Drug interactions
Trimethoprim + Warfarin
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.
Overview
Trimethoprim can increase warfarin’s anticoagulant effect, raising INR and bleeding risk; effect typically emerges within several days and is generally less pronounced than with trimethoprim-sulfamethoxazole.
Mechanism of action
Likely pharmacokinetic inhibition of warfarin metabolism (CYP2C9 involvement) and potential reduction of vitamin K–producing gut flora; magnitude variable.
Clinical management
Prefer an alternative antibiotic when feasible. If used together, check INR within 3–5 days of initiation and again after discontinuation; adjust warfarin dose based on INR. Counsel patients to report bleeding or bruising.
Drug reference
References
- Warfarin: Drug interactions — Lexicomp
- Antibiotic interactions with warfarin — UpToDate
Frequently asked questions
Can I take Trimethoprim with Warfarin?
The interaction is rated: Moderate. Trimethoprim can increase warfarin’s anticoagulant effect, raising INR and bleeding risk; effect typically emerges within several days and is generally less pronounced than with trimethoprim-sulfamethoxazole.
How serious is the interaction between Trimethoprim and Warfarin?
The interaction is rated: Moderate. Evidence level: Moderate.
How is the interaction between Trimethoprim and Warfarin managed?
Prefer an alternative antibiotic when feasible. If used together, check INR within 3–5 days of initiation and again after discontinuation; adjust warfarin dose based on INR. Counsel patients to report bleeding or bruising.
Why do Trimethoprim and Warfarin interact?
Likely pharmacokinetic inhibition of warfarin metabolism (CYP2C9 involvement) and potential reduction of vitamin K–producing gut flora; magnitude variable.
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