MAJOR — Use With Caution
FDA-documented major interaction — one of the most clinically significant warfarin-drug interactions. INR can rise dramatically. Consider alternative antibiotics.
Evidence level: STRONG
Bactrim (trimethoprim-sulfamethoxazole) is one of the most dangerous antibiotics to combine with warfarin. It powerfully blocks the enzyme that clears warfarin from your body, potentially doubling or tripling your bleeding risk.
Sulfamethoxazole inhibits CYP2C9 (warfarin S-enantiomer metabolism). Trimethoprim may also contribute. This is one of the most clinically significant warfarin-antibiotic interactions. FDA warfarin label documents this.
If possible, use a different antibiotic. If Bactrim is necessary, your warfarin dose should be reduced and INR monitored very closely (within 3-5 days).
Monitor closely. Many antibiotics amplify anticoagulant effects by disrupting gut flora that produce vitamin K. Warfarin levels may spike during antibiotic courses. If on warfarin, request more frequent INR monitoring. Take each at their normal prescribed times.
Higher risk for: elderly patients, those with liver disease, history of GI bleeding, concurrent use of multiple blood thinners, recent surgery, vitamin K intake changes, alcohol use, or low body weight.
Unusual or heavy bruising, blood in urine, dark or bloody stools, nosebleeds, bleeding gums, excessive bleeding from minor cuts
Ask if an alternative antibiotic can treat your infection. If Bactrim is necessary, request INR check at 3-5 days and again at 7 days. Discuss preemptive warfarin dose reduction.
This medication combination requires careful medical oversight. Your doctor or anticoagulation clinic can adjust doses or select alternatives with fewer bleeding risks. Frequent INR monitoring is essential.
Bactrim (trimethoprim-sulfamethoxazole) is one of the most dangerous antibiotics to combine with warfarin. It powerfully blocks the enzyme that clears warfarin from your body, potentially doubling or tripling your bleeding risk.
Monitor closely. Many antibiotics amplify anticoagulant effects by disrupting gut flora that produce vitamin K. Warfarin levels may spike during antibiotic courses. If on warfarin, request more frequent INR monitoring. Take each at their normal prescribed times.
Unusual or heavy bruising, blood in urine, dark or bloody stools, nosebleeds, bleeding gums, excessive bleeding from minor cuts
This medication combination requires careful medical oversight. Your doctor or anticoagulation clinic can adjust doses or select alternatives with fewer bleeding risks. Frequent INR monitoring is essential.
Ask if an alternative antibiotic can treat your infection. If Bactrim is necessary, request INR check at 3-5 days and again at 7 days. Discuss preemptive warfarin dose reduction.
Or browse the full interaction database (121,000+ pairs).