FIELD LOG / LENS: COMPARISON
Semax vs Selank: Comparing the Research
Two peptides from the same Russian research line, often mentioned in the same breath — and genuinely distinct. What the record says about each.
The short version
Semax vs Selank is one of the most searched questions about either peptide, and the honest answer starts with: they are not the same thing, and they were not built for the same job. Both are short lab-made peptides developed in Russia, both are usually taken as a nasal spray, and both are sold elsewhere as research chemicals. But Semax is studied mainly as a nootropic — for focus, memory, and brain protection — while Selank is studied mainly as an anxiolytic, meaning for calm and reduced anxiety.
They also come from different natural sources. Semax is built from a fragment of the hormone ACTH. Selank is built from a fragment of an immune molecule called tuftsin. They share a research family and a country of origin, not a mechanism. Below is what the literature actually compares, and where the two genuinely overlap.
Different parents, different aims
Semax (Met-Glu-His-Phe-Pro-Gly-Pro) is a seven-amino-acid analog of the ACTH(4-7) hormone fragment, stabilized with a Pro-Gly-Pro tail, and its primary studied roles are cognition and neuroprotection. Selank is a separate heptapeptide, a synthetic analog of the immune peptide tuftsin, and its primary studied role is anxiety reduction. They are siblings in the sense that both came out of the same Soviet/Russian peptide-research tradition — but their starting molecules and their intended targets differ.
This distinction matters because the two are constantly conflated online. The corpus this site draws on treats them as separate compounds, and so should any honest reading: a finding about Selank's anxiolytic profile is not evidence about Semax, and vice versa.
Where the research overlaps
There are two places the literature studies them side by side. First, enzyme inhibition: in human serum, Semax inhibited enkephalin-degrading enzymes with an IC50 of about 10 microM, while Selank's IC50 was about 20 microM — Semax roughly twice as potent in that assay [3]. Both prolong endogenous opioid signaling by this route, just to different degrees.
Second, human brain imaging. One of the few human-imaging investigations of either peptide used a resting-state functional-connectivity (connectomic) approach to study Selank and Semax together [11]. It is a notable study precisely because human data on these compounds is so scarce — most of the comparison rests on rodent and in-vitro work, and that limitation should temper any confident side-by-side claim.
Semax's distinct neurotrophic signature
The clearest way to keep the two apart is to look at what Semax does that defines it. Its signature is neurotrophic and region-specific. Intranasal Semax at 50 and 250 microg/kg raised BDNF protein in rat basal forebrain — but not cerebellum — at 3 hours, binding a specific, reversible, calcium-dependent site with a dissociation constant of 2.4 nM [1]. A single 50 microg/kg intranasal dose moved NGF and BDNF mRNA up in hippocampus while NGF mRNA fell in frontal cortex [2], and a separate study showed it regulates BDNF together with its TrkB receptor in the hippocampus [9].
This precise, directional control of the brain's own growth signals is the heart of the Semax cognition-and-neuroprotection case, and it is what people are usually reaching for when they choose Semax over Selank. The monoaminergic picture is part of the same signature: in mice Semax raised the serotonin metabolite 5-HIAA and potentiated amphetamine-evoked dopamine release without raising baseline dopamine on its own [8] — a modulating profile, not a stimulant one.
The one assay that puts them side by side
If there is a single published measurement that directly pits Semax against Selank, it is the enkephalinase study. In human serum in vitro, both peptides inhibited enkephalin-degrading enzymes — the enzymes that break down the body's natural opioid peptides — but at different potencies: Semax with an IC50 of about 10 microM, Selank about 20 microM, with Semax therefore roughly twice as potent in that particular assay [3]. Notably, the study found that pentapeptide fragments retained activity while shorter and longer fragments did not, a structural detail about how the activity is carried.
It would be a mistake to read 'twice as potent in one enzyme assay' as 'twice as good.' The assay measures one shared mechanism in a test tube, not the distinct clinical purposes the two peptides were built for. It is, however, a genuine head-to-head data point — and one of very few — which is why it anchors any careful Semax-versus-Selank reading.
How to read a Semax vs Selank comparison
When people ask which is 'better,' the literature does not answer that, because the two were tested for different outcomes. A fair reading is functional: the published Semax record leans toward neurotrophic and neuroprotective endpoints — BDNF and NGF induction [1][2], reduced cortical infarct in ischemia [4] — while Selank's reputation rests on anxiolytic endpoints that are not the subject of this site.
The shared cautions are larger than the differences. Both are research chemicals outside Russia, both rest mostly on rodent and largely Russian-language evidence, and neither has FDA or EMA approval. The human evidence for either is thin — for the two together it amounts to a single resting-state connectomic imaging study [11]. For the honest upsides and downsides people report with Semax specifically, see Semax effects; for the mechanism in depth, see the Semax research.