Drug interactions
Fosphenytoin + Metronidazole
Severity: Moderate
Fosphenytoin and Metronidazole: 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
Metronidazole can raise phenytoin concentrations, as fosphenytoin is converted to phenytoin, and toxicity such as nystagmus, ataxia and sedation has been reported. Because phenytoin has a narrow therapeutic index and saturable kinetics, even a modest rise in exposure can be clinically significant.
What this interaction means in plain words
Fosphenytoin and Metronidazole 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; Metronidazole belongs to Anti Parasitic.
Why it happens
Metronidazole appears to inhibit phenytoin metabolism, probably via CYP2C9.
What to do
Monitor total or free phenytoin levels and signs of toxicity when metronidazole is started or stopped. Reduce the phenytoin dose if levels rise.
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 Metronidazole
- Ethanol + Metronidazole Contraindicated
- Metronidazole + Pimozide Contraindicated
- Amiodarone + Metronidazole Major
- Disulfiram + Metronidazole Major
- Ergotamine + Metronidazole Major
- Lomitapide + Metronidazole Major
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 Metronidazole?
MediHelp rates the interaction as moderate. Fosphenytoin and Metronidazole 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. Metronidazole can raise phenytoin concentrations, as fosphenytoin is converted to phenytoin, and toxicity such as nystagmus, ataxia and sedation has been reported. Because phenytoin has a narrow therapeutic index and saturable kinetics, even a modest rise in exposure can be clinically significant.
How serious is the interaction between Fosphenytoin and Metronidazole?
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 total or free phenytoin levels and signs of toxicity when metronidazole is started or stopped. Reduce the phenytoin dose if levels rise.
What other medicines interact with Metronidazole?
MediHelp also lists interactions of Metronidazole with, for example, Ethanol, Pimozide, Amiodarone. The full list is on the Metronidazole page.
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