A large number of drugs are introduced every year, and new interactions between medications are increasingly reported. Consequently, it is no longer practical for physicians to rely on memory alone to avoid potential drug interactions. Multiple drug regimens carry the risk of adverse interactions. Precipitant drugs modify the object drug's absorption, distribution, metabolism, excretion or actual clinical effect. Nonsteroidal anti-inflammatory drugs, antibiotics and, in particular, rifampin are common precipitant drugs prescribed in primary care practice. Drugs with a narrow therapeutic range or low therapeutic index are more likely to be the objects for serious drug interactions. Object drugs in common use include warfarin, fluoroquinolones, antiepileptic drugs, oral contraceptives, cisapride and 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors. Many other drugs, act as precipitants or objects, and a number of drugs act as both. Regularly updated manuals of drug interactions and CD-ROM-formatted programs are useful office references.