Warfarin Institute of America
Dedicated To Your Health
PHENOBARBITAL INTERACTIONS WITH WARFARIN
THIS IS A VERY POTENT AND DANGEROUS INTERACTION. WHEN SOMETHING CONTAINING PHENOBARBITAL IS ADDED TO THE MEDICATIONS OF A PERSON TAKING WARFARIN THERE CAN BE A LARGE DROP IN THE INR. CONVERSELY, WHEN SOMETHING CONTAINING PHENOBARBITAL IS DROPPED FROM THE MEDICATIONS OF SOMEONE TAKING WARFARIN, THE INR IS LIKELY TO MAKE A LARGE INCREASE. THE INR MUST BE MONITORED OFTEN UNTIL THE WARFARIN DOSE STABILIZES IF EITHER OF THESE CHANGES IS MADE. DO NOT UNDERESTIMATE THE POTENCY OF THIS INTERACTION!
I saw a patient who had a 0.8 unit drop in her INR two weeks after Bellergal-S (phenobarbital 40 mg two times daily) was added to her warfarin. This put her well below the therapeutic range. The interaction probably took about a week to manifest itself fully. After this it took a 50% increase in the warfarin dose to get her INR back in the desired range.
This interaction is probably dose-related. This means that even small changes in the dosage will affect the INR. Monitor the INR very closely until the warfarin dose stabilizes with the INR in range.
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Last updated November 14, 2004