Rifampin and warfarin: a drug interaction.

The drug interaction between warfarin and rifampin is not well known. Rifampin has been reported to increase the warfarin requirements in human subjects ingesting these agents simultaneously. The concomitant administration of rifampin and warfarin resulted in the need for an unusually high maintenance dose of warfarin (20 mg per day) in order to produce a therapeutic effect. Withdrawal of rifampin decreased the warfarin requirement by 50%. This effect may be mediated by the ability of rifampin to induce microsomal enzymes and, thus, the catabolism of warfarin. The effect of rifampin on the warfarin requirement of our patient appeared to be maximal 5 to 7 days after the initiation of rifampin and extended a similar length of time after rifampin withdrawal. This interaction appears to be clinically significant.