Trimethoprim has 17 known drug interactions based on U.S. FDA drug labeling data. Of these, 2 are contraindicated combinations that should be avoided entirely. 3 are classified as major interactions requiring close medical supervision. Notable interactions include combinations with Dofetilide, Sepiapterin, Carospir. Patients taking Trimethoprim should inform their healthcare provider of all current medications — including over-the-counter drugs and supplements — to avoid potentially harmful combinations. Data sourced from OpenFDA and the NIH National Library of Medicine.
- Total
- 17
- Contraindicated
- 2
- Major
- 3
- Moderate
- 10
- Minor
- 2
Contraindicated (2)
- Trimethoprim + Dofetilide— Trimethoprim 160 mg BID increases dofetilide AUC by 103% and Cmax by 93%. Concomitant use is contraindicated.
- Trimethoprim + Sepiapterin— DHFR inhibitor that may reduce sepiapterin metabolism to tetrahydrobiopterin (BH4). Avoid concomitant use.
Major (3)
- Trimethoprim + Carospir— Trimethoprim may increase serum potassium, leading to severe hyperkalemia when used with CAROSPIR.
- Trimethoprim + Folic Acid— Antifolate agent that directly inhibits dihydrofolate reductase, decreasing formation of active folate derivatives.
- Trimethoprim + Spironolactone— Trimethoprim can increase serum potassium; concomitant use with spironolactone may lead to severe hyperkalemia.
Moderate (10)
- Trimethoprim + Cyclosporine— May potentiate renal dysfunction when used with sulfamethoxazole and cyclosporine. Close monitoring required.
- Trimethoprim + Dapsone— Mutual interaction where Dapsone and trimethoprim each raise the level of the other approximately 1.5 times; interaction…
- Trimethoprim + Digoxin— Increases digoxin serum concentration by 22-28%. Requires monitoring and dose adjustment.
- Trimethoprim + Lamivudine Oral— Potential interaction possible due to similar renal elimination pathways via organic cationic transport system. Monitor …
- Trimethoprim + Metformin Hydrochloride— Cationic drug eliminated by renal tubular secretion with potential for interaction with metformin by competing for commo…
- Trimethoprim + Phenytoin— Trimethoprim inhibits hepatic metabolism of phenytoin, increasing phenytoin half-life by 51% and decreasing metabolic cl…
- Trimethoprim + Phenytoin Sodium— May increase phenytoin serum levels; monitoring of phenytoin levels recommended when interaction is suspected.
- Trimethoprim + Prenatal Multivitamin Tablet And Combination Omega-3 Softgel/Mineral Capsule— May decrease plasma folate levels.
- Trimethoprim + Repaglinide— CYP2C8 inhibitor. Repaglinide dose reductions and increased glucose monitoring may be required.
- Trimethoprim + Sapropterin Dihydrochloride— May decrease endogenous BH4 levels by inhibiting dihydrofolate reductase, potentially increasing phenylalanine levels.
Minor (2)
- Trimethoprim + Terazosin— Terazosin has been used concomitantly with trimethoprim in at least 50 patients with no interactions observed.
- Trimethoprim + Terazosin Hydrochloride— Used concomitantly with terazosin in at least 50 patients with no interactions observed.