Antiepileptic — T-type Calcium Channel Blocker Pregnancy: Ethosuximide crosses the placenta; congenital abnormalities occur more frequently in infants of mothers using anticonvulsants. Weigh benefit versus risk; prefer monotherapy at the lowest effective dose with plasma monitoring. Folic acid supplementation recommended. Breast-feeding best avoided (excreted in breast milk).

Ethosuximide

Brand names: Emeside, Zarontin

Ethosuximide is an antiepileptic that is first-line specifically for absence seizures. It does not control tonic-clonic seizures, so additional cover may be needed where both seizure types coexist.

Auto-extracted from the source labelling — not yet independently clinician-verified. These values were distilled from the UK SPC (or the US label where noted) but have not had a clinician sign-off. Confirm against the current SmPC before prescribing.

Adult dose

Dose: 250 mg twice daily (initial)

Route: oral

Frequency: twice daily

Max: usually not exceeding 1500–2000 mg per day

Adults, elderly and children over 6 years: start 250 mg twice per day. Increase the daily dose by 125 mg every 7 days (outpatient) or every 4 days (clinical setting) until the optimum dose is reached; this will usually not exceed 1500–2000 mg per day (1000 mg for patients aged under six years). Dose determined individually based on serum level. Take during or after meals with liquid.

Paediatric dose

Dose: 10 mg/kg

Route: oral

Frequency: in 1–2 doses daily (initial)

Max: 1000 mg/day for patients aged under 6 years

Children under 3 years: initially 10 mg/kg body weight per day in 1–2 doses; maintenance 20–40 mg/kg body weight per day in 1–2 doses. Children 3 to 6 years: start 125 mg twice per day (use syrup if capsule strength not feasible for the child's weight). Children over 6 years: as adults, start 250 mg twice daily. US labelling: safety and effectiveness below age 3 years not established; optimal dose for most pediatric patients 20 mg/kg/day.

Dose auto-extracted from UK Summary of Product Characteristics (SPC) via the eMC; US FDA prescribing information (openFDA / DailyMed) — cross-check; US labelling may differ from UK — not yet clinician-verified. Always confirm against the product SmPC and your local formulary before prescribing.

Paediatric weight-based calculator

Patient weight (kg)

Verify in a children's formulary

US labelling (FDA)

Reference — US labelling, may differ from UK

DOSAGE AND ADMINISTRATION Zarontin is administered by the oral route. The initial dose for patients 3 to 6 years of age is one capsule (250 mg) per day; for patients 6 years of age and older, 2 capsules (500 mg) per day. The dose thereafter must be individualized according to the patient's response. Dosage should be increased by small increments. One useful method is to increase the daily dose by 250 mg every four to seven days until control is achieved with minimal side effects. Dosages exceeding 1.5 g daily, in divided doses, should be administered only under the strictest supervision of the physician. The optimal dose for most pediatric patients is 20 mg/kg/day. This dose has given …

Source: US FDA prescribing information (openFDA / DailyMed), label dated 2026-05-25. Accessed 2026-06-12. US dosing and indications can differ from UK practice — use UK sources for prescribing decisions.

Contraindications

Hypersensitivity to ethosuximide, other succinimides, and lecithin (soya lecithin) or to any of the excipients
Porphyrias

Side effects

Decreased appetite — common
Headache, ataxia, dizziness, somnolence — common
Abdominal pain, nausea, vomiting, gastrointestinal disturbance — common
Rash (erythematous), urticaria — common; Stevens-Johnson syndrome and DRESS reported
Blood dyscrasias: agranulocytosis, aplastic anaemia, leukopenia, pancytopenia, eosinophilia — uncommon/rare

Interactions

May elevate phenytoin serum levels
Valproic acid may both increase and decrease ethosuximide levels
May interact with concurrently administered antiepileptic drugs — periodic serum level monitoring may be needed

Clinical monograph

How it works

It reduces low-threshold (T-type) calcium currents in thalamic neurons, which underlie the rhythmic discharges responsible for absence seizures.

Prescribing in practice

Rare but serious blood dyscrasias and severe skin reactions can occur — advise the patient to report a sore throat, fever, mouth ulcers, bruising or rash promptly.
Gastrointestinal upset (nausea, abdominal discomfort) and drowsiness are common, especially early in treatment.
It does not control tonic-clonic seizures; withdraw gradually rather than stopping abruptly.

Monitoring

Be alert to features of blood dyscrasia or serious skin reaction and investigate promptly; monitor seizure control, gastrointestinal tolerance, and mood or suicidal ideation as with all antiepileptics.

Counselling the patient

Seek medical advice urgently if you develop a sore throat, fever, mouth ulcers, unusual bruising or a rash.
Nausea and drowsiness are common at first and often settle — taking doses with food may help.
Do not stop the medicine suddenly.

Evidence & guidelines

First-line for absence seizures (NICE NG217).

Reference: NICE NG217; NETTLES Trial (NEJM 2010); Drug verified in RxNorm (NLM); confirm dosing against the manufacturer SPC (eMC). Verify against your local formulary and current prescribing references before prescribing. The structured dose values shown have been reviewed by a clinician. Monograph status: clinician-reviewed (2026-07-04).

Curated clinical cross-links plus same-class fallbacks.

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