Phenytoin and Ethosuximide: Interaction Details
AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Phenytoin
Ethosuximide
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
What happens
Reduced ethosuximide exposure and an increased phenytoin or fosphenytoin (prodrug of phenytoin) exposure and risk of toxicity
Interaction Deep Dive
Use caution with coadministration of ethosuximide and phenytoin or fosphenytoin (prodrug of phenytoin), as concurrent use may lead to reduced ethosuximide exposure1 or increased phenytoin or fosphenytoin exposure. Consider phenytoin or fosphenytoin and ethosuximide dose adjustments to achieve optimal clinical response when these drugs are added to or withdrawn from a patient's regimen. Monitor phenytoin or fosphenytoin serum drug level during concurrent use of ethosuximide and phenytoin or fosphenytoin 2345.
Why it happens (mechanism)
Induction of ethosuximide metabolism; unknown
Literature reports
1 report — tap to read
a) In a study of 22 volunteers, the effects of chronic epileptic medication on the pharmacokinetics of ethosuximide were evaluated. The study consisted of 10 epileptic patients undergoing chronic treatment with carBAMazepine, phenytoin, or PHENobarbital, and 12 healthy control subjects taking no chronic medications. Each subject received a single 500 mg oral dose of ethosuximide after an overnight fast. Patients on chronic epileptic therapy had a decreased mean ethosuximide t(1/2) of 29 +/- 7.8 hours compared with 53.7 +/- 14.3 hours for control subjects. Patients on chronic epileptic therapy also had higher oral clearance values and slightly decreased Vd values compared with control patients. The authors postulate that the mechanism of action was due to antiepileptic medication induction of CYP3A 1.
Common questions
Can I take Phenytoin and Ethosuximide together?
Reduced ethosuximide exposure and an increased phenytoin or fosphenytoin (prodrug of phenytoin) exposure and risk of toxicity Always confirm with your pharmacist or prescriber before making any change.
How serious is the Phenytoin and Ethosuximide interaction?
It is rated major. Potentially serious — often needs a change or close monitoring.
How quickly could this interaction happen?
The documented onset is "unspecified". The timing of this interaction is not well characterized.
How strong is the evidence for this interaction?
The evidence is graded "probable". Good supporting evidence, though not definitively proven.
Questions for your pharmacist
- Does my dose of Phenytoin or Ethosuximide need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there a safer alternative to one of these medications for me?
References (5)
- Giaccone M, Bartoli A, Gatti G, et al: Effect of enzyme inducing anticonvulsants on ethosuximide pharmacokinetics in epileptic patients. Br J Clin Pharmacol 1996; 41:575-579. DOI
- Product Information: Dilantin-125(R) oral suspension, phenytoin oral suspension. Parke-Davis (per FDA), New York, NY, 2013. DailyMed
- Product Information: FOSPHENYTOIN SODIUM intravenous and intramuscular injection, fosphenytoin sodium intravenous and intramuscular injection. SAGENT Pharmaceuticals (per Dailymed), Schaumburg, IL, 2024. DailyMed
- Product Information: PHENYTOIN SODIUM intravenous and intramuscular injection, phenytoin sodium intravenous and intramuscular injection. Acella Pharmaceuticals, LLC (per Dailymed), Alpharetta, GA, 2024. DailyMed
- Product Information: PHENYTOIN SODIUM oral extended-release capsules, phenytoin sodium oral extended-release capsules. Sun Pharmaceutical Industries, Inc. (per Dailymed), Cranbury, NJ, 2023. DailyMed
Keep reading about Phenytoin
Keep reading about Ethosuximide
These medications also interact with supplements
Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
major · moderate · minor — check everything you take with our drug–supplement interaction checker.
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