What does bioavailability refer to?

Bioavailability specifically refers to the fraction of unchanged drug that reaches the systemic circulation after administration. It is a crucial pharmacokinetic parameter because it directly affects the efficacy of a drug; higher bioavailability means that a larger amount of the drug is available to exert its therapeutic effects on the body. This concept is particularly significant for oral medications, as factors such as first-pass metabolism (where the drug is metabolized in the liver before reaching the systemic circulation) can greatly influence the actual amount of the drug that enters the bloodstream.

While the rate of drug elimination, duration of drug action, and speed of drug absorption are all important aspects of pharmacokinetics, they do not define bioavailability. The elimination rate pertains to how quickly a drug is removed from the body, the duration of action focuses on how long the drug remains effective, and the speed of absorption relates to how quickly a drug enters systemic circulation, but these factors contribute to overall pharmacokinetics rather than defining bioavailability itself.