Selank vs Semax: Comparing Two Russian Research Peptides

Selank vs Semax is a comparison that comes up constantly in research-peptide discussion, and for a good reason: both were developed by the same Russian research program and share a short-chain peptide structure, which makes them easy to lump together. They are not the same molecule, and they were not studied for the same thing. This guide lines up what actually separates Selank and Semax, and walks through the reconstitution math that applies equally to both. It is written for research and educational purposes only and is not medical advice.

Where Selank and Semax come from

Both peptides trace back to the same Russian research institution, which spent decades investigating short synthetic peptides derived from naturally occurring signaling molecules. Semax is built from a fragment of adrenocorticotropic hormone (ACTH), a hormone the body produces as part of the stress-response system, with additional amino acids attached to extend its stability. It has been studied and used in Russia in registered nasal-drop form, primarily in connection with cognitive and neurological research contexts.

Selank is built differently. It's derived from tuftsin, a naturally occurring peptide involved in immune signaling, again with additional amino acids attached. Selank has a research and use history in Russia centered on anxiety-related behavior rather than the cognitive framing associated with Semax.

The shared origin story is real. The compounds themselves are not interchangeable — different parent molecule, different sequence, different area of study.

Regulatory and research status

Neither peptide carries FDA approval for any use in the United States. Both have a use history in Russia as registered nasal-drop pharmaceutical products, which is a meaningfully different regulatory environment than the U.S. research-compound market these vials are sold into. That Russian registration does not transfer to the lyophilized, U.S.-sourced research vials this guide's math applies to, and it does not substitute for the kind of FDA review process that peptides like tirzepatide or semaglutide have gone through.

Evidence for both compounds in the research literature is still developing relative to that FDA-approved tier. Treat Selank and Semax as peptides with an established use history outside the U.S. and an evolving evidence base, not as settled, universally approved compounds.

Typical vial concentrations

Both peptides commonly arrive as lyophilized powder in a sealed vial, reconstituted with bacteriostatic water before use — the same starting point as any other research peptide on this site:

Peptide	Common vial size	Reference water volume	Concentration
Selank	5 mg	2 mL	2,500 mcg/mL
Semax	5 mg	2 mL	2,500 mcg/mL

These are reference points, not fixed requirements. The peptide calculator recalculates concentration and syringe units for any vial size and water volume entered, for either compound.

Worked example: same formula, different vials

The formula behind every reconstitution on this site doesn't change based on which peptide is in the vial: concentration equals mass in mg divided by water volume in mL, and syringe units equal that solution's volume in mL multiplied by 100 on a U-100 insulin syringe.

Selank. A 5 mg vial reconstituted with 2 mL of bacteriostatic water gives 5 mg ÷ 2 mL = 2.5 mg/mL, which is 2,500 mcg/mL. A 250 mcg research reference amount from that vial works out to 250 mcg ÷ 2,500 mcg/mL = 0.1 mL, and 0.1 mL × 100 units/mL = 10 units on a U-100 syringe.

Semax. A 3 mg vial reconstituted with 3 mL of bacteriostatic water gives 3 mg ÷ 3 mL = 1 mg/mL, or 1,000 mcg/mL. A 300 mcg research reference amount from that vial works out to 300 mcg ÷ 1,000 mcg/mL = 0.3 mL, and 0.3 mL × 100 units/mL = 30 units on a U-100 syringe.

Two different vial sizes, two different water volumes, and two different unit counts — the arithmetic is the same three steps every time: find concentration, divide the target amount by concentration to get volume, multiply volume by 100 for units.

Why one calculator covers both

The step most likely to introduce error isn't drawing the syringe — it's the concentration math that happens before it. A U-100 insulin syringe always reads 100 units per mL, so how much peptide each unit represents depends entirely on how much bacteriostatic water went into the vial. That's true whether the vial holds Selank, Semax, or any other lyophilized research peptide. Get the water volume wrong once, and every draw calculated from it inherits that same error. That's the reasoning behind building a single free peptide calculator around the mass-divided-by-water-volume formula rather than a separate tool per compound. For the general mixing walkthrough, see how to calculate a peptide reconstitution ratio.

Where the comparison breaks down

The reconstitution math is identical for Selank and Semax. Their origin, sequence, and research focus are not. Semax is derived from an ACTH fragment and is more often discussed in connection with cognitive research; Selank is derived from tuftsin and is more often discussed in connection with anxiety-related research. Neither history, and neither Russian registration status, transfers to the other compound. Assuming they're functionally interchangeable because they came out of the same lab is the single most common error in how this comparison gets discussed.

Common mistakes

• Assuming shared origin means shared effects. Selank and Semax were developed by the same research group but derived from different parent molecules, with different areas of study behind each.
• Treating Russian nasal-drop registration as equivalent to FDA approval. It isn't. Neither compound has gone through FDA review, and that registration history doesn't apply to research vials sold in the U.S.
• Confusing mg and mcg. Both peptides are commonly referenced in microgram-range research amounts. 1 mg equals 1,000 mcg, and misreading one for the other produces a 1,000x error in the calculation.
• Reusing a concentration from a different vial. A 2,500 mcg/mL result only holds for the exact vial size and water volume it came from. Recalculate rather than assuming the last vial's numbers still apply.
• Shaking instead of swirling. Vigorous shaking can degrade a reconstituted peptide; swirl the vial gently instead.

Related

For the general mixing and storage steps behind every worked example on this page, see how to reconstitute peptides. Or enter a vial size, water volume, and target amount into the peptide dosage calculator to get exact syringe units for either compound.