Possible interaction between ciprofloxacin and warfarin.

A 72-year-old man had been taking warfarin for a pulmonary embolus and recurrent deep-vein thromboses. His prothrombin times (PTs) were maintained between 15 and 18 sec (PT ratio 1.25-1.5 x control) for several months on a dose of warfarin 2.5 mg/d. Six weeks prior to starting ciprofloxacin 500 mg bid, the patient's PT was 15.5 sec (Pt ratio 1.29 x control). After one week of ciprofloxacin, his PT had increased to 22 sec (PT ratio 1.83 x control). No other causes for the increase were apparent. It is recommended that patients receiving both medications have their prothrombin times carefully monitored and warfarin doses adjusted only as necessary.