Sulfamethoxazole/Trimethoprim Drug Interactions: What to Avoid and What to Tell Your Doctor

Sulfamethoxazole/trimethoprim (TMP-SMX, Bactrim, Septra) has one of the largest drug interaction profiles of any common antibiotic — with 486 known drug interactions, including 65 classified as major. This doesn't mean it can't be prescribed alongside other medications, but it does mean your prescriber and pharmacist need to know everything you're taking before you start a course.

Why Does TMP-SMX Have So Many Drug Interactions?

The combination's broad interaction profile comes from multiple mechanisms. Sulfamethoxazole is a potent inhibitor of CYP2C9, one of the liver's major drug-metabolizing enzymes. Trimethoprim inhibits CYP2C8 and the OCT2 renal transporter, which is how many drugs are cleared by the kidneys. Together, they can raise blood levels of dozens of medications that go through these same metabolic and elimination pathways.

CRITICAL: Drugs That Should NOT Be Combined With TMP-SMX

Dofetilide (Tikosyn): CONTRAINDICATED. TMP-SMX inhibits renal elimination of dofetilide, causing dangerous QTc interval prolongation and risk of Torsades de Pointes (a potentially fatal arrhythmia). This combination is absolutely contraindicated — use an alternative antibiotic.

Leucovorin (folinic acid) — for PCP treatment only: Contraindicated when using TMP-SMX to treat PCP. Combining these two has been associated with increased treatment failure and excess mortality in PCP. Do not co-administer.

Major Drug Interactions: Use With Caution and Close Monitoring

Warfarin (Coumadin): TMP-SMX significantly increases warfarin's anticoagulant effect by inhibiting CYP2C9 (warfarin's primary metabolic route) and displacing it from protein binding. Expect INR to rise 50–100% within days. Monitor INR closely and reduce warfarin dose if co-administration is necessary.

Methotrexate: TMP-SMX reduces methotrexate's renal clearance by approximately 40% and also potentiates its anti-folate effect. This can cause methotrexate toxicity — bone marrow suppression, mucositis, and hepatotoxicity. The Canadian 3E Initiative formally recommends avoiding this combination.

ACE inhibitors (lisinopril, enalapril) and ARBs (losartan, valsartan): Combining TMP-SMX with renin-angiotensin blockers can cause clinically significant hyperkalemia (dangerously high blood potassium). Trimethoprim blocks renal potassium excretion like amiloride (a potassium-sparing diuretic). Check potassium levels, especially in elderly patients and those with kidney disease.

Potassium-sparing diuretics (spironolactone, amiloride, eplerenone): Same hyperkalemia risk as with ACE inhibitors/ARBs. Monitor potassium levels.

Sulfonylureas (glipizide, glyburide, glimepiride): TMP-SMX can potentiate the hypoglycemic effect of sulfonylureas, causing dangerously low blood sugar. Diabetic patients should monitor blood glucose carefully and may need dose adjustments.

Phenytoin (Dilantin): SMX inhibits CYP2C9, which metabolizes phenytoin. Blood levels of phenytoin may rise, increasing risk of phenytoin toxicity (ataxia, confusion, nystagmus). Monitor phenytoin levels.

Digoxin: TMP-SMX increases digoxin levels, particularly in elderly patients. Monitor serum digoxin levels when co-prescribing.

Cyclosporine (transplant immunosuppressant): TMP-SMX may reduce cyclosporine levels while also increasing nephrotoxicity risk. Transplant patients on cyclosporine need careful monitoring.

Amiodarone: Both drugs prolong the QTc interval. Combining them increases the risk of cardiac arrhythmia. Use an alternative antibiotic when possible.

Food and Supplement Interactions

PABA supplements: Taking PABA (para-aminobenzoic acid) supplements directly counteracts sulfamethoxazole's mechanism by competing for the same enzyme. Avoid PABA supplements while on TMP-SMX.

Potassium supplements: May worsen hyperkalemia when combined with TMP-SMX. Take only as directed by your doctor.

St. John's Wort: Can increase photosensitivity; avoid excessive sun exposure and concurrent use during TMP-SMX treatment.

What to Tell Your Doctor Before Starting TMP-SMX

Before your provider prescribes TMP-SMX, make sure they know about:

All prescription medications — especially blood thinners, diabetes drugs, heart medications, and immunosuppressants

Over-the-counter medications, vitamins, and supplements (especially potassium, PABA, and folate)

Any sulfa allergy or history of reactions to TMP-SMX

Kidney or liver disease

Diabetes — especially if you take sulfonylureas or insulin

Pregnancy or plans to become pregnant

Once your prescription is confirmed safe for you, use medfinder to find a pharmacy near you that has it in stock. For a full guide to side effects, see our TMP-SMX side effects guide.