Mar Vista Animal Medical Center

3850 Grand View Blvd.
Los Angeles, CA 90066



(for veterinary information only)




25 MG, 50 MG, 100 MG


Seizure control in dogs and cats is usually straight 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 the phenobarbital or or even as a replacement for phenobarbital. Zonisamide is one such medication.



Zonisamide can be used in either cats or dogs for control of seizures. It is typically used twice daily in dogs, usually as a supplement to phenobarbital. Zonisamide has very long-lasting activity in cats and can be used once daily.

In dogs, serum zonisamide blood levels are frequently monitored one to two 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, it is recommended that levels be examined 3-4 hours after zonisamide administration as well as within 1 hour of the next scheduled dose. This allows for examination of both the highest and lowest zonisamide blood levels of the day.



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. Some individuals require dose adjustment for this side effect.

Case studies have been published where liver damage was reported in 2 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.



There is suspicion that the liver enzyme induction that phenobarbital is famous for will speed up the metabolism of zonisamide. In other words, concurrent use of phenobarbital may speed up the removal of zonisamide from the body so that it does not work as well. This could lead to break-through seizures and is the reason why blood levels of zonisamide are commonly measured when there is concurrent use of phenobarbital. The starting dose of zonisamide is frequently higher for dogs on concurrent phenobarbital for this reason as well.



Zonisamide should not be used in pregnancy as it can cause cardiovascular damage in the unborn.

Zonisamide can be given with or without food.

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.

Page posted: 5/9/2017
Page last reviewed: 8/23/2019