OBJECTIVE: To review available data on pharmacokinetic and pharmacogenetic influences on the response to antidepressant therapy, analyze the mechanisms for and clinical significance of pharmacokinetic and pharmacogenetic differences, and explain the implications of pharmacokinetics and pharmacogenetics for patient care. DATA SOURCES: A MEDLINE search of English-language clinical studies, abstracts, and review articles on antidepressant pharmacokinetics, pharmacogenetics, and drug interactions was used to identify pertinent literature. DATA SYNTHESIS: The pharmacokinetic profiles of selected antidepressants are reviewed and the impact of hepatic microsomal enzymes on antidepressant metabolism is considered. How phenotypic differences influence the metabolism of antidepressant drug therapy is addressed. To evaluate the clinical implications of these pharmacokinetic and pharmacogenetic considerations, the findings of studies designed to elucidate drug interactions involving antidepressant agents are discussed. CONCLUSIONS: Differences in antidepressant plasma concentrations, and possibly safety, are caused by polymorphism in the genes that encode some of the cytochrome P450 isoenzymes that metabolize antidepressants. The isoenzymes 1A2, 2C9/19, 2D6, and 3A4 are the major enzymes that catalyze antidepressant metabolic reactions. Antidepressants can be either substrates or inhibitors of these enzymes, which also metabolize many other pharmacologic agents. Although the cytochrome enzymes that metabolize antidepressants have not been fully characterized, interaction profiles of the newer antidepressants are becoming more clearly defined. Determining patient phenotypes is not practical in the clinical setting, but an awareness of the possibility of genetic polymorphism in antidepressant metabolism may help explain therapeutic failure or toxicity, help predict the likelihood of drug interactions, and help clinicians better manage antidepressant drug therapy.