Bioavailability

Bioavailability is the proportion of a given amount of a compound that reaches the bloodstream intact. Intravenous administration is 100% by definition. Subcutaneous injection of a peptide is typically high, often above 70%, with the remainder lost to local enzymes and lymphatic transit. Oral bioavailability of most peptides is close to zero, because the stomach and intestine are built to cut peptide bonds and the intestinal wall is a poor barrier for molecules this size to cross.

That is why the research literature on nearly every compound profiled here uses injection, and why oral claims deserve scrutiny. Two cases are informative. BPC-157 survives gastric juice, and some animal studies used the oral route, but its oral bioavailability in humans has not been established. Oral semaglutide exists as an approved tablet only because it is co-formulated with an absorption enhancer at a dose roughly a hundred times the injected one; the semaglutide profile covers it.

Bioavailability interacts with half-life to determine exposure. A peptide that is well absorbed but cleared in minutes may still produce only brief exposure, which is the case for most short growth-hormone secretagogues.