Why are different administration routes used for different peptides?

Subcutaneous injection is the default administration route for most peptides in this catalog. It gives predictable absorption (60–90% bioavailability for stable peptides), avoids the gastrointestinal-destruction problem of oral delivery, and can be self-administered with simple technique. Approved GLP-1 receptor agonists (semaglutide, tirzepatide), tesamorelin, thymosin alpha-1, bremelanotide (Vyleesi), afamelanotide (Scenesse implant excepted), and the vast majority of research-supply peptides use subcutaneous delivery.

Intramuscular injection is used when faster absorption or a specific muscle-tissue effect is desired. It is less common in the peptide-therapy space than subcutaneous but appears in specific clinical protocols.

Intranasal delivery bypasses the gastrointestinal tract while avoiding an injection. It works best for small peptides (Selank, Semax, DSIP) that can absorb through the nasal mucosa. Intranasal bioavailability is typically 5–20% — lower than injection but sufficient for centrally-acting neuropeptides where the target is the brain. Selank and Semax as intranasal peptides may also access the brain via the direct olfactory pathway.

Oral delivery is possible for a small number of peptides using specific formulation strategies. Oral semaglutide (Rybelsus) uses enteric coating plus a permeation enhancer (SNAC) to achieve about 1% oral bioavailability — low, but enough at large doses to produce clinical effect. Most peptides cannot be reliably delivered orally because stomach acid, intestinal peptidases, and first-pass hepatic metabolism destroy them before they reach systemic circulation.

Specific tissue targets sometimes dictate specific routes: implant delivery (Scenesse), intraocular delivery (SS-31 in AMD trials), or topical delivery (GHK-Cu cosmetic dermatology).