Pharmacology · 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
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 →
Adverse effects
Common
- Sleepiness
- Loss of appetite
- Dizziness and unsteadiness
- Agitation or irritability
- Difficulty with memory and concentration
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- Withdrawn gradually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.
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.
Related drugs
Related conditions
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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 (MedlinePlus (U.S. National Library of Medicine), 2024); DailyMed - ZONISAMIDE capsule (DailyMed (U.S. National Library of Medicine), 2026); ATCDDD - ATC/DDD Index (WHO Collaborating Centre for Drug Statistics Methodology, 2026).
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