Zonisamide is a broad-spectrum anticonvulsant medication originally developed for human epilepsy and now widely used off-label in veterinary medicine for the management of seizures in dogs. It is a sulfonamide-derived drug that works through multiple mechanisms, including blockade of voltage-gated sodium and T-type calcium channels, facilitation of GABAergic and dopaminergic neurotransmission, and inhibition of carbonic anhydrase. Zonisamide is commonly prescribed as an add-on (adjunctive) therapy for dogs whose seizures are not adequately controlled with phenobarbital or potassium bromide alone, and it is increasingly used as a first-line anticonvulsant in some cases due to its relatively favorable side effect profile.
What Is Zonisamide Used For?
Veterinarians prescribe zonisamide for:
- Idiopathic epilepsy in dogs as monotherapy or adjunctive therapy
- Refractory seizures not adequately controlled by phenobarbital, potassium bromide, or levetiracetam
- Focal and generalized seizures of various types
- Dogs that cannot tolerate phenobarbital or potassium bromide due to side effects (excessive sedation, liver toxicity, polyuria/polydipsia)
Seizure management requires close veterinary monitoring. Never adjust anticonvulsant doses or discontinue treatment without your veterinarian’s guidance.
Dosage
The typical dose of zonisamide in dogs is 5 to 10 mg/kg orally every 12 hours. Your veterinarian will determine the appropriate dose and adjust it based on seizure control, drug levels, and side effects.
| Dog’s Weight | Starting Dose (5 mg/kg BID) | Higher Dose (10 mg/kg BID) |
|---|---|---|
| 10 lbs (4.5 kg) | 22.5 mg twice daily | 45 mg twice daily |
| 25 lbs (11 kg) | 55 mg twice daily | 110 mg twice daily |
| 50 lbs (23 kg) | 115 mg twice daily | 230 mg twice daily |
| 75 lbs (34 kg) | 170 mg twice daily | 340 mg twice daily |
| 100 lbs (45 kg) | 225 mg twice daily | 450 mg twice daily |
Zonisamide is available in 25 mg, 50 mg, and 100 mg capsules. It can be given with or without food, though giving with food may reduce GI upset. Steady-state blood levels are typically reached within 3-5 days in dogs. Your veterinarian may monitor trough serum levels (target range generally 10-40 mcg/mL).
Important: When used with phenobarbital, zonisamide is metabolized faster, so higher doses or more frequent dosing may be required. Consult your veterinarian.
Side Effects
| Common Side Effects | Serious Side Effects (Contact Your Vet) |
|---|---|
| Sedation or ataxia (usually transient) | Severe vomiting or inappetence lasting >48 hrs |
| Decreased appetite | Signs of liver disease (jaundice, dark urine) |
| Mild vomiting or diarrhea | Kidney stones (rare) |
| Incoordination (first few days) | Blood abnormalities (anemia, low platelets) |
| Allergic skin reactions |
Most side effects are mild and often resolve within the first week or two of treatment as the dog adjusts. Serious hepatotoxicity and idiosyncratic reactions are rare but have been reported. Regular blood work monitoring is recommended.
Drug Interactions
- Phenobarbital: Significantly increases zonisamide metabolism (via CYP3A4 induction), reducing its half-life. Higher doses of zonisamide may be needed.
- Ketoconazole, cimetidine: May inhibit zonisamide metabolism, increasing blood levels.
- Other CNS depressants: May increase sedation when combined with zonisamide.
- Carbonic anhydrase inhibitors (acetazolamide): May increase risk of kidney stones.
Always inform your veterinarian of all medications and supplements your dog is taking.
Contraindications
Zonisamide should not be used in:
- Dogs with known hypersensitivity to sulfonamides or zonisamide
- Dogs with severe liver or kidney disease (use with extreme caution and close monitoring)
Use with caution in dehydrated dogs or dogs prone to urinary stone formation, as zonisamide is a carbonic anhydrase inhibitor and may alkalinize urine. Adequate hydration should be maintained.
Storage
- Store at controlled room temperature, 68-77 degrees F (20-25 degrees C).
- Protect from moisture and light.
- Keep in original packaging.
- Keep out of reach of children and pets.
Frequently Asked Questions
Can zonisamide be used as a sole anticonvulsant? Yes. While commonly used as add-on therapy, zonisamide is increasingly prescribed as first-line monotherapy, particularly in dogs where phenobarbital’s side effects (hepatotoxicity, PU/PD, excessive sedation) are a concern. Consult your veterinarian about the best approach for your dog.
How long does it take zonisamide to work? Steady-state blood levels are typically reached within 3-5 days in dogs. Seizure frequency should be assessed over several weeks before concluding whether the dose is effective. Your veterinarian will monitor seizure logs and blood levels.
What happens if I miss a dose? Give the missed dose as soon as you remember. If it is close to the next scheduled dose, skip the missed dose and resume the regular schedule. Missing doses can lower blood levels and increase seizure risk. Contact your veterinarian if you miss multiple doses.
Is zonisamide expensive? Generic zonisamide is relatively affordable compared to some other anticonvulsants. Costs vary by dose and pharmacy but are typically in the range of $20-60 per month for most dogs.
This content is for informational purposes only and is not a substitute for professional veterinary advice. Always consult your veterinarian before starting, changing, or stopping any medication for your dog.