Many drug interactions are a result of inhibition or induction of cytochrome P450 enzymes (CYP450). The CYP3A subfamily is involved in many clinically significant drug interactions, including those involving nonsedating antihistamines and cisapride, that may result in cardiac dysrhythmias. CYP3A4 and CYP1A2 enzymes are involved in drug interactions involving theophylline. CYP2D6 is responsible for the metabolism of many psychotherapeutic agents. The protease inhibitors, which are used to treat patients infected with the human immunodeficiency virus, are metabolized by the CYP450 enzymes and consequently interact with a multitude of other medications. By understanding the unique functions and characteristics of these enzymes, physicians may better anticipate and manage drug interactions and may predict or explain an individual's response to a particular therapeutic regimen.