Cotrimoxazole and Warfarin: Drug Interaction Guide
Quick answer: This is a major interaction — avoid combining Cotrimoxazole and Warfarin unless specifically directed by your doctor.
How Cotrimoxazole interacts with Warfarin
Sulfamethoxazole inhibits CYP2C9 and can reduce vitamin K availability through alteration of gut flora, markedly increasing warfarin effect.
What you should do
Avoid if a suitable antibiotic is available. If used, reduce warfarin as needed and check INR early and repeatedly during treatment.
Key facts
Severity
Major — avoid combination
Mechanism
Sulfamethoxazole inhibits CYP2C9 and can reduce vitamin K availability through alteration of gut flora, markedly increas...
Action
Avoid if a suitable antibiotic is available. If used, reduce warfarin as needed and check INR early and repeatedly durin...
Frequently asked questions
Can I take Cotrimoxazole and Warfarin together?
No — this combination should generally be avoided. Avoid if a suitable antibiotic is available. If used, reduce warfarin as needed and check INR early and repeatedly during treatment.
What is the severity of this interaction?
The interaction between Cotrimoxazole and Warfarin is classified as Major — avoid combination. Sulfamethoxazole inhibits CYP2C9 and can reduce vitamin K availability through alteration of gut flora, markedly increasing warfarin effect.
What should I do if I'm prescribed both?
Avoid if a suitable antibiotic is available. If used, reduce warfarin as needed and check INR early and repeatedly during treatment. Always consult your prescribing clinician — do not stop or change medications without medical guidance.
Last reviewed: by iMedic Medical Editorial Team. Our editorial process.