Prescribing patterns for combinations of warfarin and potentially dangerous interacting drugs in a large general practice population

The objectives of this study were to find what proportion of patients in a large general practice population were prescribed warfarin during a 1‐month period and to determine what proportion were simultaneously prescribed potentially dangerous interacting drugs. Data from prescriptions issued by GPs for the month of January 1991 to the segment of the population of the Eastern Health Board area entitled to free medical care were linked with individual age and sex data. Age and sex‐specific prescription rates for warfarin were calculated and the frequency of prescribing interacting drugs was computed for warfarin‐treated patients. The subjects of the study were residents of the Eastern Health Board who are entitled to free health care‐333 621 persons in all. Results were as follows. The crude warfarin prescription rate was 3.9 per 1000 (95% Cl: 3.7–4.1) and the rate increased with age, especially for men aged 65–74 in whom it was 15.5 per 1000 (95% Cl: 13.4–17.5). Of 722 patients on warfarin for more than 1‐month, 31.6% (95% Cl: 28.2–35.0) were prescribed at least one interacting drug concurrently, and 5.7% (95% Cl: 4.1–7.6) were prescribed more than one interacting drug. Non‐steroidal anti‐inflammatory drugs, including aspirin, accounted for 35.9% of the interacting drugs. It is concluded that combinations of warfarin and potentially interacting drugs are relatively commonly prescribed in this large general practice population.