Bioavailability.

The fraction of a dose that reaches systemic circulation in active form.

Bioavailability is the proportion of an administered dose that reaches the bloodstream intact and active. By definition an intravenous dose is fully bioavailable; everything else is compared against that. This is the central practical problem of peptide science. The digestive tract exists to break amino-acid chains into their components, so an orally swallowed peptide meets stomach acid, then proteases, then a gut wall that is selectively permeable to small molecules and largely closed to large ones. The result is that oral bioavailability for most peptides is a small fraction of one percent, which is why almost every peptide of interest is studied by injection, and why injection is the norm rather than a stylistic choice. The exceptions show how hard the problem is: making an oral GLP-1 agonist work required co-formulating it with an absorption enhancer to protect it and help it cross, and even then only a small share of the dose gets through. Nasal, buccal, and transdermal routes each face their own version of the same barrier, since peptides are generally too large and too water-loving to cross skin. The misunderstanding worth correcting is that a peptide sold as an oral capsule or a topical spray is delivering the same molecule to the same place as an injected one. Absent evidence for that specific formulation, the assumption usually runs the other way.