Inappropriate medication is a major cause of adverse drug reactions in elderly patients.

To determine the extent to which adverse drug reactions (ADRs) in elderly patients admitted to hospital are due to inappropriate prescribing, we examined 416 successive admissions of elderly patients to a teaching hospital. Interacting drug combinations and drugs with relative contra-indications (CIs) were common, but not as important in producing ADRs as drugs with absolute CIs or unnecessary drugs. Forty-eight patients (11.5% of admissions) were taking a total of 51 drugs with absolute CIs (3.8% of prescriptions). One hundred and seventy-five drugs were discontinued on or shortly after admission in 113 (27%) patients because they were deemed to be unnecessary. One hundred and three patients (27.0% of those on medication) experienced 151 ADRs of which 75 (49.7%) were due to drugs with absolute CIs and/or that were unnecessary, a significantly higher rate of ADRs (p less than 0.001) than observed for all prescriptions. Of 26 (6.3%) admissions attributed to ADRs, 13 (50%) were due to inappropriate prescriptions. The admission rate per prescription was significantly higher (p less than 0.001) for inappropriate than for appropriate drugs. We conclude that much drug-related morbidity in the elderly population may be avoidable, as it is due to inappropriate prescribing.