Drug interactions
Phenytoin + Zafirlukast
Severity: Moderate
Phenytoin and Zafirlukast: 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
Phenytoin is metabolized mainly by CYP2C9 and has a narrow therapeutic index, so inhibition by zafirlukast may raise its levels. Toxicity may present as nystagmus, ataxia and confusion.
What this interaction means in plain words
Phenytoin and Zafirlukast 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.
Phenytoin belongs to Central Nervous System; Zafirlukast belongs to Respiratory.
Why it happens
Inhibition of CYP2C9-mediated phenytoin metabolism by zafirlukast.
What to do
Monitor phenytoin concentrations and for signs of toxicity when zafirlukast is started or stopped. Adjust the phenytoin dose accordingly.
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 Zafirlukast
- Pimozide + Zafirlukast Contraindicated
- Lomitapide + Zafirlukast Major
- Nilotinib + Zafirlukast Major
- Warfarin + Zafirlukast Major
- Apalutamide + Zafirlukast Moderate
- Carbamazepine + Zafirlukast Moderate
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 Zafirlukast?
MediHelp rates the interaction as moderate. Phenytoin and Zafirlukast 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. Phenytoin is metabolized mainly by CYP2C9 and has a narrow therapeutic index, so inhibition by zafirlukast may raise its levels. Toxicity may present as nystagmus, ataxia and confusion.
How serious is the interaction between Phenytoin and Zafirlukast?
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 concentrations and for signs of toxicity when zafirlukast is started or stopped. Adjust the phenytoin dose accordingly.
What other medicines interact with Zafirlukast?
MediHelp also lists interactions of Zafirlukast with, for example, Pimozide, Lomitapide, Nilotinib. The full list is on the Zafirlukast page.
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