Section 01 · Monograph

Ethosuximide

Reviewed Nov 2026FDA label

ZarontinT-type Ca2+ blocker

Indications

Reduces low-threshold (T-type) calcium currents in thalamocortical neurons
This action suppresses the thalamocortical 3 Hz spike-and-wave discharge characteristic of absence seizures, raising the seizure threshold specifically for this circuit

Increase by 250mg every 4-7 days as needed/tolerated until seizure control is achieved
Maximum recommended dose is generally 1.5g/day in divided doses; doses above this require close medical supervision
Not effective for and may worsen generalized tonic-clonic or other non-absence seizure types — do not use as monotherapy if mixed seizure types are present without an additional appropriate agent

Children 3-6 years: initiate at 250mg once daily
Children ≥6 years and adults: initiate at 500mg/day (typically as 250mg twice daily)
Target pediatric dosing is often described as approximately 20mg/kg/day, with a common therapeutic plasma range of roughly 40-100mcg/mL used to guide titration

May elevate serum phenytoin levels
Valproic acid has been reported to both increase and decrease ethosuximide levels — periodic serum level monitoring recommended when co-administered with other antiepileptic drugs
General CNS depressant additive effects with alcohol and other sedating agents

Common: gastrointestinal upset (nausea, vomiting, anorexia, abdominal pain), drowsiness, dizziness, headache, hiccups
Blood dyscrasias (rare but serious): leukopenia, agranulocytosis, pancytopenia, eosinophilia
Lupus-like syndrome, including cases of systemic lupus erythematosus
Serious dermatologic reactions including Stevens-Johnson syndrome (rare)
Neuropsychiatric effects including behavioral changes and suicidal ideation/behavior (class effect of ASMs)

Periodic complete blood counts, particularly if signs of infection, fever, sore throat, or bruising/bleeding develop
Periodic liver function tests and urinalysis
Serum ethosuximide levels, especially when used with other antiepileptic drugs or when response is inadequate
Monitor for mood changes, depression, and suicidal thoughts/behavior
Monitor seizure frequency/type, since ethosuximide is not effective against and may exacerbate generalized tonic-clonic seizures

Crosses the placenta; birth defects have been reported in offspring of women treated with ethosuximide during pregnancy, though a causal relationship has not been established
Discontinuation risks recurrence of absence seizures or status epilepticus — weigh individual seizure risk against potential teratogenic risk
Encourage enrollment in the North American Antiepileptic Drug (NAAED) Pregnancy Registry (1-888-233-2334)
Ethosuximide is excreted in human breast milk; effects on the nursing infant are not well characterized — use only if benefits clearly outweigh risks