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[Pharmacology](https://durgical.com/en/pharmacology/) · [Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/)

# Zonisamide

A sulfonamide antiseizure medicine with a very long half-life. It blocks sodium and T-type calcium channels and inhibits carbonic anhydrase.

Updated 2026-11-01

- Class: [Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/)
- Ion channel: Voltage-gated sodium channels and T-type calcium channels
- ATC code: N03AX15

## Mechanism of action

Zonisamide blocks voltage-gated sodium channels and reduces T-type calcium currents, stabilizing neuronal membranes and suppressing neuronal hypersynchronization. It is also a carbonic anhydrase inhibitor, which explains metabolic acidosis, kidney stones and reduced sweating.

- Blocks voltage-gated sodium channels
- Reduces T-type calcium currents
- Neuronal membranes stabilize and synchrony falls
- Also inhibits carbonic anhydrase (acidosis, stones)

## Pharmacokinetics

**Absorption**
Absorbed by mouth

**Metabolism**
In the liver, partly by CYP3A4; inducers shorten its half-life

**Half-life**
Very long: about 63 hours in plasma

## Indications

- Add-on treatment of focal seizures in adults [Epilepsy and seizures →](https://durgical.com/en/conditions/epilepsy-and-seizures/)

## Adverse effects

### Common

- Sleepiness
- Loss of appetite
- Dizziness and unsteadiness
- Agitation or irritability
- Difficulty with memory and concentration

### Serious (rare, important)

- Severe sulfonamide reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis
- Reduced sweating with high body temperature, especially in children
- Metabolic acidosis and kidney stones
- Suicidal thoughts

## Contraindications

- Hypersensitivity to sulfonamides or zonisamide

## Key interactions

**Liver enzyme inducers (carbamazepine, phenytoin, phenobarbital)**
Faster clearance and shorter half-life of zonisamide

**Other carbonic anhydrase inhibitors (topiramate, acetazolamide)**
More metabolic acidosis, hyperammonaemia and kidney stones

## What is monitored

- Serum bicarbonate
- Sweating and body temperature in children
- Skin reactions and fever
- Kidney function and signs of stones

## Administration and safety

- Given by mouth as capsules
- Good fluid intake helps prevent kidney stones
- Withdrawn gradually

## Nursing care

- Ask about allergy to sulfonamide medicines before the first dose
- Teach to report rash, fever, sore throat or mouth sores at once
- Encourage plenty of fluids and watch for flank pain
- Watch children for reduced sweating in hot weather

## Pregnancy and breastfeeding

Human data are limited; as with other antiseizure medicines, benefits and risks are weighed with specialist care.

## Memory aid

Zonisamide: «Zone of the sulfa» – a sulfonamide, so ask about sulfa allergy; and like topiramate, it makes stones and stops sweat.

## Test yourself

Think of the answer, then tap the question to see it.

## Why is zonisamide contraindicated in people allergic to sulfa drugs?

Zonisamide is a sulfonamide; severe reactions such as Stevens-Johnson syndrome can occur when a sulfonamide is given again.

## Which antiseizure medicine shares zonisamide's kidney stones, acidosis and reduced sweating?

Topiramate; both inhibit carbonic anhydrase.

## What does zonisamide's long half-life mean for steady state?

With a plasma half-life of about 63 hours, it takes many days to reach steady state after each change.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/zonisamide/)

## Related drugs

- [Topiramate](https://durgical.com/en/pharmacology/topiramate/)

## Related conditions

- [Epilepsy and seizures](https://durgical.com/en/conditions/epilepsy-and-seizures/)

[← Back to the class: Antiseizure medicines](https://durgical.com/en/pharmacology/classes/antiseizure-medicines/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [Zonisamide: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a603008.html) (MedlinePlus (U.S. National Library of Medicine), 2024); [DailyMed - ZONISAMIDE capsule](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=e8f368cb-6a07-4178-95fa-1d5a209171ed) (DailyMed (U.S. National Library of Medicine), 2026); [ATCDDD - ATC/DDD Index](https://atcddd.fhi.no/atc_ddd_index/?code=N03AX15) (WHO Collaborating Centre for Drug Statistics Methodology, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/pharmacology/zonisamide/
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