Detailed Explanation
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.