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Zonisamide (Zonegran)
Revised: November 30, 2023
Published: August 09, 2017

(For veterinary information only)

The size of the tablet/medication is NOT an indication of a proper dose. Never administer any drug without your veterinarian's input. Serious side effects or death can occur if you use drugs on your pet without your veterinarian's advice. 

It is our policy not to give dosing information over the internet.

Brand Name: Zonegran

Available as: 25 mg, 50 mg, 100 mg capsules


Seizure control in dogs and cats is usually straightforward forward with most patients being predictably responsive to phenobarbital, but this is not always the case. Some animals cannot be adequately controlled with only one medication, or there may be reasons why phenobarbital is not an appropriate choice. For these patients, there are several other medications that can be used either to supplement or replace phenobarbital. Zonisamide is one such medication.

How this Medication is Used

Zonisamide can be used in either cats or dogs to control seizures. It is typically used twice daily in dogs, usually as a supplement to phenobarbital. It has long-lasting activity in cats and can be used once daily, although use in cats is still new and the full potential for side effects is still being worked out.

In dogs, serum zonisamide blood levels are frequently monitored 1 to 2 weeks after starting zonisamide or after any dosage adjustment. Usually, the test is timed against medication administration so as to get the lowest drug levels of the day.

If zonisamide is used in combination with phenobarbital and seizures have recurred, levels should be examined three to four hours after zonisamide administration as well as within one hour of the next scheduled dose. This allows for examination of both the highest and lowest zonisamide blood levels of the day.

Side Effects

The most common side effect is sedation/drowsiness, and there is concern that tolerance can develop, which means that as time goes by, a higher dose is required to achieve the same effect.

Vomiting and appetite loss were the next most common reactions after sedation. Will probably require a dose adjustment if combined with phenobarbital for seizure control. Expect blood test monitoring to be recommended.

Zonisamide is considered hazardous in pregnancy. Women considering pregnancy should wear gloves when handling zonisamide and wash their hands after handling zonisamide.

Case studies have been published where liver damage was reported in two dogs. Another case report involved a dog that developed a condition called renal tubular acidosis. Because zonisamide is relatively new to veterinary use, the risk of these serious conditions is unknown, but because of these cases, relevant blood testing is recommended prior to starting zonisamide as well as periodically throughout therapy.

Zonisamide may reduce thyroid levels.

Interactions with Other Drugs

Phenobarbital can induce liver enzymes, which means that other drugs can be removed from the body faster than they would without any phenobarbital. Since zonisamide and phenobarbital are commonly used together, an increase in the zonisamide dosing is generally needed to offset its faster removal.

When zonisamide is combined with carbonic anhydrase inhibitors (such as acetazolamide or dorsolamide - common glaucoma medications), there is an increased risk for kidney stones and for the previously mentioned renal tubular acidosis.

Concerns and Cautions

Zonisamide should not be used during pregnancy as it can cause cardiovascular damage in the unborn. Women considering pregnancy should wear gloves when handling zonisamide and wash their hands after handling zonisamide.

Zonisamide can be given with or without food. Usually given twice daily for dogs and once daily for cats.

Zonisamide should not be abruptly discontinued, or rebound seizures could occur.

Listlessness, appetite loss, vomiting, or yellow pigment changes in the eyes should be reported to the prescribing veterinarian at once.

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