oral bioavailability

Oral bioavailability is the fraction of a swallowed dose that survives the gauntlet of the gut and liver to reach the general circulation intact. It is the everyday, most practically important form of bioavailability, because the oral route — a simple tablet or capsule — is by far the most convenient and widely used way to take medicine.

A swallowed drug faces two big losses before it can act. First, it must be absorbed: it has to dissolve and permeate the gut wall, and any unabsorbed fraction is lost. Second, the blood draining the gut goes straight to the liver, the body's chemical processing plant, which can metabolize a large share of the drug before it ever reaches the rest of the body — the first-pass effect. Oral bioavailability is the product of surviving both hurdles.

This makes oral bioavailability a demanding, integrated target for drug design. A molecule must be soluble enough to dissolve, permeable enough to cross the gut, and metabolically stable enough to escape the liver — properties that often pull in opposite directions. Drugs that fail any one of these may need to be given by injection instead, or redesigned, reformulated, or delivered as a prodrug.