This chapter discusses various aspects of drug interactions. A drug interaction results when the effects of a drug are altered in some way by the presence of another drug, by food, or by environmental exposure. The risk of developing an adverse drug reaction secondary to a drug-drug interaction increases significantly with the number of medications a patient is receiving. Interactions affecting drug absorption may result in changes in the rate of absorption, the extent of absorption, or a combination of both. Interactions resulting in a reduced rate of absorption are not typically clinically important for maintenance medications as long as the total amount of drug absorbed is not affected. On the other hand, for acutely administered medications, such as sedative-hypnotics or analgesics, a reduction in the rate of absorption may cause an unacceptable delay in the onset of the drug's pharmacologic effect. A series of events lead to increased synthesis of cytochrome P450 (CYP) isoenzymes, with a resultant augmentation of their catalytic activity. This enzyme induction may increase the intestinal and hepatic clearance of drugs, subsequently altering serum concentrations.