What is known and Objective: Warfarin is a potent anticoagulant with many drug-drug interactions, including antimicrobials. There is limited data on the frequency of prescription of high-risk antimicrobials to patients on warfarin. To examine the frequency of prescriptions for potentially interacting antimicrobials in ambulatory patients on warfarin and the impact of warfarin on the prescription of high-risk antimicrobials. Methods: A retrospective cohort study of patients with pharmacy benefits who had ≥1 claim for an oral antimicrobial between 1 January 2008 and 31 December 2008 was conducted, utilizing a pharmacy benefits database. Demographic data including age, gender, chronic disease score (CDS) and geographic location were determined. Warfarin users were defined as any patient with ≥1 claim for warfarin during the follow-up period. Antimicrobials considered high risk for potential interaction with warfarin based on existing literature included trimethoprim/ sulfamethoxazole, levofloxacin, ciprofloxacin, metronidazole and fluconazole. Multivariate analysis was used to determine the impact of warfarin use and other factors on high-risk antimicrobial prescription. Results and Discussion: A total of 4 568 150 patients with ≥1 claim for antimicrobials during 2008 were analysed. Of them, 110 192 (2.4%) also had one or more claims for warfarin. Among all antimicrobial prescriptions in warfarin users, 42.6% were for high-risk antimicrobials. The mean number of antimicrobial prescriptions was 3.0 in warfarin users versus 2.4 in warfarin non-users (P-value <0.001). After adjusting for age, gender, CDS and geography, the odds of exposure to high-risk antimicrobials was 42% lower (OR 0.58; P-value <0.001) in warfarin users compared with warfarin non-users. What is new and Conclusions: A high percentage (42.6%) of antimicrobial prescriptions among warfarin users were for high-risk antimicrobials that carry excess bleeding risk. Although clinicians were somewhat less likely to prescribe high-risk antimicrobials to warfarin users compared with nonusers, the incidence of co-prescription remains high.
- Drug-drug interactions