Bioavailability (F) Definition:
The fraction of administered drug that enters the systemic blood circulation unchanged.
Bioavailability is a pharmacokinetic parameter represented by the symbol "F" and is usually presented as % of the dose. For example, a bioavailability of 70% means 70% of the administered dose reaches the systemic circulation. Oral bioavailability (bioavailability of an orally administered drug) is most often determined by administering a drug orally and intravenously on separate days and measuring the plasma concentrations over time. The area under the plasma drug concentration time curve (AUC) is then determined for each route of administration and the bioavailability is the ratio of AUCoral to AUCintravenous (F = AUCoral / AUCintravenous). The most common reasons for a reduced bioavailability are drug interactions, altered drug metabolism, variation in drug transporters and degradation (by stomach acid or gut bacteria). Some drugs are not absorbed following oral administration and must be given parenterally.
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