Interaction of rifampin and warfarin.

A 72-year-old man who had been taking rifampin daily for several months was concurrently administered warfarin daily for ten weeks. During this period, the prothrombin time (PT) rose remarkably little as the dosage of warfarin was increased. With difficulty, satisfactory anticoagulation was achieved by giving warfarin 20 mg daily. On discontinuation of rifampin therapy, the PT increased significantly, and subsequent stabilization of the PT within therapeutic range required treatment with warfarin 7.5 mg daily.