Does Phenytoin Interact With Sulfamethoxazole Trimethoprim?

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…

Severity: Moderate

Severity basis: drug interactions section (keywords: may increase, may decrease). Combining Phenytoin (Anticonvulsant) and Sulfamethoxazole Trimethoprim (Sulfonamide antibiotic) is classified as a moderate interaction.

What to Do

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

From the FDA Label

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).

Phenytoin

Anticonvulsant

Brands: Dilantin

View Phenytoin safety profile

Sulfamethoxazole Trimethoprim

Sulfonamide antibiotic

Brands: Bactrim, Septra

View Sulfamethoxazole Trimethoprim safety profile

Check these against your full medication list in our free Interaction Checker

This information is educational — not medical advice.

This page is provided for general educational purposes and summarizes publicly available data from sources such as the U.S. Food & Drug Administration. It is not a substitute for the judgment of a licensed clinician and should not be used to start, stop, or change any medication. It may be incomplete or out of date, and individual circumstances vary. Always talk with your prescriber or pharmacist about your specific medications and health conditions. If you think you may have a medical emergency, call 911.

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Other Phenytoin Interactions

Other Sulfamethoxazole Trimethoprim Interactions

Sulfamethoxazole Trimethoprim + Azathioprine(minor)