EpilepsyRx

Ethosuximide

Zarontin

Seizure aggravation / coverage
Treats absence seizures; does not provide broad protection against generalized tonic-clonic seizures.

DailyMed-verified antiseizure indications

Indication / use

Label / evidence summary
Zarontin and ethosuximide oral solution: control of absence (petit mal) epilepsy.
Role
Maintenance
Class
Thalamic T-type calcium-current reduction: a proposed antiabsence mechanism

Indications checked 26 September 2026. Product, route, age, and treatment-role limits apply. A DailyMed listing alone does not establish FDA approval.

Formulations & strengths

U.S. products unless another country is named. Strengths are per unit or stated volume.

Brand & formulation labels

Each link names the product and formulation it covers. Archived labels do not establish current availability.

Dosing & administration

Adult / product-specific schedule
Start 500 mg/day; increase by 250 mg every 4–7 days. Usual 1,000–1,500 mg/day; doses >1,500 mg/day require strict supervision.
Pediatric
Label initial doses: ages 3–6, 250 mg/day; age 6 and older, 500 mg/day (label age bands overlap at 6). Individualize; increase by 250 mg every 4–7 days. Optimal pediatric dose often 20 mg/kg/day; doses >1,500 mg/day require strict supervision.
Titration / repeat dosing
Age 3–6 years: start 250 mg daily; age ≥6 years: start 500 mg daily. Increase by 250 mg every 4–7 days to response; usual optimum about 20 mg/kg/day.

Dose adjustment & concentrations

Renal impairment
No explicit label algorithm; titrate cautiously and use concentrations/clinical response in significant impairment.
Hepatic impairment
No explicit label algorithm; monitor and titrate cautiously in significant impairment.
Serum reference information
40–100 mcg/mL

Safety

Boxed warning
No boxed warning.
Contraindications
Hypersensitivity to succinimides.
Serious precautions & monitoring
Useful when absence seizures occur without other generalized seizure types. Monitor for blood dyscrasias, hepatic/renal effects, and serious skin reactions.

Common / selected adverse effects

  • Gastrointestinal symptoms
  • Drowsiness/lethargy
  • Headache
  • Dizziness
  • Hiccups
  • Euphoria

Drug interactions: toxicity & clinical action

Partner / combinationClinical effectClinical action
ValproateEthosuximide concentrations may increase or decrease.Check response, GI/CNS toxicity and levels when relevant after changes.
PhenytoinEthosuximide may raise phenytoin concentrations.Monitor phenytoin for dose-related toxicity.

Pregnancy & contraception

Fetal / neonatal risk
Ethosuximide crosses the placenta. Birth defects have been reported, but the available reports do not establish causation or a reliable drug-specific risk estimate.

Clinical evidence

Research summaries describe studied populations and outcomes; they do not add indications or constitute treatment recommendations.

Direct pivotal trial

Trial summary

Study design / duration
Original approval predates modern Phase 3 standards. The later double-blind Childhood Absence Epilepsy comparative trial randomized 453 children to ethosuximide, valproate, or lamotrigine with response-driven titration.
Primary endpoint
No single pivotal endpoint is identified in the cited source.
Results
Marketed before modern Phase 3 standards. In the later randomized Childhood Absence Epilepsy trial, freedom from treatment failure at 16–20 weeks was 53% with ethosuximide, 58% with valproate, and 29% with lamotrigine; this was comparative effectiveness evidence, not the original registration program.

Trial publications

Comparative evidence

Findings
Childhood absence randomized double-blind trial: freedom from treatment failure at 16–20 weeks was 53% ethosuximide, 58% valproate and 29% lamotrigine; this composite is not a seizure-freedom rate.

Mechanism of action

  • Classic recordings in isolated thalamic neurons show reduced low-threshold T-type calcium current, offering an explanation for suppression of thalamocortical burst activity and spike-wave seizures.
  • This is an experimental mechanistic model, not an exclusive proven human target: results differ across preparations, and other ionic-current effects have been reported.

Pharmacology

Adult values unless specified. Vd/F denotes apparent oral distribution volume.

Pharmacokinetics

Bioavailability
Practically complete oral absorption.
Elimination half-life
Adults: 38.3–66.6 hours in the cited single-dose study; children: approximately 25–42 hours across capsule/solution studies.
Volume of distribution
Approximately 0.7 L/kg (apparent).
Active metabolite(s)
No established active metabolite; hydroxylated metabolites are described as probably inactive in the UK SmPC.
Active-metabolite half-life
Not applicable — no clinically established active metabolite.
Protein binding
Not appreciably bound to plasma proteins (UK SmPC).
Metabolism / elimination
Extensive hepatic oxidation, primarily CYP3A4

The ranges come from small studies and vary between individuals.

Molecular structure

Molecular structure of Ethosuximide
Principal compound; salt and product forms may differ.
NIH PubChem structure and record ↗

Product history

Initial U.S. approval
1960
Brands
Zarontin
Manufacturer / marketer
Pfizer (Zarontin)
Generic availability
Yes
Related drugs
Succinimide ASM; historically related to methsuximide.

Sources & review status

Representative sources are selected product labels, not an exhaustive list of generic manufacturers. Consult the exact product and formulation prescribed. Brand-name package inserts are linked separately.

Brand reference: Zarontin capsules ↗

Indications and DailyMed PDFs checked: 2026-09-26. Other profile sources reviewed: 2026-09-15 · label revision noted at that review: 5/2026. This is a draft reference; final review is pending.

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