Drug interactions
Fosphenytoin + Zafirlukast
Severity: Moderate
Fosphenytoin 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
Fosphenytoin is converted to phenytoin, which is cleared mainly by CYP2C9, and zafirlukast inhibits this enzyme. Phenytoin has a narrow therapeutic index and nonlinear kinetics, so a modest decrease in clearance may produce toxicity.
What this interaction means in plain words
Fosphenytoin 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.
Fosphenytoin belongs to Central Nervous System; Zafirlukast belongs to Respiratory.
Why it happens
Inhibition of CYP2C9 by zafirlukast reduces phenytoin metabolism.
What to do
Monitor phenytoin concentrations (free levels if albumin is low) and for nystagmus, ataxia or confusion when starting or stopping zafirlukast; reduce the fosphenytoin dose if needed.
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 Fosphenytoin
- Dronedarone + Fosphenytoin Contraindicated
- Fosphenytoin + Ledipasvir-sofosbuvir Contraindicated
- Fosphenytoin + Lorlatinib Contraindicated
- Fosphenytoin + Lurasidone Contraindicated
- Fosphenytoin + Ranolazine Contraindicated
- Fosphenytoin + Sofosbuvir-velpatasvir-voxilaprevir Contraindicated
Frequently asked questions
Can I take Fosphenytoin with Zafirlukast?
MediHelp rates the interaction as moderate. Fosphenytoin 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. Fosphenytoin is converted to phenytoin, which is cleared mainly by CYP2C9, and zafirlukast inhibits this enzyme. Phenytoin has a narrow therapeutic index and nonlinear kinetics, so a modest decrease in clearance may produce toxicity.
How serious is the interaction between Fosphenytoin 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 (free levels if albumin is low) and for nystagmus, ataxia or confusion when starting or stopping zafirlukast; reduce the fosphenytoin dose if needed.
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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