Pharmacoepidemiologic study of warfarin prescription in a Brazilian tertiary hospital.

Warfarin is among the ten drugs most commonly involved in adverse drug reactions, has a narrow therapeutic index and complex dosage regimen, exhibits enormous variability dose-response and high risk drug-drug interactions. To analyze the profile of pharmacoepidemiological drug prescriptions for warfarin in patients admitted to a Brazilian tertiary hospital. In the cross sectional study the electronic prescriptions of the January 01, 2004-December 31, 2010 of patients using warfarin of the Clinical Hospital of the Faculty of Medicine of Ribeirao Preto were analyzed. Sociodemographic characteristics, clinical data, laboratory test results, and drug therapy data were collected. We identified 3,048 patients who received 154,161 drug prescriptions, 42,120 of which contained warfarin. The mean age was 55.8 ± 19.3 years, 48.2% were elderly, and 4.3% had specific cerebrovascular disease diagnoses. The average International Normalized Ratio (INR; 2.4 ± 1.7) and warfarin dose (5.1 ± 1.8 mg/day) were within therapeutic protocol recommendations. However, approximately 14.0 and 8.5% of the patients had mean INR greater than 3.0 and 3.5, respectively. In addition, 66.4 and 6.1% of the patients received polypharmacy and clopidogrel, respectively, which can increase the risk of drug-drug interactions and bleeding. The average number of drugs per prescription was 7.2 ± 3.8, and 31,595 drug prescriptions (74.0%) consisted of five or more drugs. The study found a high prevalence of elderly patients, polypharmacy, and concomitant use of anticoagulant and antiplatelet drugs, which may favor the occurrence of adverse events.