Drug absorption and bioavailability

Abstract

Most drugs are prescribed as oral preparations or extravascular injections (other than intravenous injections) for the treatment of systemic diseases. These drugs must therefore be absorbed in order to be transported to the target tissues to produce their pharmacological actions. Consequently, absorption plays a key role in determining whether or not a drug produces a clinical effect and how fast it occurs. The rate and extent to which a drug is absorbed systemically are related to its time-to-peak concentration (T max) and fractional bioavailability (F). Often the two pharmacokinetic terms, absorption and bioavailability, are considered synonymously, but there is actually a subtle difference between them. It is possible for drugs to be well absorbed orally because of good lipid solubility and yet not have a good oral bioavailability because of extensive presystemic loss. While the intravenous bioavailability of drugs is always 100%, the oral bioavailability is usually less than 100% because of incomplete absorption and/or first-pass elimination. Many factors influence the oral bioavailability of a drug: Some are related to the drug while others to the patient. To overcome poor bioavailability, we can increase the dose administered, change the pharmaceutical formulation, or use a different route of administration.