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 × 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 × control). After one week of ciprofloxacin, his PT had increased to 22 sec (PT ratio 1.83 × 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.
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