Frequency, severity and risk factors for adverse drug reactions in adult out-patients: a prospective study.

To provide information on the frequency of adverse drug reactions in ambulatory patients we used intensive telephone surveillance to detect suspected reactions in 1026 patients seen at an internal medicine group practice over a 1-year period. Two hundred and ninety-two suspected reactions were detected. The majority of suspected reactions were mild, causing predominantly worry or discomfort. We used a published algorithm to assess the suspected reactions for drug causation with the following results: 40 (14%) unlikely; 193 (66%) possible; 56 (19%) probable; and 3 (1%) definite. The rate of probable or definite reactions was 49/1026 (5%) per patient and 58/3330 (2%) per drug course. Surprisingly, neither the age of the patient nor the number of other drugs he was taking modified the risk of a reaction to an individual newly-started drug. Our data suggest that fear of adverse reaction should not usually be a major factor in therapeutic decision making for an ambulatory patient even when the patient is old or already on multiple drugs.