生物利用性：概要

After a drug is administered, the amount of administered drug that enters the systemic circulation and is available for distribution to its site of action is called bioavailability.

The bioavailability of an orally administered drug is affected by GI absorption and biotransformation by the liver.

In the case of a drug that is intravenously administered, the drug directly enters the systemic circulation, and the bioavailability is assumed to be 100%, by definition.

For an orally administered drug, a comparison of the plasma concentration of the drug administered through the IV route helps to determine the bioavailability of that drug.

A plot of the drug's plasma concentration versus time for the same drug administered in equivalent doses, both orally and intravenously, can be generated.

The total area under the curve for each route of administration represents the extent of drug absorption for that route.

The ratio of the areas calculated gives an estimate of the bioavailable fraction of the orally administered drug.