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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 bioavailability is 100 %.

However, when a medication is administered via other routes ( such as orally ), its bioavailability generally decreases ( due to incomplete absorption and first-pass metabolism ) or may vary from patient to patient.

Bioavailability is one of the essential tools in pharmacokinetics, as bioavailability must be considered when calculating dosages for non-intravenous routes of administration.