Drug Interaction Report

Methsuximide and Fosphenytoin: Interaction Details

A pharmacist-reviewed analysis of the Methsuximide and Fosphenytoin interaction: the documented clinical effects, severity rating, onset, mechanism, and management considerations, with cited references throughout.

AI-assisted, pharmacist-reviewed · Source data updated Aug 8, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature

Fosphenytoin

Cerebyx
+

Methsuximide

Celontin Celontin®
Dr. Brian Staiger, PharmD, BCPS
Medically reviewed by
Dr. Brian Staiger, PharmD, BCPS
Updated Aug 8, 2026
Editorial policy · Data sources
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Interaction severity
Major
Potentially serious — often needs a change or close monitoring.
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.

Of 12 documented Methsuximide interactions, 6 are rated major — including this one.
Worried about symptoms right now? Contact your pharmacist or prescriber, or call Poison Control at 1-800-222-1222 (US). Call 911 for an emergency.
Onset
delayed
Evidence
probable
Severity
Major

What happens

Increased phenytoin or fosphenytoin (prodrug of phenytoin) exposure and an increased risk of phenytoin or fosphenytoin toxicity (ataxia, hyperreflexia, nystagmus, tremor)

Interaction Deep Dive

Concomitant use of phenytoin or fosphenytoin (prodrug of phenytoin) with methsuximide may result in increased phenytoin or fosphenytoin exposure13. Concomitant methsuximide therapy with other anticonvulsants such as PHENobarbital, primidone, and phenytoin has resulted in significant elevations in plasma concentrations of the second anticonvulsant and subsequent side effects 5. Consider phenytoin or fosphenytoin dose adjustments to achieve optimal clinical response when methsuximide is added to or withdrawn from a patient's regimen. Monitor phenytoin or fosphenytoin serum drug level during concurrent use of methsuximide and phenytoin or fosphenytoin 13.

Why it happens (mechanism)

Inhibition of phenytoin metabolism

Literature reports

2 reports — tap to read

a) Increased plasma concentrations of phenytoin and PHENobarbital after the addition of methsuximide is presumably due to competitive inhibition of hydroxylation of the phenyl ring, the principal route of metabolism of all three drugs 4.

b) Concomitant methsuximide therapy with other anticonvulsants such as PHENobarbital, primidone, and phenytoin has resulted in significant elevations in plasma concentrations of the second anticonvulsant and subsequent side effects 5.

Common questions

Can I take Methsuximide and Fosphenytoin together?

Increased phenytoin or fosphenytoin (prodrug of phenytoin) exposure and an increased risk of phenytoin or fosphenytoin toxicity (ataxia, hyperreflexia, nystagmus, tremor) Always confirm with your pharmacist or prescriber before making any change.

How serious is the Methsuximide and Fosphenytoin interaction?

It is rated major. Potentially serious — often needs a change or close monitoring.

How quickly could this interaction happen?

The documented onset is "delayed". Effects tend to build up gradually over days to weeks.

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 Methsuximide or Fosphenytoin 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)

  1. Product Information: FOSPHENYTOIN SODIUM intravenous and intramuscular injection, fosphenytoin sodium intravenous and intramuscular injection. SAGENT Pharmaceuticals (per Dailymed), Schaumburg, IL, 2024. DailyMed
  2. Product Information: PHENYTOIN SODIUM intravenous and intramuscular injection, phenytoin sodium intravenous and intramuscular injection. Acella Pharmaceuticals, LLC (per Dailymed), Alpharetta, GA, 2024. DailyMed
  3. Product Information: PHENYTOIN SODIUM oral extended-release capsules, phenytoin sodium oral extended-release capsules. Sun Pharmaceutical Industries, Inc. (per Dailymed), Cranbury, NJ, 2023. DailyMed
  4. Browne TR, Feldman RG, Buchanan RA, et al: Methsuximide for complex partial seizures: efficacy, toxicity, clinical pharmacology, and drug interactions. Neurology 1983; 33:414-418.
  5. Rambeck B: Pharmacological interactions of methsuximide with phenobarbital and phenytoin in hospitalized epileptic patients. Epilepsia 1979; 20:147-156.
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Fosphenytoin

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Beyond drug–drug

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:

Fosphenytoin + 1,4-butanediol American Hellebore Arsenic Beer Cesium Ephedra Gamma-hydroxybutyrate (ghb) Grapefruit + 197 more
Methsuximide + 1,4-butanediol Activated Charcoal Beer Gamma-hydroxybutyrate (ghb) Kava L-tryptophan Wine 5-htp + 97 more

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This information is for education, not a substitute for professional medical advice. Do not start, stop, or change any medication without talking to your pharmacist or prescriber.