Bioavailability quantifies the extent and rate at which the active ingredient of a drug formulation reaches the systemic circulation, providing a measure of the drug's effective exposure after administration. For drugs given intravenously, bioavailability is by definition 100%, as the entire dose enters the blood directly. For other routes of administration, bioavailability may be substantially lower due to incomplete absorption, first-pass metabolism in the gut wall or liver, or degradation before reaching the systemic circulation.

Two components define bioavailability: extent and rate. The extent of bioavailability, expressed as a percentage, indicates what proportion of the administered dose ultimately reaches the circulation. The rate of bioavailability describes how quickly the drug enters the circulation, influencing the time to onset of action and peak drug levels. Both components are pharmacologically important, as two formulations with identical extent of absorption may produce very different clinical effects if they differ substantially in absorption rate.

Bioavailability assessments are essential for formulation development and for demonstrating bioequivalence between generic and brand-name products. Absolute bioavailability studies compare a non-intravenous formulation to an intravenous reference to determine the true fraction absorbed. Relative bioavailability studies compare different formulations of the same drug, as required for generic approval or when changing formulations during development. Bioequivalence is established when the confidence interval for the ratio of key pharmacokinetic parameters falls within the regulatory acceptance range, typically 80-125% for Cmax and AUC.