Potential Warfarin‐Ciprofloxacin Interaction in Patients Receiving Long‐Term Anticoagulation

This study prospectively evaluated the potential interaction between the oral anticoagulant warfarin and the quinolone antimicrobial agent ciprofloxacin. After a 10‐day placebo lead‐in phase, 16 patients stabilized with long‐term warfarin therapy were randomized to receive ciprofloxacin 500 mg or a matching placebo twice/day for 10 days. International normalized ratios (INRs) measured by both standard laboratory analysis and by Coumatrak (finger‐stick) methods were evaluated at 3‐ to 5‐day intervals. No patient experienced a significant increase in INR. No patient experienced a bleeding event. These data support the fact that a warfarin‐ciprofloxacin interaction does not routinely occur at this dosage and duration of ciprofloxacin therapy.