Wednesday July 29, 2009


Case: 34 year old female with history of recurrent DVT now on chronic Coumadin therapy, presented with black tarry stool and probable GI bleed. There was on change on her usual coumadin dose of 5 mg/day on which she had therapeutic INR of 2.8 since last 2 years. Today her INR is 7.8. One week ago, she has a bout of severe UTI (urinary tract infection) and started on antibiotics by her primary care physician?




Answer: Quinolone (Ciprofloxacin/levofloxacin) - Coumadin interaction

Coumadin-Quinolone are among top ten drug-drug interactions. The exact mechanism for the warfarin-quinolone drug interaction is unknown. Reduction of intestinal flora responsible for vitamin K production by antibiotics is probable as well as decreased metabolism and clearance of warfarin. It can notoriously increase effects of warfarin, with potential for life threatening bleeding.

Best approach is to avoid quinolone in Coumadin dependent patient but if required, INR should be monitored daily during co-administration of warfarin with a quinolone.