warfarin and levofloxacin

Levofloxacin may increase the anticoagulant effect of warfarin. The exact mechanism is unclear, but proposed mechanisms include inhibition of warfarin metabolism by CYP2C9 and reduced production of vitamin K by gut flora, leading to elevated INR and bleeding risk.

Patients may experience a significant increase in INR and prothrombin time, with risk of serious or fatal bleeding, including intracranial haemorrhage. Onset can occur within a few days of starting levofloxacin and may persist for several days after discontinuation.

If co-prescription is necessary, monitor INR frequently (e.g., every 2-3 days) during the first week of levofloxacin therapy and for at least one week after stopping. Consider reducing warfarin dose based on INR. Advise patients to report any signs of bleeding.

Evidence from case reports, case series, and retrospective studies. The interaction is well-recognized, but the frequency and magnitude vary. Some studies show a modest increase in INR (average 0.5-1.0), while others report dramatic elevations. Consider regional variations and patient-specific factors such as age, renal function, and concurrent medications.