Sounds like 'eth-oh-sux-ih-mide'

Key points about ethosuximide

  • Ethosuximide is used to treat epilepsy.
  • Ethosuximide is also called Zarontin.
  • Find out how to take it safely and possible side effects. 
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Ethosuximide is used to treat epilepsy by preventing absence (petit mal) seizures. It's called an anti-seizure medication. Ethosuximide works by controlling the neurotransmitters (chemicals) in your brain. Read more about anti-seizure medication.

January 2024: The brand of ethosuximide 250mg capsules is changing from Zarontin to Ethosuximide Essential Generics. The Ethosuximide Essential Generics packaging and capsules look different. Each capsule has the same amount of active ingredient as Zarontin and works in the body in the same way.

The Ethosuximide Essential Generics capsules are made by the same manufacturer as Zarontin, but at a different site.

It’s important to keep taking ethosuximide because suddenly stopping epilepsy medicines can cause an increase in seizures. Your healthcare provider will answer your questions about this brand change.

You may have already used this brand. Ethosuximide Essential Generics capsules were supplied in 2022 when there was a shortage of Zarontin. 

In Aotearoa New Zealand, ethosuximide is available as 250 mg capsules and a 250 mg per 5 mL liquid syrup.

  • The dose of ethosuximide will be different for different people, depending on your age and response to the medication or if you are taking other medicines.
  • Your doctor will start you on a low dose and increase your dose slowly over a few weeks.
  • Always take your ethosuximide exactly as your doctor has told you. The pharmacy label on your medicine will tell you how much to take, how often to take it, and any special instructions.

  • Timing: Ethosuximide is usually taken twice a day, usually about 10–12 hours part. Try to take it at the same time(s) each day, as this will help you to remember to take your doses regularly.
  • Food and drink: Swallow your capsules whole with a drink of water. You can take ethosuximide with or without food.
  • Missed dose: If you forget to take your dose, take it as soon as you remember if it's within 4 hours of when your dose was due. But if more than 4 hours have passed since when the dose was due, just take the next dose at the right time. Do not take extra doses to make up for a forgotten dose. If you're not sure what to do, ask your healthcare provider.
  • Keep taking ethosuximide every day:  See tips to help you remember to take your medicines regularly.  It may take a few weeks before you notice the full benefits of ethosuximide. Don't stop taking ethosuximide suddenly; talk to your doctor or nurse before stopping.

For information on how to give ethosuximide to children, see ethosuximide information for parents and carers.(external link) 

Here are some things to know when you're taking ethosuximide. Other things may be important as well, so ask your healthcare provider what you should know about.

  • Alcohol: Avoid drinking alcohol while you're taking ethosuximide. Alcohol can increase your chance of side effects such as drowsiness.
  • Tell all your healthcare providers that you're taking ethosuximide. This includes your doctors, nurses, pharmacists and dentists.
  • Driving: Avoid driving and doing other tasks or actions that need you to be alert until you see how this medicine affects you.
  • Other medicines: Ethosuximide can interact with other medicines. Tell your doctor or pharmacist about all medicines you're taking including over the counter medicines, herbal and complementary medicines or recreational drugs. 
  • Pregnant or breastfeeding: Tell your doctor if you're pregnant, planning to become pregnant, or breastfeeding.

Like all medicines, ethosuximide can cause side effects, although not everyone gets them. Often side effects go away once your body gets used to the new medicine.

Side effects What should I do?
  • Feeling sick (nausea)
  • Being sick (vomiting) 
  • Runny poo (diarrhoea)
  • Stomach ache
  • This is quite common when you start ethosuximide.
  • Tell your doctor if these are ongoing.
  • Feeling sleepy, drowsy or tired
  • Feeling dizzy or lightheaded 
  • This is quite common when you start ethosuximide.
  • Be careful when driving or using tools until you know how this medicine affects you.
  • These effects put you at risk of falls, and injuries especially if you are an older person. Tell your doctor if you are concerned.
  • Do not drink alcohol – it makes these effects worse.
  • Headache
  • Muscle weakness
  • Hiccups
  • These may happen when you first start taking ethosuximide.
  • Tell your doctor if these bother you.
  • Loss of coordination, trouble concentrating
  • Tell your doctor.
  • Skin rash, skin peeling or blisters
  • Stop taking ethosuximide and see your doctor immediately.
  • Mood changes, low mood, thoughts or talk of suicide and self-harm
  • Tell your doctor immediately or phone Healthline free on 0800 611 116.
  • Signs of problems with your blood cells, such as mouth ulcers, fever, chills, sore throat, unexplained bruising or bleeding
  • Ethosuximide can affect your blood cells – this is serious but very rare.
  • Tell your doctor immediately or phone Healthline free on 0800 611 116.

Read more about medicines and side effects and reporting a reaction that you think might be a side effect.

The following links have more information on ethosuximide.

Ethosuximide(external link) New Zealand Formulary, Patient Information (adults)
Ethosuximide(external link) New Zealand Formulary, Patient Information (children)
Zarontin(external link) Medsafe Consumer Information, NZ


  1. Ethosuximide(external link) NZ Formulary, NZ

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Credits: Sandra Ponen, Pharmacist, Healthify He Puna Waiora. Healthify is brought to you by Health Navigator Charitable Trust.

Reviewed by: Angela Lambie, Pharmacist, Auckland

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