Drug interactions
Fluvoxamine + Phenytoin
Severity: Major
Fluvoxamine and Phenytoin: 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
Fluvoxamine inhibits CYP2C9 and CYP2C19, which clear phenytoin, so phenytoin levels may rise to toxic concentrations. Because phenytoin has narrow therapeutic index and saturable kinetics, small changes may matter.
What this interaction means in plain words
Fluvoxamine and Phenytoin 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.
Fluvoxamine belongs to Central Nervous System; Phenytoin belongs to Central Nervous System.
Why it happens
Inhibition of CYP2C9 and CYP2C19 by fluvoxamine reduces phenytoin metabolism.
What to do
Measure phenytoin levels when starting or stopping fluvoxamine and reduce the dose as needed. Watch for nystagmus, ataxia and confusion.
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 Fluvoxamine
- Agomelatine + Fluvoxamine Contraindicated
- Alosetron + Fluvoxamine Contraindicated
- Fezolinetant + Fluvoxamine Contraindicated
- Fluvoxamine + Isocarboxazid Contraindicated
- Fluvoxamine + Mavacamten Contraindicated
- Fluvoxamine + Methylene blue Contraindicated
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 Fluvoxamine with Phenytoin?
MediHelp rates the interaction as major. Fluvoxamine and Phenytoin 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. Fluvoxamine inhibits CYP2C9 and CYP2C19, which clear phenytoin, so phenytoin levels may rise to toxic concentrations. Because phenytoin has narrow therapeutic index and saturable kinetics, small changes may matter.
How serious is the interaction between Fluvoxamine and Phenytoin?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Measure phenytoin levels when starting or stopping fluvoxamine and reduce the dose as needed. Watch for nystagmus, ataxia and confusion.
What other medicines interact with Fluvoxamine?
MediHelp also lists interactions of Fluvoxamine with, for example, Agomelatine, Alosetron, Fezolinetant. The full list is on the Fluvoxamine page.
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