Drug interactions
Phenobarbital + Warfarin
Severity: Major
Phenobarbital and Warfarin: major interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Phenobarbital induces hepatic enzymes and speeds warfarin metabolism, lowering INR and anticoagulant effect. The effect develops over weeks, and on stopping phenobarbital warfarin levels can rise with bleeding risk.
What this interaction means in plain words
Phenobarbital and Warfarin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Why it happens
Induction of CYP2C9 and other hepatic enzymes by phenobarbital, increasing warfarin clearance.
What to do
Monitor INR frequently when phenobarbital is started or stopped, and increase warfarin dose as required. Reduce the dose again with close INR checks if phenobarbital is withdrawn.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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Other interactions of Warfarin
- Defibrotide + Warfarin Contraindicated
- Ketorolac + Warfarin Contraindicated
- Abciximab + Warfarin Major
- Acetylsalicylic acid + Warfarin Major
- Adagrasib + Warfarin Major
- Amiodarone + Warfarin Major
Frequently asked questions
Can I take Phenobarbital with Warfarin?
MediHelp rates the interaction as major. Phenobarbital and Warfarin can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Phenobarbital induces hepatic enzymes and speeds warfarin metabolism, lowering INR and anticoagulant effect. The effect develops over weeks, and on stopping phenobarbital warfarin levels can rise with bleeding risk.
How serious is the interaction between Phenobarbital and Warfarin?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor INR frequently when phenobarbital is started or stopped, and increase warfarin dose as required. Reduce the dose again with close INR checks if phenobarbital is withdrawn.
What other medicines interact with Warfarin?
MediHelp also lists interactions of Warfarin with, for example, Defibrotide, Ketorolac, Abciximab. The full list is on the Warfarin page.
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