Subcutaneous (SC) injection

Subcutaneous administration places a solution into the layer of fatty tissue between the skin and the muscle, typically at the abdomen or thigh, using a short fine needle such as the one on a U-100 insulin syringe. From there the compound is absorbed into local capillaries and lymphatics over minutes to hours, which gives a smoother profile than intravenous injection and a slower onset than intramuscular.

It is the route the research literature uses for almost every peptide profiled here, from the once-weekly incretins (semaglutide, tirzepatide) to the daily growth-hormone secretagogues (ipamorelin, CJC-1295). The approved versions of the incretin drugs are supplied in subcutaneous pens for the same reason. Where a study used a different route, the compound's profile says so under "Research protocols".

Rotating sites matters because repeated injection at one spot can cause lipohypertrophy, a lump of fatty tissue that absorbs unevenly. The subcutaneous injection guide covers site selection, angle and rotation, and the article on subcutaneous versus intramuscular injection explains when the literature uses each.