Moxifloxacin and warfarin: additional evidence for a clinically relevant interaction.

Several case reports have been published regarding an interaction between fluoroquinolones and warfarin; however, the exact mechanism of the interaction has yet to be established. We describe three patients who were receiving warfarin and experienced increases in international normalized ratio (INR) when moxifloxacin was added. Two of the patients had stable anticoagulation regimens and then experienced persistent elevations in their INRs shortly after therapy with moxifloxacin was started. The third patient experienced a dramatic rise in INR after the first dose of moxifloxacin was administered. These cases strengthen the evidence that an interaction does exist despite manufacturer-generated statements to the contrary. Increased awareness of this potential interaction is necessary to ensure appropriate monitoring and improve therapeutic decision making.