semax affects the expression of genes related to the immune and vascular systems in rat brain focal ischemia: genome-wide transcriptional analysis. BMC Genomics. 2014;15:228. DOI: 10.1186/1471-2164-15-228
6. Zozulya AA, Neznamov GG, Syunyakov TS, et al. Efficacy and possible mechanisms of action of a new peptide anxiolytic selank in the therapy of generalized anxiety disorders and neurasthenia. PubMed 18454096
7. Doyno CR, White CM. Sedative-hypnotic agents that affect gamma-aminobutyric acid receptors: focus on phenibut and Selank as poorly studied products. Pharmacotherapy. 2021. PubMed 34396551
8. Semax and selank inhibit the enkephalin-degrading enzymes from human serum. Bioorganicheskaia Khimiia. 2001. PubMed 11443939
9. Filatova E, Kasian A, Kolomin T, et al. GABA, Selank, and Olanzapine Affect the Expression of Genes Involved in GABAergic Neurotransmission in IMR-32 Cells. Front Pharmacol. 2017;8:89. DOI: 10.3389/fphar.2017.00089

Published by Pureline Biolabs Ltd, a UK-registered supplier of laboratory research materials, company number 17236739. Last reviewed September 2026. Pureline did not conduct the studies listed above; our contribution is the organisation of the evidence, the limits placed around its interpretation, and the publication of analytical records for our own stock. Factual corrections are welcome through the contact page.