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OBJECTIVE: To estimate the prevalence of questionable and rational high-risk prescribing among elderly people of the three drug groups most commonly implicated in drug-related illness: cardiovascular drugs, psychotropic drugs and nonsteroidal anti-inflammatory drugs (NSAIDs). DESIGN: Retrospective prevalence study; all prescription and billing records for the period Jan. 1 to Dec. 31, 1990, for the study sample were retrieved from the relevant provincial databases of the Régie de l'assurance-maladie du Québec. SETTING: Quebec. PARTICIPANTS: Regionally stratified random sample of 63,268 elderly medicare registrants who made at least one visit to physician in 1990 and were not living in a health care institution for the entire year. MAIN OUTCOME MEASURE: Prescription information was examined for three types of high-risk prescribing: rational and questionable drug combinations, excessive treatment duration and drugs relatively contraindicated for use in elderly people. RESULTS: Overall, 52.6% of the patients experienced one or more events of high-risk prescribing, and 45.6% experienced at least one that was questionable. High-risk prescribing was most prevalent for psychotropic drugs, and questionable prescribing was more frequent than rational prescribing in this drug group. An estimated 30.8% of the total elderly population in Quebec received benzodiazepines for more than 30 consecutive days, 12.9% received a long-acting benzodiazepine, and 13.0% received a questionable high-risk psychotropic drug combination. The prevalence of high-risk prescribing was higher among the women than among the men and increased with age until 75 to 84 years. There were significant unexplained differences between regions in the regional prevalence of high-risk prescribing, particularly of psychotropic drugs. CONCLUSION: The prevalence of questionable high-risk prescribing, especially of psychotropic drugs, is substantial among elderly people. This may be a potentially important and avoidable risk factor for drug-related illness in elderly people.