Trimethoprim Sulfamethoxazole and Warfarin: Drug Interaction Guide
Quick answer: This is a major interaction — avoid combining Trimethoprim Sulfamethoxazole and Warfarin unless specifically directed by your doctor.
How Trimethoprim Sulfamethoxazole interacts with Warfarin
Sulfamethoxazole inhibits CYP2C9 metabolism of (S)-warfarin and displaces it from albumin, substantially raising INR with bleeding risk.
What you should do
Anticipate empirical warfarin dose reduction of 25-50% and check INR within 3 days of starting the antibiotic; consider doxycycline if appropriate.
Key facts
Severity
Major — avoid combination
Mechanism
Sulfamethoxazole inhibits CYP2C9 metabolism of (S)-warfarin and displaces it from albumin, substantially raising INR wit...
Action
Anticipate empirical warfarin dose reduction of 25-50% and check INR within 3 days of starting the antibiotic; consider ...
Frequently asked questions
Can I take Trimethoprim Sulfamethoxazole and Warfarin together?
No — this combination should generally be avoided. Anticipate empirical warfarin dose reduction of 25-50% and check INR within 3 days of starting the antibiotic; consider doxycycline if appropriate.
What is the severity of this interaction?
The interaction between Trimethoprim Sulfamethoxazole and Warfarin is classified as Major — avoid combination. Sulfamethoxazole inhibits CYP2C9 metabolism of (S)-warfarin and displaces it from albumin, substantially raising INR with bleeding risk.
What should I do if I'm prescribed both?
Anticipate empirical warfarin dose reduction of 25-50% and check INR within 3 days of starting the antibiotic; consider doxycycline if appropriate. Always consult your prescribing clinician — do not stop or change medications without medical guidance.
Last reviewed: by iMedic Medical Editorial Team. Our editorial process.