Drug interactions
Metronidazole + Phenytoin
Severity: Moderate
Metronidazole and Phenytoin: 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 serum phenytoin concentrations and has been linked to phenytoin toxicity with nystagmus, ataxia and drowsiness. Because phenytoin has a narrow therapeutic index with saturable metabolism, small changes in clearance can matter.
What this interaction means in plain words
Metronidazole and Phenytoin 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 belongs to Anti Parasitic; Phenytoin belongs to Central Nervous System.
Why it happens
Metronidazole inhibits CYP2C9-mediated hydroxylation of phenytoin, decreasing its clearance.
What to do
Measure phenytoin levels (free levels if albumin is low) within several days of starting metronidazole and reduce the phenytoin dose as needed. Recheck after metronidazole is stopped.
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 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 Metronidazole with Phenytoin?
MediHelp rates the interaction as moderate. Metronidazole and Phenytoin 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 serum phenytoin concentrations and has been linked to phenytoin toxicity with nystagmus, ataxia and drowsiness. Because phenytoin has a narrow therapeutic index with saturable metabolism, small changes in clearance can matter.
How serious is the interaction between Metronidazole and Phenytoin?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Measure phenytoin levels (free levels if albumin is low) within several days of starting metronidazole and reduce the phenytoin dose as needed. Recheck after metronidazole is stopped.
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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