Drug interactions
Phenytoin + Valproic acid
Severity: Major
Phenytoin and Valproic acid: major 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
Valproate displaces phenytoin from albumin and may inhibit its metabolism, so free phenytoin rises while total levels may fall or appear normal. Phenytoin also lowers valproate levels through induction.
What this interaction means in plain words
Phenytoin and Valproic acid 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 belongs to Central Nervous System; Valproic acid belongs to Central Nervous System.
Why it happens
Valproate displaces phenytoin from protein binding and inhibits CYP2C9 metabolism, while phenytoin induces valproate metabolism.
What to do
Measure free phenytoin rather than total, and monitor for toxicity such as nystagmus or ataxia. Check valproate levels and adjust both doses as needed.
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?
Pro+ members
Unlock the full page
Create an account and start a Pro+ trial to unlock the full management advice for Phenytoin and Valproic acid, the additional notes, the evidence detail and the whole-prescription interaction checker.
- Full clinical management advice
- Additional notes
- Evidence detail
- Interaction checker for a whole prescription
Try Pro+ free for 7 days on the web. No card needed.
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
Other interactions of Valproic acid
- Ertapenem + Valproic acid Contraindicated
- Imipenem / cilastatin / relebactam + Valproic acid Contraindicated
- Imipenem / cilastatin + Valproic acid Contraindicated
- Meropenem / vaborbactam + Valproic acid Contraindicated
- Meropenem + Valproic acid Contraindicated
- Calcium / magnesium / potassium / sodium oxybates + Valproic acid Major
Frequently asked questions
Can I take Phenytoin with Valproic acid?
MediHelp rates the interaction as major. Phenytoin and Valproic acid 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. Valproate displaces phenytoin from albumin and may inhibit its metabolism, so free phenytoin rises while total levels may fall or appear normal. Phenytoin also lowers valproate levels through induction.
How serious is the interaction between Phenytoin and Valproic acid?
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 free phenytoin rather than total, and monitor for toxicity such as nystagmus or ataxia. Check valproate levels and adjust both doses as needed.
What other medicines interact with Phenytoin?
MediHelp also lists interactions of Phenytoin with, for example, Artemether / lumefantrine, Bictegravir, Cabotegravir. The full list is on the Phenytoin page.
Full monographs, dosing and an interaction checker in the MediHelp app
Open the complete reference, check multi-drug interactions and use dosing tools in the MediHelp app.
