An article in the current Archives of Internal Medicine identifies the sulfonamide antibiotic cotrimoxazole (Bactrim® and several generics) as conferring a 3.84 odds ratio of upper gastrointestinal bleeding when co-prescribed with warfarin in patients >66. Likewise ciprofloxacin (Ciprol®) co-therapy generated an odds ratio of 1.94, also significant. These antibiotics are primarily used for urinary tract infection and may reduce vitamin K-producing intestinal flora and may also compete with warfarin metabolism through the cytochrome P450 enzyme 2C9. Co-therapy with amoxicillin, ampicillin, nitrofurantoin or norfloxacin did not significantly raise patient bleeding risk. Fischer HD, Juurlink DN, Mamdani MM, Kopp A, Laupacis A. Hemorrhage during warfarin therapy associated with cotrimoxazole and other urinary tract anti-infective agents. Arch Intern Med 2010; 170: 617–21.