Bioavailability refers to the proportion of an unaltered drug that, after administration, enters the systemic circulation and can be distributed to the desired action site. Factors such as gastrointestinal (GI) absorption and liver biotransformation influence the bioavailability of a drug when it is administered orally. When a drug is administered intravenously, it enters the systemic circulation directly; by definition, its bioavailability is assumed to be 100%. The bioavailability of an orally administered drug can be determined by comparing its plasma concentration with that of the same drug administered intravenously. A graph can be plotted that shows the plasma concentration of a drug over time, given in equivalent doses both orally and intravenously. The total area under this graph for each method of administration signifies the degree of drug absorption via that particular route. By calculating the ratio of these areas, an estimate of the bioavailable fraction of an orally administered drug can be obtained.
