Semax and Selank came out of the same Moscow institute. Both are seven amino acids long. Both end in the exact same three. So most people comparing Semax vs Selank figure it's one peptide with two names. Nope.
The tail is shared. The front piece decides everything.
Think of amino acids as beads on a string. Each of these is a four-bead natural fragment with the same three-bead tail stuck on the end. The tail was added to slow down how fast the molecule gets chewed up. The front piece is what makes it Semax or Selank, and it sent their research in two different directions.
The proof: Semax vs Selank, same blank, different cut
Here's what's actually going on.
Picture two keys cut from the same blank. The blank is Pro-Gly-Pro, three amino acids that also show up as a fragment of collagen. Its developers report that it makes short peptides harder for the body's enzymes to break down. The cut at the front decides which lock the key fits.
Semax's cut comes from a stress hormone. Its front four beads are copied from ACTH, the hormone that tells the adrenal glands to make cortisol. In 2006 the institute that built it gave rats a single dose and measured BDNF, a growth-factor protein in the brain. In the hippocampus, the brain's memory hub, BDNF rose by up to 1.4 times (Dolotov et al., Brain Research 2006).
Selank's cut comes from the immune system. Its front four beads are tuftsin, a tiny piece of an antibody. In 2016 researchers at the same institute gave rats either Selank or GABA, the brain's main calming messenger, then checked 84 genes tied to nerve signaling. An hour later, the gene changes from the two tracked each other (Volkova et al., Frontiers in Pharmacology 2016).
The tail slows the scissors. It doesn't stop them. Back in 1991, the same institute watched Semax break down in rat blood. Enzymes still clipped the first two beads off its front end. The rest held up better than ACTH's own longer fragment did (Potaman et al., BBRC 1991). So "stable" here means slower, not bulletproof.
So why should you care? A COA can tell the twins apart. Semax weighs about 814 daltons and Selank about 752 (daltons are just the unit chemists weigh molecules in). The LC-MS identity test on a certificate of analysis (COA) is a molecular scale. That 62-dalton gap is how a lab shows which twin is in the vial. (Somebody at that institute really liked that tail.)
"They're used as medicines in Russia, so they're proven, right?" Fair question. Both are used clinically there. Neither is approved by the FDA. Most of the work comes from Russian labs and animal models, and independent replication elsewhere is thin. That's why both sit at C on our A-to-E evidence ladder. Used in one country is a fact. It isn't the same as proven.
The big learning lesson
Keep reading: How to read a peptide certificate of analysis · What "third-party tested" really means · The FDA "peptide ban" of 2026, explained
Frequently asked questions
What's the difference between Semax and Selank?
Both are seven-amino-acid peptides ending in the same Pro-Gly-Pro tail. Semax's front piece comes from the hormone ACTH and is studied mainly around BDNF, a brain growth factor. Selank's comes from tuftsin, a piece of an antibody, and is studied mainly around the GABA system and stress-related behavior in animals.
Is Semax the same as Selank?
No. They share a design and a tail, but the front pieces differ, they weigh different amounts (about 814 vs 752 daltons) and their research points at different systems.
Are Semax and Selank FDA approved?
No. Both are used as medicines in Russia, but neither is approved by the FDA. In July 2026 an FDA advisory committee voted to recommend Semax for the pharmacy compounding list. That vote isn't binding and isn't an approval.