title: Levofloxacin and warfarin interaction.
abstract: OBJECTIVE
  To report 4 cases of hypoprothrombotic response resulting from addition of levofloxacin therapy to chronic warfarin therapy and to review related literature to support or refute a warfarin-levofloxacin interaction.
  
  
  METHODS
  Four patients, 34-81 years old, were prescribed levofloxacin concomitantly with stable warfarin therapy. Three patients had a target international normalized ratio (INR) range of 2.0-3.0 and experienced an increase in INR to 3.5, 8.12, and 11.5 on days 11, 5, and 4 of a 10-day course of levofloxacin, respectively. The fourth patient experienced minor bleeding, with a slightly elevated INR on the second day of levofloxacin therapy that required up to a 19% warfarin dose reduction during levofloxacin treatment.
  
  
  CONCLUSIONS
  An initial premarketing clinical trial concluded that levofloxacin had no effect on warfarin's pharmacokinetics and pharmacodynamic response. Two case reports have since documented an increase in INR in patients taking long-term warfarin on completion of levofloxacin therapy. Our case reports provide further evidence of a significant increase in INR observed during concomitant levofloxacin therapy. The proposed mechanism of this interaction is displacement of warfarin from protein binding sites, reduction in gut flora producing vitamin K, and decreased warfarin metabolism.
  
  
  CONCLUSIONS
  Prolonged prothrombin response in patients undergoing chronic warfarin therapy has been well documented with many antibiotics, including fluoroquinolones. Recognition of newer antibiotics' effects on warfarin therapy is important to guide safe use and monitoring of anticoagulation therapy. Our case studies demonstrate significant elevations in INR values during and up to 1 day after levofloxacin therapy in patients undergoing stable warfarin therapy.
authors.affiliation: The University of Oklahoma Health Sciences Center, Oklahoma City, OK 73190, USA.
authors.forename: Cade B
authors.initials: CB
authors.lastname: Jones
authors.forename: Susan E
authors.initials: SE
authors.lastname: Fugate
mesh_terms: D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000890:Anti-Infective Agents / Q000008:administration & dosage / Q000009:adverse effects*; D000925:Anticoagulants / Q000008:administration & dosage / Q000009:adverse effects*; D004334:Drug Administration Schedule; D004357:Drug Synergism; D005260:Female; D006801:Humans; D019934:International Normalized Ratio; D064704:Levofloxacin; D008297:Male; D008875:Middle Aged; D015242:Ofloxacin / Q000008:administration & dosage / Q000009:adverse effects*; D011517:Prothrombin Time; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects*
chemical_list: D000890:Anti-Infective Agents; D000925:Anticoagulants; D014859:Warfarin; D064704:Levofloxacin; D015242:Ofloxacin