OBJECTIVE: To analyse the prevalence of potential drug interactions, and to identify patients particularly prone to drug interaction.

DESIGN: Database study (Odense University Pharmacoepidemiologic Database).

SETTING: Individuals exposed to polypharmacy in 1999 were examined for potential drug interactions.

SUBJECTS: Inhabitants of the County of Funen (n = 471 732).

MAIN OUTCOME MEASURES: Prevalence of potential drug interactions.

RESULTS: One-third of the population were exposed to polypharmacy and among these 15% were exposed to drugs carrying a risk of harmful interaction. Among the elderly with polypharmacy, 25% aged 60-79 years and 36% over 80 years received drugs carrying the risk of interaction. Among individuals exposed to potential drug interaction, 62% were exposed only to one drug interaction and 38% to two or more different drug interactions. The drugs accounting for the highest number of potential interactions were diuretics, NSAIDs, ACE-inhibitors, digoxin, oral antidiabetics, calcium channel blockers, anticoagulants and beta-blockers. When focusing only on major drug interactions, potassium-sparing diuretics and oral anticoagulants were the most frequently involved drugs.

CONCLUSION: Elderly patients exposed to polypharmacy should be kept under intensified monitoring as they are at increased risk of clinically significant drug interactions.