Definition
Bioavailability, abbreviated F, is the proportion of an administered compound that reaches systemic circulation in active form. By definition, intravenous administration gives 100%, and other routes are compared against it using the area under the plasma concentration–time curve. For most peptides, oral bioavailability is very low, often below 1–2%, because gastric acid and digestive proteases degrade them and their size and polarity limit absorption across the intestinal wall. Strategies studied to improve it include absorption enhancers such as SNAC, used in oral semaglutide, as well as cyclisation, D-amino acids and nanoparticle carriers. Bioavailability is measured in pharmacokinetic studies and varies with species, formulation and route.