warfarin and erythromycin

Erythromycin inhibits the hepatic cytochrome P450 enzyme CYP3A4, which is one of the pathways responsible for the metabolism of warfarin. This inhibition reduces the clearance of warfarin, leading to increased plasma concentrations and enhanced anticoagulant effect.

The interaction can cause a significant rise in the international normalised ratio (INR), increasing the risk of serious bleeding, including gastrointestinal and intracranial haemorrhage. The onset is usually within a few days of starting erythromycin, but may be delayed.

Avoid concomitant use if possible. If co-prescription is necessary, monitor the INR closely at baseline and within 3 to 5 days of starting erythromycin. Consider a reduction in warfarin dose and continue monitoring after erythromycin is stopped.

This interaction is well established and documented in multiple clinical reports and studies. It is a recognised cause of elevated INR and bleeding when macrolides are used in warfarin-treated patients.