What does bioavailability refer to?

Bioavailability specifically refers to the proportion of a drug that enters the systemic circulation when it is administered. This measure is significant because it determines the extent and rate at which the active ingredient or active moiety is available at the site of action after administration. For example, when a drug is given intravenously, its bioavailability is considered to be 100% since it directly enters the bloodstream. However, for orally administered drugs, bioavailability might be less than 100% due to factors like absorption variability and first-pass metabolism. This concept is critical in pharmacokinetics as it influences dosage calculations and therapeutic effectiveness of drugs. Understanding bioavailability helps in recognizing how different routes of administration can affect drug efficacy in clinical settings.