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Does Phenytoin Interact with Valproic Acid?

Phenytoin and Valproic Acid have a moderate drug interaction according to U.S. FDA drug labeling data. Enzyme-inducing drug that can increase valproate clearance and double clearance rate. Increased monitoring of valproate and concomitant drug concentrations and dosage adjustment indicated. The mechanism involves hepatic enzyme induction increasing glucuronosyltransferase levels. Patients taking both medications should consult their healthcare provider before starting, stopping, or changing the dosage of either drug. This information is based on official FDA drug labeling sourced from OpenFDA and the NIH National Library of Medicine.

Severity
Moderate
Phenytoin Class
Anti-epileptic Agent
Valproic Acid Class
Anti-epileptic Agent
Management
Monitor for effects
Data Source
U.S. FDA via OpenFDA

How This Interaction Works

The interaction between Phenytoin and Valproic Acid occurs because hepatic enzyme induction increasing glucuronosyltransferase levels. Clinically, this means enzyme-inducing drug that can increase valproate clearance and double clearance rate. increased monitoring of valproate and concomitant drug concentrations and dosage adjustment indicated. This is classified as a moderate interaction. While it may not require stopping either medication, your healthcare provider should be aware you are taking both so they can monitor for potential effects and adjust treatment if necessary.

What To Tell Your Doctor or Pharmacist

If you are taking Phenytoin and your doctor is considering prescribing Valproic Acid (or vice versa), make sure to:

  • Inform your doctor and pharmacist of all current medications, including over-the-counter drugs and supplements
  • Ask whether the benefits of combining these medications outweigh the risks for your specific situation
  • Ask what symptoms to watch for that would indicate the interaction is causing problems
  • Ask whether any dosage adjustments are needed
  • Never stop or change either medication without first consulting your healthcare provider
💊 Phenytoin(Phenytoin Infatabs)+💊 Valproic Acid(Valproic Acid)

Severity & Interaction Details

🟡
moderate
Be aware — moderate interaction
Documented interaction with manageable risk. May require dose adjustment or monitoring.
Severity scale
MinorContra
On record
Yes
Drug A class
Anti-epileptic Agent
Drug B class
Anti-epileptic Agent
Source
NLP:valproic acid

What this means in plain English

Enzyme-inducing drug that can increase valproate clearance and double clearance rate. Increased monitoring of valproate and concomitant drug concentrations and dosage adjustment indicated.

About Phenytoin and Valproic Acid

Phenytoin and Valproic Acid both belong to the Anti-epileptic Agent class. Both medications are taken by mouth. Valproic Acid additionally carries an FDA boxed warning, the FDA's strongest safety alert.

What the FDA label says

"Hepatic enzyme induction increasing glucuronosyltransferase levels"

📖 Source: NLP:valproic acid

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Data integrity: Every interaction shown is extracted directly from official U.S. FDA drug labeling via openFDA and the U.S. National Library of Medicine (NIH) — no interactions are invented, inferred, or manually edited. Maintained by CheckDrugInteractions.com. See our methodology or report a correction at support@checkdruginteractions.com.

Data sourced from U.S. FDA drug labeling via openFDA and the NIH National Library of Medicine, supplemented with a small number of well-established interactions from curated clinical references. For informational purposes only. Always consult your pharmacist or physician.