warfarin and omeprazole

ModerateUsually manageable, but it must be recognised and monitored.

Mechanism

Omeprazole inhibits CYP2C19, which is involved in the metabolism of R-warfarin, and may also weakly inhibit CYP2C9, the enzyme responsible for metabolising the more potent S-warfarin. This can reduce warfarin clearance and increase its anticoagulant effect.

What it means clinically

Co-administration may lead to an increase in INR and a heightened risk of bleeding. The effect is variable; some patients may experience a clinically significant rise in INR, while others may not.

What to do

Monitor INR closely, especially when starting or stopping omeprazole. Consider alternative acid-suppressant therapy with a proton pump inhibitor that has less CYP2C9/2C19 interaction (e.g., pantoprazole) if appropriate. Adjust warfarin dose as required.

Evidence

Well-established interaction. The effect is modest but can be clinically relevant in susceptible patients. Guidelines recommend monitoring INR when these drugs are co-prescribed.

The two drugs

warfarinVitamin K antagonistRead the monograph omeprazoleProton pump inhibitorsRead the monograph

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Other interactions involving warfarin or omeprazole

Educational reference for students and clinicians in training. It does not replace professional judgement, local protocols or current prescribing information.