Interactions between warfarin and fluoroquinolones

Case: 34 years old female with history of recurrent DVT, now on chronic Coumadin therapy, presented with black tarry stool and probable GI bleed. There was no change in her usual coumadin dose of 5 mg/day, on which she had a therapeutic INR of 2.5 for the last 2 years. Today her INR is 7.8. One week ago, she had a bout of severe UTI (urinary tract infection) and was started on antibiotics by her primary care physician. Which antibiotic seems to be the probable culprit? 

 

Answer: Quinolone (Ciprofloxacin/levofloxacin) - Coumadin interaction 

 Coumadin-Quinolone is among the top ten drug-drug interactions. The exact mechanism for the warfarin-quinolone drug interaction is unknown. The reduction of intestinal flora responsible for vitamin K production by antibiotics is the most probable cause. Also, decreased metabolism and clearance of warfarin are described. Quinolone can notoriously increase the effects of warfarin, with potential for life-threatening bleeding. The best approach is to avoid quinolone in Coumadin-dependent patients, but if required, INR should be monitored daily during co-administration of warfarin with quinolone. 

 #pharmacology 

 Reference: 

 Carroll DN, Carroll DG. Interactions between warfarin and three commonly prescribed fluoroquinolones. Ann Pharmacother. 2008 May;42(5):680-5. doi: 10.1345/aph.1K605. Epub 2008 Apr 15. PMID: 18413687.