Clarithromycin × Warfarin

Clarithromycin is one of the enzyme inhibitors that slow the liver metabolism of warfarin and increase its anticoagulant effect and bleeding risk (warfarin SmPC). The US Food and Drug Administration (FDA) lists clarithromycin as an inhibitor of CYP3A4, which clears one of the two warfarin isomers (warfarin FDA label). Serious bleeding and marked rises in the international normalised ratio (INR) and prothrombin time can occur with this combination (clarithromycin SmPC).

Bleeding gums, nosebleeds, bruising and pinpoint skin bleeds, blood in the urine, black stools, heavy periods. In severe cases, gastrointestinal or intracranial bleeding.

The combination can be used with frequent coagulation checks. Check the international normalised ratio (INR) and prothrombin time frequently throughout the clarithromycin course (clarithromycin SmPC). The US Food and Drug Administration (FDA) advises close INR monitoring when any antibiotic is started or stopped (warfarin FDA label). The European Society of Cardiology (ESC) lists macrolides among drugs that call for careful INR monitoring in patients on vitamin K antagonists (ESC 2024). Adjust the warfarin dose according to the INR (warfarin SmPC). Ask the patient to report any signs of bleeding (warfarin FDA label).