The fraction (%) of the initial dose of an administered or active drug that reaches the systemic circulation unaltered.

Bioavailability is influenced by the physicochemical characteristics of the drug and physiological factors of the patient. This is particularly relevant for drugs administered orally. Bioavailability calculations are based on the bioavailability of IV administered drugs as they are considered 100% bioavailable. In addition to the route of administration, bioavailability Is also affected by the drug dose administered, to which it is inversely proportional i.e., drugs with a low bioavailability require larger dose to reach the minimum effective concentration.