Recent technologies have resulted in an explosion of information concerning the cytochrome P-450 isoenzymes and increased awareness of life-threatening interactions with such commonly prescribed drugs as cisapride and some antihistamines. Knowledge of the substrates, inhibitors, and inducers of these enzymes assists in predicting clinically significant drug interactions. In addition to inhibition and induction, microsomal drug metabolism is affected by genetic polymorphisms, age, nutrition, hepatic disease, and endogenous chemicals. Of the more than 30 human isoenzymes identified to date, the major ones responsible for drug metabolism include CYP3A4, CYP2D6, CYP1A2, and the CYP2C subfamily.