Bioavailability : Definition and methods

Bioavailability is defined in § 320.1 as:

the rate and extent to which the active ingredient or active moiety is absorbed from a drug product and becomes available at the site of action. For drug products that are not intended to be absorbed into the bloodstream, bioavailability may be assessed by measurements intended to reflect the rate and extent to which the active ingredient or active moiety becomes available at the site of action.

It is the fraction of an administered dose of unchanged drug that reaches the systemic circulation, so when medication is administered intravenously, its bioavailability is 100%. However, when a medication is administered via other routes (such as orally), its bioavailability decreases (due to incomplete absorption and first-pass metabolism).

They may look at absolute bioavailability (relative to an intravenous formulation) or relative bioavailability (with an alternate reference dosage form with better absorption). The time to absorption and rate of absorption are studied to obtain Area Under Curve (AUC) data.