Warfarin-5-FU interaction - A consecutive case series

Five patients from a single institution received concomitant warfarin and 5 -fluorouracil (5-FU) during a 3-year period. The mean weekly warfarin dose before starting chemotherapy was 40.66 mg and during chemotherapy it was 24 mg (p=0.0026). All patients required a warfarin dosage reduction (range 18 -74%, mean 44%). Two patients were hospitalized, one with a major retroperi toneal bleed, the other for fresh-frozen plasma administration and observat ion. Maximum international normalized ratios (INRs) ranged from 3.66-23.7. This series confirms a common, clinically significant interaction between w arfarin and 5-FU. An interaction between capecitabine, the orally available prodrug of S-FU, and warfarin also has been reported. We recommend weekly monitoring of prothrombin time and INR for all patients receiving concomita nt warfarin and 5-FU or capecitabine.