The two major concerns in drug safety are adverse drug reactions and drug interactions. When multiple drug therapies are prescribed, drug interactions become an important consideration for patients and physicians. The life of a drug is reviewed with emphasis on absorption, distribution, metabolism, and excretion. Pharmacokinetic and pharmacodynamic mechanisms for drug interactions are reviewed. The contributions of P-glycoprotein, pharmacogenetic variation, and genetic polymorphisms to drug interactions are highlighted. Prediction of drug interactions is possible with knowledge of which agents are likely to cause alterations in drug metabolism. (J Am Acad Dermatol 2002;47:467-84.)

Learning objective: At the conclusion of this learning activity, participants should have an understanding of the life of a drug. This knowledge should help predict important potential drug interactions.