[Interaction of rifampicin with phenprocoumon (author's transl)]

Prothrombin time and plasma phenprocoumon levels were serially controlled, at short intervals, in three tubercular patients receiving both tuberculostatic drugs and phenprocoumon. The required phenprocoumon dose was continuously adapted to the prothrombin time values. During and after treatment with rifampicin optimal levels of prothrombin time were difficult or even impossible to achieve in two of the three patients. This confirms the known loss of effectiveness of the coumarins induced by rifampicin. This action has a slower onset and disappears more gradually than after acenocoumarol or warfarin. It is, therefore, likely that these two latter drugs are more suitable in patients treated with rifampicin than phenprocoumon.