Procoagulant effect of phenobarbital in rats

Daily administration of phenobarbital, 75 mg/kg ip for 4 days, to adult male Sprague-Dawley rats produced a pronounced increase of prothrombin complex activity (PCA) in plasma, i.e. a decrease of the prothrombin time. This effect persisted for 4 to 5 days after the last dose of phenobarbital. The rate constant for the decline of PCA after administration of a PCA synthesis-blocking dose of warfarin was not affected by treatment with phenobarbital but the rate of synthesis of PCA was increased appreciably. Thus, the phenobarbital-induced increase of PCA was caused by increased synthesis of one or more clotting factors.