Phenobarbital and Warfarin: Drug Interaction Guide

Quick answer: This combination requires caution. Phenobarbital and Warfarin can be used together with adjustments and monitoring.

Interaction severity: Moderate — caution required

How Phenobarbital interacts with Warfarin

Phenobarbital induces CYP2C9 and CYP3A4, increasing warfarin clearance and reducing anticoagulant effect; on withdrawal, INR can rise rapidly causing bleeding.

What you should do

Monitor INR closely during initiation, dose changes, or discontinuation of phenobarbital and adjust warfarin dose accordingly.

Key facts

Severity

Moderate — caution required

Mechanism

Phenobarbital induces CYP2C9 and CYP3A4, increasing warfarin clearance and reducing anticoagulant effect; on withdrawal,...

Action

Monitor INR closely during initiation, dose changes, or discontinuation of phenobarbital and adjust warfarin dose accord...

Important medical disclaimer: This page provides educational information about drug interactions for general reference. It is not a substitute for professional medical advice, diagnosis, or treatment. Always discuss your specific medications with a qualified healthcare provider before starting, stopping, or changing any treatment. About our editorial team.

Frequently asked questions

Can I take Phenobarbital and Warfarin together?

With caution. Monitor INR closely during initiation, dose changes, or discontinuation of phenobarbital and adjust warfarin dose accordingly.

What is the severity of this interaction?

The interaction between Phenobarbital and Warfarin is classified as Moderate — caution required. Phenobarbital induces CYP2C9 and CYP3A4, increasing warfarin clearance and reducing anticoagulant effect; on withdrawal, INR can rise rapidly causing bleeding.

What should I do if I'm prescribed both?

Monitor INR closely during initiation, dose changes, or discontinuation of phenobarbital and adjust warfarin dose accordingly. Always consult your prescribing clinician — do not stop or change medications without medical guidance.

Last reviewed: by iMedic Medical Editorial Team. Our editorial process.