Interactions between warfarin and antibiotics, especially cotrimoxazole

Canadian authors have asked whether, in patients over 65 years treated continuously with warfarin, treatment of urinary tract infections with certain antibiotics did not alter the risk of gastrointestinal bleeding. Their paper, published in Archives of Internal Medicine April 12, 2010, shows that the incidence of gastrointestinal bleeding was significantly higher in patients treated with cotrimoxazole (trimethoprim plus sulfamethoxazole, Bactrim *) than in patients not treated with antibiotics. Ciprofloxacin also increased the frequency of bleedings but lesser than coytrimoxazole. Other antibiotics including amoxicillin, ampicillin, norfloxacin and nitrofurantoin have not resulted in significant increase in the incidence of gastrointestinal bleeding.

This study confirms that cotrimoxazole and ciprofloxacin enhance the anticoagulant effect of warfarin