Does Phenytoin Interact With Sulfamethoxazole Trimethoprim?

Use this combination with caution. Your prescriber may monitor you more closely or adjust doses.

Interactions like this one can happen when two medicines have overlapping effects, or when one changes how the other is absorbed, broken down, or cleared from the body. Phenytoin is a anticonvulsant and Sulfamethoxazole Trimethoprim is a sulfonamide antibiotic; together they are documented as a moderate interaction. How much this matters depends on your dose, your other medications, and your overall health.

If you have been prescribed both Phenytoin and Sulfamethoxazole Trimethoprim, do not start, stop, or change either medication on your own — some medicines are unsafe to stop suddenly. Tell your prescriber and pharmacist that you take both so they can decide whether the combination is right for you, adjust the dose, monitor you more closely, or suggest an alternative.

Table 1: Drugs That Affect Phenytoin Concentrations Interacting Agent Examples Drugs that may increase phenytoin serum levels Antiepileptic drugs Ethosuximide, felbamate, oxcarbazepine, methsuximide, topiramate Azoles Fluconazole, ketoconazole, itraconazole, miconazole, voriconazole Antineoplastic agents Capecitabine, fluorouracil Antidepressants Fluoxetine, fluvoxamine, sertraline Gastric acid reducing agents H 2 antagonists (cimetidine), omeprazole Sulfonamides Sulfamethizole, sulfaphenazol…

Source: FDA structured product labeling (set_id 035a8d4e-2063-4240-83cb-d7eebcabe301).