The anticoagulant effects of warfarin were modestly and markedly reduced,respectively, in 2 patients stabilised on warfarin when they were given griseofulvin 1 g daily in divided doses. Griseofulvin 1 g daily had no effect on prothrombin time in one healthy subject given warfarin, whereas 2 g daily caused a marked reduction in prothrombin time. Another healthy subject showed no interaction, even when griseofulvin dosage was raised to 4 g daily for 2 weeks (See reference number 1).
In another study there was no change in mean prothrombin time in 10 patients stabilised on warfarin when they were given griseofulvin 1 g daily in divided doses for 2 weeks. Four of patients had an equivocal average reduction in prothrombin time of 4.2 seconds (See reference number 2). One case report describes decreased anticoagulant effects in a man stabilised on warfarin when he took griseofulvin 250mg twice daily,which took 12 weeks to develop fully (See reference number 3). He eventually needed a 41 % increase in his daily dose of warfarin. Another report very briefly mentions a case of a coagulation defect in a patient taking warfarin and griseofulvin (See reference number 4).
Not understood. It has been suggested that griseofulvin acts as a liver enzyme inducer, which increases metabolism of warfarin, thereby reducing its effects (See reference number 1,3).
This interaction is poorly documented and not well established. It possibly affects some patients. Because of uncertainty, prothrombin times of all patients taking warfarin who are given griseofulvin should be monitored, and suitable warfarin dosage increases made as necessary.
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