Trimethoprim and Warfarin: Drug Interaction Guide
Quick answer: This is a major interaction — avoid combining Trimethoprim and Warfarin unless specifically directed by your doctor.
How Trimethoprim interacts with Warfarin
Trimethoprim-sulfamethoxazole inhibits CYP2C9 and can reduce vitamin K-producing gut flora, increasing warfarin effect and bleeding risk.
What you should do
Avoid when possible; if used, pre-emptively plan close INR monitoring and consider temporary warfarin dose reduction.
Key facts
Severity
Major — avoid combination
Mechanism
Trimethoprim-sulfamethoxazole inhibits CYP2C9 and can reduce vitamin K-producing gut flora, increasing warfarin effect a...
Action
Avoid when possible; if used, pre-emptively plan close INR monitoring and consider temporary warfarin dose reduction.
Frequently asked questions
Can I take Trimethoprim and Warfarin together?
No — this combination should generally be avoided. Avoid when possible; if used, pre-emptively plan close INR monitoring and consider temporary warfarin dose reduction.
What is the severity of this interaction?
The interaction between Trimethoprim and Warfarin is classified as Major — avoid combination. Trimethoprim-sulfamethoxazole inhibits CYP2C9 and can reduce vitamin K-producing gut flora, increasing warfarin effect and bleeding risk.
What should I do if I'm prescribed both?
Avoid when possible; if used, pre-emptively plan close INR monitoring and consider temporary warfarin dose reduction. Always consult your prescribing clinician — do not stop or change medications without medical guidance.
Last reviewed: by iMedic Medical Editorial Team. Our editorial process.