Semax and Selank are routinely discussed together, and for good reason — they share a design principle, a country of origin and a research tradition. But they are built from entirely different parent molecules: one from a pituitary hormone, the other from a fragment of an antibody. That difference is where any meaningful comparison has to start.
What the two have in common
Both are synthetic heptapeptides. Both were developed in Russia, at the Institute of Molecular Genetics of the Russian Academy of Sciences and associated institutions. Both end in the same three residues: Pro-Gly-Pro.
That shared tail is not coincidence, and understanding it explains most of what the two compounds have in common. Short peptides are ordinarily destroyed within minutes by peptidases, which is the central obstacle to studying them at all. Proline resists cleavage by many of those enzymes because its ring structure constrains the backbone geometry they need to engage. Appending a proline-containing tail therefore extends how long the molecule survives.
So both compounds are the same engineering move applied twice: take a short biologically interesting fragment, bolt Pro-Gly-Pro onto the end, and you have something stable enough to work with. What differs is the fragment they started from.
Different parents, different biology
Semax is built on ACTH(4–7) — Met-Glu-His-Phe — four residues lifted from adrenocorticotropic hormone. ACTH is a pituitary hormone whose main job is triggering cortisol release from the adrenal cortex, and it belongs to the melanocortin family. Semax retains the fragment but not the hormonal action: it does not stimulate cortisol release.
Selank is built on tuftsin — Thr-Lys-Pro-Arg — which comes from an entirely different place. Tuftsin is a four-residue sequence found within the heavy chain of immunoglobulin G, the most abundant antibody class in blood, and its described role is immunological rather than endocrine.
This is the substantive difference, and it is routinely lost in summaries that describe both as "nootropic peptides." One derives from the hormonal axis that governs stress responses. The other derives from the immune system. Their shared tail and shared origin story make them look like siblings; their parent molecules make them something closer to unrelated compounds that happen to have been built by the same workshop.
| Semax | Selank | |
| Sequence | Met-Glu-His-Phe-Pro-Gly-Pro | Thr-Lys-Pro-Arg-Pro-Gly-Pro |
| Single-letter | MEHFPGP | TKPRPGP |
| Parent fragment | ACTH(4–7) | Tuftsin |
| Parent's origin | Pituitary hormone (melanocortin family) | Immunoglobulin G heavy chain |
| Residue count | 7 | 7 |
| Shared motif | C-terminal Pro-Gly-Pro — both |
| Molecular formula | C37H51N9O10S | C33H57N11O9 |
| Molecular weight | 813.93 g/mol | 751.89 g/mol |
| Contains sulfur | Yes (N-terminal Met) | No |
| CAS number | 80714-61-0 | 129954-34-3 |
| Developed at | Russian Academy of Sciences institutions — both |
| Format supplied | 10 mg per vial — both |
| Category | Neuro research — both |
Reported mechanisms compared
Neither compound has an established mechanism, and anything you read asserting otherwise is overstating the evidence. What exists is a set of reported observations, some overlapping between the two.
Both have been reported to raise brain-derived neurotrophic factor in the hippocampus of rats. Both have been described as inhibiting enzymes that degrade enkephalins and other endogenous regulatory peptides, which would indirectly raise levels of those peptides rather than acting on a receptor directly. That overlap is worth noting precisely because it complicates any attempt to distinguish them experimentally on those endpoints.
Where the reports diverge: Semax has additionally been proposed to interact with melanocortin receptors, consistent with its ACTH parentage, and reports describe rapid effects on the TrkB receptor alongside BDNF. Selank has additionally been described as influencing monoamine neurotransmitter concentrations and serotonin metabolism, and — tracking its tuftsin origin — as having immunological effects, including modulation of interleukin-6 expression and T helper cell cytokine balance.
A caution that applies to both: a large share of the published work on these compounds is Russian-language, produced around domestically regulated medicines, and has not been widely replicated by independent laboratories elsewhere. That is a statement about the structure of the evidence base, not about its quality. But it does mean English-language summaries frequently restate findings several steps removed from the primary source, and claims about either compound should be traced back rather than taken from secondary descriptions — including this page.
The blend, and when a blend is appropriate
We list the two together as a Semax and Selank blend, 10 mg of each in one vial. It exists because researchers working across both compounds frequently want them on hand together, and because a single vial is simpler to inventory than two.
It is worth being straightforward about the experimental trade-off. A blend is convenient, but in a controlled experiment it confounds attribution: if you observe an effect, the blend cannot tell you which compound produced it, or whether the result depends on both being present. For any study whose question is "what does this compound do," separate vials are the correct choice, with the blend reserved for work specifically asking about the combination, or for preliminary exploration before a controlled design is built.
Our guide to peptide blends covers this reasoning in more depth across our other multi-compound formulations.
Choosing between them for a given study
The useful question is not which compound is "better" — they are not graded versions of one thing. It is which parent biology your question sits closer to.
If the work concerns melanocortin receptor pharmacology, the hormonal axis, or neurotrophin responses tied to that pathway, Semax is the compound whose parentage matches. If the work concerns immunological signalling, monoamine systems, or anything following from tuftsin's role in immune function, Selank is the closer fit.
If the question is specifically about the Pro-Gly-Pro stabilisation strategy itself — peptidase resistance, half-life, degradation products — then the two together are genuinely informative, because they isolate the variable: same tail, different body. That is one of the few cases where studying both in parallel answers something neither does alone.
Regulatory status
Neither compound is authorised by Health Canada for any use. Semax is a prescription medicine in Russia and appears on that country's list of vital and essential medicines; Selank was developed at the same institution and is likewise unapproved in most jurisdictions outside Russia. Neither is FDA approved.
Handling: one difference that matters
Both are supplied as lyophilised powder, stable frozen and markedly less so once reconstituted, and both should be brought to room temperature before opening so that moisture does not condense inside the vial.
There is one practical asymmetry. Semax begins with methionine, the residue most readily oxidised, and an N-terminal methionine is particularly exposed. Selank contains no sulfur at all and so is not subject to that specific degradation route. If you are running both, Semax is the one that warrants more care with air exposure, and the one where an unexpected mass shift in a spectrum most likely indicates oxidation rather than contamination.
For both, avoid repeated freeze-thaw cycling and log lot numbers against runs. Reconstitution solvent and working concentration should follow the protocol of the study rather than any general figure.
Verification and certificates of analysis
Every lot of both compounds is tested by an independent third-party laboratory using high-performance liquid chromatography and mass spectrometry before it is listed, and the report is published openly rather than supplied on request after purchase. Reports are in the NexPept Certificate of Analysis library, with the batch number on each vial matching a published certificate.
If you are comparing suppliers generally, our quality standards page sets out how batches are tested and documented, what research peptides are explains how research-grade material differs from a drug or supplement, and buying peptides in Canada covers what to check before ordering. The broader guide to research peptides in Canada goes into sourcing and regulation.
All three listed separately — 10 mg vials, batch-tracked, third-party tested by HPLC and MS. Ships from Ontario with same-day dispatch on orders placed before 2 PM EST.
Semax 10 mg Selank 10 mg The blendRelated research compounds
Researchers working across neuropeptide questions frequently hold these alongside Semax and Selank:
Frequently asked questions
What is the single biggest difference between Semax and Selank?
Their parent molecules. Semax is built from a fragment of ACTH, a pituitary hormone. Selank is built from tuftsin, a fragment of an immunoglobulin G antibody. One comes from the endocrine system, the other from the immune system. Everything they share — length, the Pro-Gly-Pro tail, country of origin — is downstream of a shared design method, not shared biology.
Why do both end in Pro-Gly-Pro?
It is a stabilisation strategy. Proline resists many peptidases because its ring structure constrains the backbone geometry those enzymes require, so a proline-containing terminus slows degradation of the whole molecule. Both compounds apply the same approach to different starting fragments.
Do they have overlapping mechanisms?
Partly, as far as the reports go. Both have been described as raising BDNF in rat hippocampus and as inhibiting enzymes that degrade enkephalins. Semax is additionally linked to melanocortin receptors and TrkB; Selank to monoamine systems and to immunological effects including interleukin-6 and T helper cytokine balance. None of this is settled mechanism — both compounds' modes of action remain formally unknown.
Should I use the blend or separate vials?
Separate vials for any study asking what an individual compound does, because a blend cannot tell you which component produced an observed effect. The blend suits work specifically about the combination, or preliminary exploration before a controlled design is set.
Which one is stronger?
Not a question the two support. They are not graded versions of one compound — they derive from different parent molecules and are reported to act on different systems. The useful question is which parent biology your research question sits closer to.
Is either approved in Canada?
No. Neither has Health Canada authorisation and neither is FDA approved. Semax is a prescription medicine in Russia; that has no bearing on Canadian status. Material supplied by NexPept is for laboratory research use only and is not for human or veterinary use.
Is there a handling difference between them?
Yes, one worth knowing. Semax has an N-terminal methionine, which oxidises readily, so it needs more care with air exposure. Selank contains no sulfur and is not subject to that route. Otherwise both follow the same practice: frozen, protected from light and moisture, brought to room temperature before opening, aliquoted to avoid freeze-thaw cycling.
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More from NexPept: browse the full research catalogue, read our other research articles, or check the frequently asked questions for ordering and documentation details.