Drug interactions
Fosphenytoin + Ketoconazole
Severity: Moderate
Fosphenytoin and Ketoconazole: 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
Ketoconazole may increase phenytoin concentrations, though evidence is limited. Phenytoin can also lower ketoconazole levels through enzyme induction, risking antifungal failure.
What this interaction means in plain words
Fosphenytoin and Ketoconazole 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; Ketoconazole belongs to Anti Fungals.
Why it happens
Possible inhibition of CYP2C9/2C19 metabolism of phenytoin by ketoconazole, plus phenytoin induction of CYP3A4 reducing ketoconazole exposure.
What to do
Monitor phenytoin serum levels and for toxicity such as nystagmus and ataxia. Monitor antifungal response and consider an alternative antifungal.
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 Ketoconazole
- Alfuzosin + Ketoconazole Contraindicated
- Alprazolam + Ketoconazole Contraindicated
- Avanafil + Ketoconazole Contraindicated
- Conivaptan + Ketoconazole Contraindicated
- Dihydroergotamine intranasal + Ketoconazole Contraindicated
- Dihydroergotamine + Ketoconazole Contraindicated
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 Ketoconazole?
MediHelp rates the interaction as moderate. Fosphenytoin and Ketoconazole 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. Ketoconazole may increase phenytoin concentrations, though evidence is limited. Phenytoin can also lower ketoconazole levels through enzyme induction, risking antifungal failure.
How serious is the interaction between Fosphenytoin and Ketoconazole?
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 levels and for toxicity such as nystagmus and ataxia. Monitor antifungal response and consider an alternative antifungal.
What other medicines interact with Ketoconazole?
MediHelp also lists interactions of Ketoconazole with, for example, Alfuzosin, Alprazolam, Avanafil. The full list is on the Ketoconazole page.
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