Reviewed on 3/28/2022

What Is Ethosuximide and How Does It Work?

Ethosuximide is a prescription medication used for treating symptoms of absence seizures.

  • Ethosuximide is available under the following different brand names: Zarontin

What Are Side Effects Associated with Using Ethosuximide?

Common side effects of Ethosuximide include:

  • upset stomach,
  • nausea,
  • stomach pain,
  • loss of appetite,
  • diarrhea,
  • weight loss,
  • hiccups,
  • swelling in the tongue or gums,
  • headache,
  • dizziness,
  • trouble concentrating, and
  • feeling tired.

Serious side effects of Ethosuximide include:

  • mood or behavior changes,
  • anxiety,
  • panic attacks,
  • trouble sleeping,
  • impulsive behavior,
  • irritableness,
  • agitation,
  • hostile behavior,
  • aggression,
  • restlessness,
  • hyperactivity (mental or physical),
  • depression,
  • thoughts of hurting yourself,
  • fever,
  • chills,
  • flu symptoms,
  • sore throat,
  • weakness,
  • confusion,
  • hallucinations,
  • unusual thoughts or behavior,
  • extreme fear,
  • worsening seizures,
  • joint pain or swelling with fever,
  • swollen glands,
  • muscle aches,
  • chest pain,
  • vomiting,
  • patchy skin color,
  • swollen glands,
  • fly symptoms,
  • easy bruising or bleeding,
  • severe tingling or numbness,
  • muscle weakness,
  • upper stomach pain,
  • yellowing of the skin or eyes (jaundice),
  • chest pain,
  • new or worsening cough,
  • trouble breathing,
  • fever,
  • sore throat,
  • swelling of the face or tongue,
  • burning in the eyes, and
  • skin pain followed by a red or purple skin rash that spreads (especially in the face or upper body) and causes blistering and peeling

Rare side effects of Ethosuximide include:

  • none 

This is not a complete list of side effects and other serious side effects or health problems that may occur as a result of the use of this drug. Call your doctor for medical advice about serious side effects or adverse reactions. You may report side effects or health problems to FDA at 1-800-FDA-1088.

What Are Dosages of Ethosuximide?

Adult and pediatric dosage


  • 250mg


  • 250mg/5mL

Absence Seizures

Adult dosage

  • 500 orally once daily, increase by 250 mg every 4-7 days; generally not to exceed 1.5 g/day
  • Therapeutic range: 40-100 mg/L (may require 4-7 days to reach steady-state)

Pediatric dosage

  • Children below 3 years: Safety and efficacy not established
  • Children between 3-6 years: 250 mg orally once daily initially; if needed, may increase by 250 mg every 4-7 days; usual maintenance dose 20 mg/kg/day  
  • Children above 6 years: As adults, 500 mg orally once daily initially; may increase by 250 mg every 4-7 days; generally not to exceed 1.5 g/day in divided doses
  • Therapeutic range: 40-100 mg/L (may require 4-7 days to reach steady-state)

Dosage Considerations – Should be Given as Follows: 

  • See “Dosages”

What Other Drugs Interact with Ethosuximide?

If your medical doctor is using this medicine to treat your pain, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

  • Ethosuximide has severe interactions with no other drugs.
  • Ethosuximide has serious interactions with the following drugs:
  • Ethosuximide has moderate interactions with at least 71 other drugs.
  • Ethosuximide has minor interactions with at least 30 other drugs.

This information does not contain all possible interactions or adverse effects. Visit the RxList Drug Interaction Checker for any drug interactions. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns.

What Are Warnings and Precautions for Ethosuximide?


  • Hypersensitivity

Effects of drug abuse

  • None

Short-Term Effects

  • See “What Are Side Effects Associated with Using Ethosuximide?”

Long-Term Effects

  • See “What Are Side Effects Associated with Using Ethosuximide?”


  • Do not discontinue rapidly; proceed slowly when increasing or decreasing dosage, as well as when adding or eliminating other medications; abrupt withdrawal of anticonvulsant medication may precipitate absence (petit mal) status
  • When used alone in mixed types of epilepsy, therapy may increase the frequency of grand mal seizures in some patients
  • Blood dyscrasias may occur; perform periodic blood counts; should sign and/or symptoms of infection (e.g., sore throat, fever) develop, consider blood counts
  • Cases of systemic lupus erythematosus reported with the use of the drug; the physician should be alert to this possibility
  • Serious dermatologic reactions reported including Stevens-Johnson syndrome (SJS); onset usually within 28 days, but can occur later; discontinue therapy at the first sign of rash, unless the rash is not drug-related; if signs or symptoms suggest SJS, use of this drug should not be resumed; consider alternative therapy
  • Drug reaction with Eosinophilia and Systemic Symptoms (DRESS), also known as multi-organ hypersensitivity, reported; some fatal or life-threatening; early manifestations of hypersensitivity (e.g. fever, lymphadenopathy) may be present even though rash is not evident; if such signs or symptoms appear, the patient should be evaluated immediately and therapy discontinued if an alternative etiology for the signs or symptoms cannot be established
  • The drug is capable of producing morphological and functional changes in the animal liver; in humans, abnormal liver and renal function studies reported; ethosuximide should be administered with extreme caution to patients with known liver or renal disease; periodic urinalysis and liver function studies are advised for all patients receiving the drug
  • May cause CNS depression
  • Suicidal behavior and ideation
    • Anyone considering prescribing this drug or any other AED must balance the risk of suicidal thoughts and behavior with the risk of untreated illness
    • Antiepileptic drugs increase the risk of suicidal thoughts or behavior in patients taking them for any indication; monitor for the emergence or worsening of depression, suicidal thoughts or behavior, and/or any unusual changes in mood or behavior
    • Epilepsy and many other illnesses for which AEDs are prescribed are themselves associated with morbidity and mortality and an increased risk of suicidal thoughts and behavior
    • Should suicidal thoughts and behavior emerge during treatment, the prescriber needs to consider whether the emergence of these symptoms in any given patient may be related to the illness being treated
    • Behaviors of concern should be reported immediately to healthcare providers
  • Thrombocytopenia
    • Drug-induced immune thrombocytopenia (DITP) reported; in reported cases, the onset of symptoms occurred 1 to 3 weeks after initiation of therapy; one patient had a recurrence of symptoms within 1 day of a subsequent re-challenge with the drug
    • When DITP is suspected, discontinue therapy; monitor serial platelet counts, and treat as appropriate; if possible, assess the presence of drug-dependent antiplatelet antibodies; avoid future use of the drug in patients with a history of ethosuximide-induced DITP
    • Pregnancy and Lactation
    • The drug crosses the placenta; reports suggest an association between the use of anticonvulsant drugs by women with epilepsy and elevated incidence of birth defects in children born to these women; data are more extensive concerning phenytoin and phenobarbital, but these are also the most commonly prescribed anticonvulsants; less systematic or anecdotal reports suggest a possible similar association with the use of all known anticonvulsant drugs
    • Cases of birth defects reported with this drug; reports suggesting an elevated incidence of birth defects in children of drug-treated epileptic women cannot be regarded as adequate to prove a definite cause and effect relationship
    • There are intrinsic methodological problems in obtaining adequate data on drug teratogenicity in humans; the possibility also exists that other factors, eg, genetic factors or the epileptic condition itself, maybe more important than drug therapy in leading to birth defects
    • The great majority of mothers on anticonvulsant medication deliver normal infants; it is important to note that anticonvulsant drugs should not be discontinued in patients in whom the drug is administered to prevent major seizures because of the strong possibility of precipitating status epilepticus with attendant hypoxia and threat to life; in individual cases where severity and frequency of seizure disorder are such that removal of medication does not pose a serious threat to the patient; discontinuation of drug may be considered before and during pregnancy
    • Although it cannot be said with any confidence that even minor seizures do not pose some hazard to the developing embryo or fetus; the prescribing physician will wish to weigh these considerations in treating or counseling epileptic women of childbearing potential
    • Pregnancy Registry: Pregnant women exposed to ethosuximide are encouraged to enroll themselves by calling 1-888-233-2334


  • The drug is excreted in human breast milk; because effects of ethosuximide on a nursing infant are unknown, use caution when administering to a nursing mother; the drug should be used in nursing mothers only if the benefits outweigh the risks
Medscape. Ethosuximide.

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