warfarin and ciprofloxacin

Ciprofloxacin inhibits the hepatic metabolism of warfarin, specifically the CYP1A2 isoenzyme responsible for metabolising the less potent R-warfarin enantiomer, and may also reduce vitamin K production by intestinal flora. This leads to increased warfarin plasma concentrations and enhanced anticoagulant effect.

The INR can rise significantly, increasing the risk of serious bleeding, including gastrointestinal and intracranial haemorrhage. The onset may be within a few days of starting ciprofloxacin and can persist for several days after discontinuation.

Prescribers should avoid concurrent use if possible, especially in patients with other bleeding risk factors. If co-prescription is necessary, closely monitor the INR at baseline, after 2 to 3 days, and then regularly throughout treatment. Adjust the warfarin dose as needed and warn the patient to seek medical attention if signs of bleeding occur.

This interaction is well-established and documented in multiple case reports and clinical studies. It is listed in standard interaction compendia.