Drug interactions
Phenytoin + Sulfadiazine
Severity: Moderate
Phenytoin and Sulfadiazine: moderate interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Sulfadiazine can raise phenytoin concentrations, which may lead to phenytoin toxicity such as nystagmus, ataxia and confusion. Because phenytoin has a narrow therapeutic index and saturable metabolism, small changes in clearance can matter.
What this interaction means in plain words
Phenytoin and Sulfadiazine can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Why it happens
Sulfonamides inhibit CYP2C9-mediated hepatic metabolism of phenytoin.
What to do
Monitor phenytoin serum concentrations (free levels if albumin is low) and for signs of toxicity when starting or stopping sulfadiazine. Reduce the phenytoin dose if levels rise.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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Other interactions of Phenytoin
- Artemether / lumefantrine + Phenytoin Contraindicated
- Bictegravir + Phenytoin Contraindicated
- Cabotegravir + Phenytoin Contraindicated
- Doravirine + Phenytoin Contraindicated
- Elbasvir / grazoprevir + Phenytoin Contraindicated
- Elvitegravir / cobicistat / emtricitabine / tenofovir df + Phenytoin Contraindicated
Frequently asked questions
Can I take Phenytoin with Sulfadiazine?
MediHelp rates the interaction as moderate. Phenytoin and Sulfadiazine can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Sulfadiazine can raise phenytoin concentrations, which may lead to phenytoin toxicity such as nystagmus, ataxia and confusion. Because phenytoin has a narrow therapeutic index and saturable metabolism, small changes in clearance can matter.
How serious is the interaction between Phenytoin and Sulfadiazine?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor phenytoin serum concentrations (free levels if albumin is low) and for signs of toxicity when starting or stopping sulfadiazine. Reduce the phenytoin dose if levels rise.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
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