Drug interactions
Phenytoin + Sulfamethoxazole
Severity: Moderate
Phenytoin and Sulfamethoxazole: 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
Sulfamethoxazole can raise phenytoin concentrations and cause toxicity such as nystagmus, ataxia and confusion. Case reports and pharmacokinetic data support a clinically relevant effect.
What this interaction means in plain words
Phenytoin and Sulfamethoxazole 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
Sulfamethoxazole inhibits CYP2C9, reducing phenytoin metabolism.
What to do
Monitor phenytoin levels and for signs of toxicity during co-therapy. Reduce the phenytoin dose if needed, or use another antibiotic.
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 Sulfamethoxazole?
MediHelp rates the interaction as moderate. Phenytoin and Sulfamethoxazole 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. Sulfamethoxazole can raise phenytoin concentrations and cause toxicity such as nystagmus, ataxia and confusion. Case reports and pharmacokinetic data support a clinically relevant effect.
How serious is the interaction between Phenytoin and Sulfamethoxazole?
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 levels and for signs of toxicity during co-therapy. Reduce the phenytoin dose if needed, or use another antibiotic.
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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