Why Semax and Selank Are Sprayed, Not Injected
Both are ACTH/tuftsin-derived peptides built to cross into the brain — and the nasal route is doing real mechanistic work, not just added convenience.
Most peptides on this site are injected because that's how a peptide reliably gets into the bloodstream at all — oral doses are broken down by digestive enzymes before they can act. Semax and Selank are the exception, and the reason is specific to what they're trying to reach: the brain.
The blood-brain barrier is selective about what it lets through, and a peptide injected into general circulation still has to cross that barrier to have a central effect. The nasal cavity offers a more direct route: the olfactory and trigeminal nerve pathways that line the nasal passage connect relatively directly to the central nervous system, bypassing a significant amount of the systemic circulation a subcutaneous injection would have to travel through first. That's the actual reason both compounds are formulated as nasal sprays rather than injectables — it isn't about convenience, it's about the route that gets a CNS-targeted peptide where it's trying to go.
Semax is derived from a fragment of ACTH (adrenocorticotropic hormone) with the hormonal activity engineered out, leaving a fragment studied for effects on neurotrophic factors like BDNF — which is the basis for its focus/cognitive-performance association. Selank is a synthetic analog of tuftsin, an endogenous immunomodulatory peptide, and is studied separately for anxiolytic-adjacent effects without the sedation profile of classic anti-anxiety pharmaceuticals.
Because they're engineered from different source molecules with different proposed effects — one focus-leaning, one calm-leaning — but share the same nasal-delivery logic and a similar short-acting profile, they're frequently run together rather than as substitutes for each other.