Drug interactions
Phenytoin + Sulfisoxazole
Severity: Moderate
Phenytoin and Sulfisoxazole: 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
Sulfisoxazole may increase phenytoin exposure and cause signs of phenytoin toxicity such as nystagmus, ataxia and drowsiness. The narrow therapeutic index of phenytoin makes this clinically relevant.
What this interaction means in plain words
Phenytoin and Sulfisoxazole 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 metabolism of phenytoin and may displace it from protein binding.
What to do
Check phenytoin levels and clinical signs of toxicity during and after sulfisoxazole therapy. Reduce the phenytoin dose if concentrations increase.
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 Sulfisoxazole?
MediHelp rates the interaction as moderate. Phenytoin and Sulfisoxazole 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. Sulfisoxazole may increase phenytoin exposure and cause signs of phenytoin toxicity such as nystagmus, ataxia and drowsiness. The narrow therapeutic index of phenytoin makes this clinically relevant.
How serious is the interaction between Phenytoin and Sulfisoxazole?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Check phenytoin levels and clinical signs of toxicity during and after sulfisoxazole therapy. Reduce the phenytoin dose if concentrations increase.
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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