bioavailability

Bioavailability answers a practical question: of the drug you swallowed, how much actually made it into the bloodstream intact and ready to work? If a 100 mg tablet delivers the equivalent of 70 mg into systemic circulation, its bioavailability is 70%. The rest was never absorbed, or was chewed up by metabolism on the way in.

Formally, bioavailability (often written F) is the fraction of an administered dose of unchanged drug that reaches the systemic circulation, capturing both the rate and the extent of that delivery. Extent is measured from the area under the plasma concentration-time curve (AUC); rate shows up in how high and how soon the peak concentration (Cmax at Tmax) appears. An intravenous dose is by definition 100% available, which is why it serves as the reference.

For oral drugs, bioavailability is the product of the fraction that survives the gut lumen and dissolves, the fraction absorbed across the gut wall, and the fraction that escapes first-pass metabolism in the gut wall and liver. A drug can be completely absorbed yet have low bioavailability if first-pass metabolism is heavy — propranolol and morphine are familiar examples. So bioavailability is broader than absorption: it is what the body can ultimately use.