Mesalazine Warfarin

An interaction of mesalazine with warfarin has been reported (108). 

A 55-year-old woman taking warfarin 5 mg/day for deep venous thrombosis was also given mesalazine... 

A single case report describes reduced warfarin effects in a patient given mesalazine. Another single case report describes a marked reduction in the response to warfarin when mesalazine... 

A woman stabilised on warfarin 5 mg daily, with INRs between 2 and 3, started taking mesalazine 800 mg three times daily for the treatment of a caecal ulcer. Four weeks later she presented to hospital with left leg pain, which was diagnosed as an acute popliteal vein thrombosis, and at the same time it was found that her prothrombin time and INR had fallen to 11.3 seconds and 0.9, respectively. The patient was treated with intravenous heparin. Over the next 10 days INRs of up to 1.7 were achieved by increasing the doses of warfarin up to 10 mg daily, but a satisfactory DSTR of 2.1 was only reached when the mesalazine was stopped. The report says that serum warfarin levels were not detectable during the use of mesalazine. For diseussion of a patient who had a reduction in the response to warfarin when switehed from mesalazine to sulfasalazine, see below. 

Not understood. Sulfasalazine is broken down in the colon to a sulfonamide, sulfapyridine, and 5-aminosalicylic acid (mesalazine). Some sulfonamides , (p.376) are known inhibitors of warfarin metabolism, and increase the effects of warfarin. In contrast, in the case with sulfasalazine a marked decrease was noted. 

Not established. The case of a reduction in effect of warfarin on starting mesalazine appears to be the first and only report of an interaction, which suggests that it is unlikely to be of general importance. Similarly, the case when mesalazine was switched to sulfasalazine is an unexplained and isolated case, and its validity has been debated. There are no other reports in the literature and this possible interaction also seems unlikely to be of general importance. Consider these cases in the event of an unexpected response to treatment. 

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