Drug interactions and multiple drug administration.

Effects of multiple drug administration on adverse drug reactions were studied in 10,518 patients hospitalized on a general medical service during a five-year period. Nine index drug groups, including analgesic, antacid, antiarrhythmic, antimicrobic, anticoagulant, antihypertensive, anti-inflammatory, diuretic, and sedative-tranquilizer drugs, were selected for study. The average number of adverse drug reactions for the anticoagulant and antihypertensive drug groups was higher (p less than 0.05) than for all other drug groups when classified by the number of drugs being taken concurrently (i.e., 0 to 5, 6 to 10, etc.). The rate of reaction for anticoagulant and antihypertensive drug groups was higher (p less than 0.001) than the rate for other drug groups studied. These data suggest a higher risk of adverse drug reactions for patients receiving multiple drugs. The increased risk may result from drug interactions.