bioavailability
Bioavailability answers a simple question: of the dose you gave, how much actually made it into the bloodstream intact and ready to work? If you swallow 100 mg but only 40 mg reaches the circulation unchanged, the bioavailability is 40 percent. The rest was lost — never absorbed, or chemically chewed up before it arrived.
By convention, a drug injected straight into a vein has 100 percent bioavailability, because it is placed directly into the blood with nothing lost. Every other route is measured against this benchmark. Bioavailability is usually estimated by comparing the total drug exposure (the area under the concentration-time curve) after the test route with that after an intravenous dose.
The number captures the combined toll of incomplete absorption and any first-pass loss in the gut wall and liver. It matters enormously in practice: a drug with low and variable bioavailability gives unpredictable blood levels, making it hard to dose safely. Improving bioavailability is a constant goal in formulation and in the chemistry of designing absorbable molecules.
A drug that is well absorbed but extensively cleared by the liver on first pass may have only 10 to 20 percent oral bioavailability, even though nearly all of it crossed the gut wall.
Good absorption does not guarantee high bioavailability.
Bioavailability is denoted F, a fraction between 0 and 1 (or a percentage); it folds together absorption and first-pass metabolism into a single end-to-end yield.