Factors contributing to variability in drug pharmacokinetics. I. Absorption.

Apart from the physical and chemical properties of a drug, and also the dosage form in which it is presented, many other factors may affect the absorption of orally administered compounds and give rise to variable systemic availability. Three such factors, gastrointestinal (GI) disease, drug-drug interactions, and drug-food interactions are considered here. Although information regarding these factors, particularly GI disease, is scarce and sometimes conflicting, it is clear that they may give rise to variable drug absorption. The wide spectrum of effects on different drugs precludes the application of general rules and guidelines in drug therapy in most cases. Drugs, dosage forms, and various interactions should be considered individually. Also the types of interactions described in this review must be combined with other factors to be discussed later in this series when considering the influence of variable absorption, distribution, metabolism, and excretion on drug pharmacokinetics and clinical efficacy.