Phenytoin and Warfarin: Drug Interaction Guide
Quick answer: This is a major interaction — avoid combining Phenytoin and Warfarin unless specifically directed by your doctor.
How Phenytoin interacts with Warfarin
Phenytoin causes biphasic effects: initial inhibition of CYP2C9 raises INR, followed by enzyme induction reducing warfarin effect; phenytoin levels also rise due to displacement and CYP inhibition.
What you should do
Increase frequency of INR monitoring (every 3-7 days) for at least 4 weeks after any change; check phenytoin levels and adjust both agents as required.
Key facts
Severity
Major — avoid combination
Mechanism
Phenytoin causes biphasic effects: initial inhibition of CYP2C9 raises INR, followed by enzyme induction reducing warfar...
Action
Increase frequency of INR monitoring (every 3-7 days) for at least 4 weeks after any change; check phenytoin levels and ...
Frequently asked questions
Can I take Phenytoin and Warfarin together?
No — this combination should generally be avoided. Increase frequency of INR monitoring (every 3-7 days) for at least 4 weeks after any change; check phenytoin levels and adjust both agents as required.
What is the severity of this interaction?
The interaction between Phenytoin and Warfarin is classified as Major — avoid combination. Phenytoin causes biphasic effects: initial inhibition of CYP2C9 raises INR, followed by enzyme induction reducing warfarin effect; phenytoin levels also rise due to displacement and CYP inhibition.
What should I do if I'm prescribed both?
Increase frequency of INR monitoring (every 3-7 days) for at least 4 weeks after any change; check phenytoin levels and adjust both agents as required. Always consult your prescribing clinician — do not stop or change medications without medical guidance.
Last reviewed: by iMedic Medical Editorial Team. Our editorial process.