Drug interactions
Fosphenytoin + Voriconazole
Severity: Major
Fosphenytoin and Voriconazole: major interaction.
Last updated Sources: US FDA label
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, the active form of fosphenytoin, induces voriconazole metabolism and can reduce voriconazole concentrations enough to cause antifungal failure. Voriconazole may also raise phenytoin levels, leading to phenytoin toxicity.
What this interaction means in plain words
Fosphenytoin and Voriconazole can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Fosphenytoin belongs to Central Nervous System; Voriconazole belongs to Anti Fungals.
Why it happens
Phenytoin induces CYP2C19, CYP2C9 and CYP3A4, lowering voriconazole exposure, while voriconazole inhibits CYP2C9 and CYP2C19, which clear phenytoin.
What to do
Avoid unless benefit outweighs risk. If coadministered, increase the voriconazole dose as labeled, monitor voriconazole troughs, and monitor phenytoin levels and toxicity signs.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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References
Other interactions of Voriconazole
- Avanafil + Voriconazole Contraindicated
- Butabarbital + Voriconazole Contraindicated
- Carbamazepine + Voriconazole Contraindicated
- Conivaptan + Voriconazole Contraindicated
- Dihydroergotamine intranasal + Voriconazole Contraindicated
- Dihydroergotamine + Voriconazole 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 Voriconazole?
MediHelp rates the interaction as major. Fosphenytoin and Voriconazole can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Phenytoin, the active form of fosphenytoin, induces voriconazole metabolism and can reduce voriconazole concentrations enough to cause antifungal failure. Voriconazole may also raise phenytoin levels, leading to phenytoin toxicity.
How serious is the interaction between Fosphenytoin and Voriconazole?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Avoid unless benefit outweighs risk. If coadministered, increase the voriconazole dose as labeled, monitor voriconazole troughs, and monitor phenytoin levels and toxicity signs.
What other medicines interact with Voriconazole?
MediHelp also lists interactions of Voriconazole with, for example, Avanafil, Butabarbital, Carbamazepine. The full list is on the Voriconazole page.
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