In pharmacology, bioavailability (BA) is a subcategory of absorption and is the fraction of an administered dose of unchanged drug that reaches the systemic circulation, one of the principal pharmacokinetic properties of drugs.
By definition, when a medication is administered intravenously, its BA is 100%. However, when a medication is administered via other routes (such as orally), its BA generally decreases (due to incomplete absorption and first-pass metabolism) or may vary from patient to patient.
BA is one of the essential tools in pharmacokinetics, as bioavailability must be considered when calculating dosages for non-intravenous routes of administration.
It is a measurement of the rate and extent to which a drug reaches at the site of action. It is denoted by the letter f (or, if expressed as a percentage, by F).