The cytochrome P450 superfamily plays an important role in drug metabolism. Many environmental and genetic factors regulating the activity of individual enzymes, as well as their drug substrates, have been described. In several cases the cytochrome P450 enzymes are genetically polymorphic, resulting in phenotypically distinct poor and extensive metabolizers for some drugs. Poor metabolizers are susceptible to concentration-related adverse drug reactions, whilst extensive metabolizers are susceptible to drug interactions; inhibitory interactions may result in toxicity due to increased plasma concentrations. Altered drug metabolism is an important cause of adverse drug reactions.