Human cytochrome P450 (CYP) 3A4 is the most abundant hepatic and intestinal phase I enzyme that metabolizes approximately 50% marketed drugs. The crystal structure of bound and unbound CYP3A4 has been recently constructed, and a small active site and a peripheral binding site are identified. A recent study indicates that CYP3A4 undergoes dramatic conformational changes upon binding to ketoconazole or erythromycin with a differential but substantial (>80%) increase in the active site volume, providing a structural basis for ligand promiscuity of CYP3A4. A number of important drugs have been identified as substrates, inducers and/or inhibitors of CYP3A4. The ability of drugs to act as inducers, inhibitors, or substrates for CYP3A is predictive of whether concurrent administration of these compounds with a known CYP3A substrate might lead to altered drug disposition, efficacy or toxicity. The substrates of CYP3A4 considerably overlap with those of P-glycoprotein (P-gp). To date, the identified clinically important CYP3A4 inhibitors mainly include macrolide antibiotics (e.g., clarithromycin, and erythromycin), anti-HIV agents (e.g., ritonavir and delavirdine), antidepressants (e.g. fluoxetine and fluvoxamine), calcium channel blockers (e.g. verapamil and diltiazem), steroids and their modulators (e.g., gestodene and mifepristone), and several herbal and dietary components. Many of these drugs are also mechanism-based inhibitors of CYP3A4, which involves formation of reactive metabolites, binding to CYP3A4 and irreversible enzyme inactivation. A small number of drugs such as rifampin, phenytoin and ritonavir are identified as inducers of CYP3A4. The orphan nuclear receptor, pregnane X receptor (PXR), have been found to play a critical role in the induction of CYP3A4. The inhibition or induction of CYP3A4 by drugs often causes unfavorable and long-lasting drug-drug interactions and probably fatal toxicity, depending on many factors associated with the enzyme, drugs and the patients. The study of interactions of newly synthesized compounds with CYP3A4 has been incorporated into drug development and detection of possible CYP3A4 inhibitors and inducers during the early stages of drug development is critical in preventing potential drug-drug interactions and side effects. Clinicians are encouraged to have a sound knowledge on drugs that behave as substrates, inhibitors or inducers of CYP3A4, and take proper cautions and close monitoring for potential drug interactions when using drugs that are CYP3A4 inhibitors or inducers.