THE EFFECT OF PHENOBARBITAL ADMINISTRATION ON THE CONTROL OF COAGULATION ACHIEVED DURING WARFARIN THERAPY IN MAN

¹ Departments of Pharmacology and Medicine, University of Vermont College of Medicine, and The Mary Fletcher Hospital, Burlington, Vermont

Difficulties experienced with the therapeutic control of coagulation in man, in some cases associated with the concurrent administration of phenobarbital, led to a study of the effect of phenobarbital in patients on long term anticoagulation with warfarin. Phenobarbital in a daily dose of 2 mg/kg was given for a period of 4 wk to 16 outpatients on chronic warfarin therapy. A decrease in mean prothrombin time was evident after the first week and continued during the study even though warfarin dosage was increased. During a similar 4-wk experiment with placebo, mean prothrombin time and warfarin dosage remained unchanged from control values. It is concluded that antagonism to the anticoagulant effect of warfarin can occur when phenobarbital, in doses commonly employed for sedation, is given concomitantly.