Bioavailability of Drugs

SYSTEMIC drugs administered orally or parenterally must reach the general circulation in their pharmacologically active form to be distributed throughout the body and to exert therapeutic effects at their sites of action. Physicians widely assume that this condition is reliably met by most drugs and drug products. In actual fact, complete entry into the general circulation is assured only when the drug product is injected intravascularly. Other routes of administration, and particularly the oral route, often allow appearance in the systemic circulation of only some fraction of the administered drug. A major advance in pharmacotherapy during the last few years . . .