Trimethoprim and Warfarin Interaction
Drug interaction information between Trimethoprim and Warfarin.
Trimethoprim and Warfarin have a documented minor interaction in FDA labeling.
FDA drug labeling documents a minor-severity interaction between Trimethoprim and Warfarin. This page displays the available source-record category and label text. The category is not an individual risk estimate or medication-combination direction.
How They Interact
Trimethoprim slows down how your body processes warfarin, which can make your blood thinner and increase your risk of bleeding.
FDA Label Information
It has been reported that sulfamethoxazole and trimethoprim may prolong the prothrombin time in patients who are receiving the anticoagulant warfarin (a CYP2C9 substrate).
Trimethoprim Also Interacts With
- Theophylline major
- Prenatal Multivitamin moderate
- Spironolactone moderate
- Spironolactone (Acne) moderate
- Metformin minor
Warfarin Also Interacts With
- Fluoxetine major
- Tamoxifen major
- Ibuprofen moderate
- Aspirin moderate
- Diclofenac moderate
Frequently Asked Questions
What SPL severity band is on file for Trimethoprim + Warfarin?
Compiled FDA labeling labels this pair minor. That is a source-record field, not an individual medication-combination decision.
Which drug in this pair has the larger labeled-interaction portfolio?
Warfarin has 163 labeled pairs on file vs Trimethoprim's 22. In the minor band specifically: Trimethoprim has 18 (also Metformin, Glipizide, Methotrexate, Pioglitazone); Warfarin has 133 (also Atorvastatin, Amlodipine, Omeprazole, Famotidine).
What mechanism field is on file for Trimethoprim and Warfarin?
Trimethoprim slows down how your body processes warfarin, which can make your blood thinner and increase your risk of bleeding.
Warfarin carries more major-labeled pairs than Trimethoprim
SPL source-record counts only. This page's band is minor · classes on file: Trimethoprim (Dihydrofolate Reductase Inhibitor); Warfarin (Vitamin K Antagonist (Anticoagulant)).
Warfarin has 2 major-labeled pairs vs Trimethoprim's 1. This page itself is in the minor band, not the major band.
1 major · 3 moderate · 18 minor · 22 total
- Other minor peers for Trimethoprim
- Metformin
- Glipizide
- Methotrexate
- Pioglitazone
2 major · 28 moderate · 133 minor · 163 total
- Other minor peers for Warfarin
- Atorvastatin
- Amlodipine
- Omeprazole
- Famotidine
Sources: FDA Drug Labels (SPL) via openFDA (2026). This is for informational purposes only and is not medical advice. This registry does not provide individual medication-combination directions.
Read our methodology - how this data is sourced, computed, and verified.