In pharmacology, bioavailability (BA or F) is a subcategory of absorption and is the fraction (%) of an administered drug that reaches the systemic circulation. By definition, when a medication is administered intravenously, its bioavailability is 100%. However, when a medication is administered via routes other than intravenous, its bioavailability is generally[TH] lower than that of intravenous due to intestinal endothelium absorption and first-pass metabolism. Thereby, mathematically, bioavailability equals the ratio of comparing the area under the plasma drug concentration curve versus time (AUC) for the extravascular formulation to the AUC for the intravascular formulation. AUC is utilized because AUC is proportional to the dose that has entered the systemic circulation.

Bioavailability of a drug is an average value; to take into account of population variability, deviation range is given in To ensure that the drug taker who has poor absorption is dosed appropriately, the bottom value of the deviation range is employed to represent real bioavailability to calculate drug dose needed for the drug taker to achieve systemic drug concentrations similar to the intravenous formulation. To dose without the prerequisite of drug taker's absorption state, the bottom value of the deviation range is used in order to ensure the anticipated efficacy will be met unless the drug is associated with narrow therapeutic window