# Zonisamide for Cats: Dosage and Side Effects Zonisamide for cats side effects are usually mild, but some cats develop vomiting, lethargy, or wobbliness that needs veterinary attention. This seizure medication is used most often when phenobarbital alone does not control seizures or when a cat cannot safely take phenobarbital [1]. In a study of 57 cats, zonisamide reduced seizure frequency by a median of about one seizure per month [1]. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - Zonisamide is an oral antiseizure drug given to cats, often as an add-on to phenobarbital or as sole therapy when other drugs are not an option [1][2]. - In one study of 57 cats, zonisamide reduced seizure frequency by a median of one seizure per month and seizure days by a median of one per month [1]. - Your veterinarian calculates the dose from your cat's weight, kidney and liver health, and other medications, so no single dose fits every cat. - Common side effects include sleepiness, vomiting, and unsteady walking. Report any of these to your veterinarian. - Never stop zonisamide suddenly. Stopping a seizure drug abruptly can trigger a cluster of seizures or status epilepticus. ## What Is Zonisamide and What Do Veterinarians Use It For? Zonisamide is a sulfonamide-class antiseizure medication. It is not a sulfonamide antibiotic, but it belongs to the same chemical family, which matters for one specific side effect discussed below. In veterinary medicine, it is used most often in cats whose seizures are not well controlled by phenobarbital alone, or in cats that cannot safely receive phenobarbital [1]. A 2024 study of 57 cats with a history of seizures found that zonisamide reduced seizure frequency across the whole group, including cats with idiopathic epilepsy and cats taking zonisamide as their only seizure medication [1]. A separate study from a referral hospital in Japan followed 20 cats with idiopathic epilepsy. Three cats received zonisamide alone, and two of those three reached the treatment goal of fewer than one seizure every three months [2]. Two of three cats on a combination of phenobarbital and zonisamide also reached that goal [2]. That said, zonisamide is not a first-choice drug for every cat. Many veterinarians still start with phenobarbital because it has a longer track record in cats. Zonisamide tends to come into the picture when seizures persist, when phenobarbital causes unacceptable side effects, or when a cat has liver problems that make phenobarbital risky. ## How Zonisamide Works in Cats Zonisamide works through several mechanisms at once. It blocks sodium channels in nerve cells, which stabilizes the electrical activity that can spiral into a seizure. It also affects calcium channels and certain neurotransmitter systems. The exact way these effects combine to reduce seizures in cats is not fully understood, but the result is a raised seizure threshold. Zonisamide is metabolized mainly by liver enzymes in the CYP450 family [3]. That detail matters because other drugs can change how fast zonisamide is cleared from the body. In dogs, phenobarbital increases zonisamide clearance and shortens its half-life [3]. The reverse also happens: when zonisamide was added to phenobarbital in dogs, phenobarbital blood levels rose in 9 out of 10 dogs, and five dogs needed a phenobarbital dose reduction [3]. This interaction has not been confirmed in cats, but veterinarians watch for it because the two drugs are often combined. ## Zonisamide for Cats Dosage: Who Decides and How It Is Given ### The dose is calculated by your veterinarian There is no single zonisamide dose that fits every cat. Your veterinarian calculates the dose based on your cat's body weight, kidney and liver function, seizure pattern, and other medications. In the 57-cat study, cats received oral zonisamide, and the researchers compared seizure frequency before and after treatment instead of publishing a fixed dose for all cats [1]. Do not adjust the dose on your own, and do not use a dose you found online for dogs. ### How to give the medication Zonisamide comes as capsules that are usually given by mouth, often twice daily. You can hide the capsule in a small amount of food or use a pill popper. If your cat spits it out, do not re-dose unless your veterinarian tells you to. Giving a second dose after a partial dose can push blood levels too high. ### What to do about a missed dose If you miss a dose, give it as soon as you remember, unless it is almost time for the next one. In that case, skip the missed dose and go back to your regular schedule. Never double up to make up for a missed dose. If you miss more than one dose in a row, call your veterinarian, because seizure control can slip quickly. ### Do not stop suddenly Stopping zonisamide abruptly can trigger withdrawal seizures, including clusters or continuous seizures. If you need to stop the drug for any reason, your veterinarian will guide you through a taper. ## Zonisamide for Cats Side Effects: What to Watch For This is the part most owners search for, and it is worth reading closely. In the 57-cat study, researchers tracked adverse clinical and laboratory effects in cats receiving zonisamide [1]. Side effects in cats tend to be manageable, but some require a call to your veterinarian. | Side effect | How common or serious | What to do | |---|---|---| | Sleepiness, lethargy, or wobbliness | Common, usually mild and worst in the first weeks | Note the timing and call your vet if it does not improve or your cat seems sedated | | Vomiting or poor appetite | Common, can be mild or a sign of drug intolerance | Call your vet if vomiting repeats or your cat stops eating for more than a day | | Unsteady walking or stumbling | Can be mild or a sign of blood levels that are too high | Call your vet, especially if it comes on suddenly | | Bloodwork changes (liver or kidney values) | Possible, which is why monitoring bloodwork is done | Keep scheduled recheck appointments | | Low thyroid hormone from sulfonamide drugs | Rare but documented with sulfonamide antibiotics in cats | Report weight gain, dull coat, or cold intolerance to your vet | | Severe allergic reaction (facial swelling, hives, trouble breathing) | Rare but an emergency | Stop the drug and seek emergency care now | The thyroid concern deserves a note. Zonisamide is a sulfonamide, and sulfonamide drugs have caused reversible hypothyroidism in cats. One report described a cat that developed hypothyroidism after sulfonamide treatment for an infection, with low thyroxine and high thyroid stimulating hormone, and the thyroid values returned to normal after the drug was stopped [4]. Another report described a cat with goitrous hypothyroidism three months after starting a potentiated sulfonamide, with thyroid changes that resolved once the drug was discontinued [5]. These cases involved sulfonamide antibiotics, not zonisamide, so the risk with zonisamide is not proven. Still, tell your veterinarian if your cat develops unexplained weight gain, lethargy, or a dull coat while on zonisamide. If you want a broader look at how seizure drugs affect cats, see [What Are the Side Effects of Seizure Medication in Cats?](/knowledge/veterinary-medicine/pet-medication-guide/seizure-medication-side-effects-cats). If your cat also takes gabapentin for seizures or pain, [What Are the Side Effects of Gabapentin for Cats?](/knowledge/veterinary-medicine/pet-medication-guide/gabapentin-side-effects-cats) covers what to expect. ## Monitoring Bloodwork While Your Cat Takes Zonisamide Your veterinarian will likely recommend periodic blood tests. These checks look at liver values, kidney values, and sometimes zonisamide blood levels. The 57-cat study reported clinicopathologic adverse effects, which is one reason monitoring is part of routine care [1]. If your cat also takes phenobarbital, monitoring matters even more. In dogs, adding zonisamide raised phenobarbital blood levels in 9 out of 10 dogs, and five dogs reached levels above the reported hepatotoxic threshold, requiring a phenobarbital dose reduction [3]. Your veterinarian may check phenobarbital levels after starting zonisamide, even if the phenobarbital dose has not changed. Keep every recheck appointment, even if your cat seems fine. Seizure control and drug safety are judged partly on bloodwork, and problems can show up before you notice symptoms at home. ## Who Should Not Take Zonisamide and What Interacts With It Zonisamide is not right for every cat. Your veterinarian may avoid it or use it with extra caution if your cat has kidney disease, liver disease, or a history of reactions to sulfonamide drugs. Cats with certain metabolic or blood disorders may also need a different medication. Drug interactions matter. Phenobarbital changes how the liver processes zonisamide, and zonisamide can raise phenobarbital levels [3]. Other seizure medications, including levetiracetam and diazepam, have been used alongside zonisamide in cats, and one study described cats on zonisamide plus levetiracetam plus diazepam [2]. That does not mean any combination is safe for your cat. It means your veterinarian needs a complete list of everything your cat takes, including supplements and over-the-counter products. Tell your veterinarian about every medication, even ones you think are harmless. Some drugs and supplements affect liver enzymes and can change zonisamide levels. ## Zonisamide Overdose and When to Call for Help If your cat eats extra zonisamide capsules, call for help right away. Signs of overdose can include severe sedation, wobbliness, vomiting, tremors, or difficulty breathing. Do not wait to see if symptoms appear. Call your veterinarian or an animal poison control hotline immediately. Have the bottle handy so you can tell them the strength of the capsules and how many your cat may have swallowed. Do not induce vomiting unless a veterinarian tells you to. ## Limitations and When to Contact a Veterinarian Individual cats respond differently to zonisamide, and only your veterinarian can judge your cat's dose, monitoring schedule, and risk level. Call the same day if your cat vomits more than once, refuses food for more than a day, seems more sleepy or wobbly than usual, or has a seizure that looks different from the usual pattern. Go to an emergency clinic now if your cat has a seizure lasting more than a few minutes, has two or more seizures in a row without fully recovering, has trouble breathing, or develops facial swelling, hives, or collapse. Keep routine visits for bloodwork and dose checks, even when your cat seems stable. Seizure control is a long game, and small adjustments often make a big difference. ## Frequently Asked Questions ### Is zonisamide safe for cats? Zonisamide is used in cats and was studied in a group of 57 cats with seizures, where it reduced seizure frequency and produced manageable side effects [1]. Safety depends on your cat's liver and kidney health, other medications, and how well your cat tolerates the drug. Your veterinarian decides whether the benefits outweigh the risks for your cat. ### How is zonisamide given to a cat? It is given by mouth, usually as a capsule, often twice daily. Your veterinarian sets the exact dose based on your cat's weight and health. Hide the capsule in food or use a pill popper, and never change the dose without veterinary guidance. ### Can zonisamide be used alone for cat seizures? Yes, in some cases. In one study, three cats with idiopathic epilepsy received zonisamide as their only seizure medication, and two of them reached the treatment goal [2]. Many cats take it alongside phenobarbital or other drugs instead [1][2]. Your veterinarian will choose the approach that fits your cat. ### What should I do if my cat misses a dose of zonisamide? Give the missed dose when you remember, unless the next dose is almost due. If it is almost time, skip the missed dose and continue on schedule. Never give two doses at once, and call your veterinarian if you miss more than one dose. ### Can I stop zonisamide suddenly if my cat seems fine? No. Stopping a seizure medication abruptly can trigger withdrawal seizures, including clusters. If zonisamide needs to stop, your veterinarian will guide a gradual taper. Call your vet before making any change, even if your cat has been seizure-free for months. ## Related Articles - [What Are the Side Effects of Gabapentin for Cats?](/knowledge/veterinary-medicine/pet-medication-guide/gabapentin-side-effects-cats) - [What Are the Side Effects of Seizure Medication in Cats?](/knowledge/veterinary-medicine/pet-medication-guide/seizure-medication-side-effects-cats) - [Mirtazapine Appetite Stimulant: Dose for Cats and Dogs Guide](/knowledge/veterinary-medicine/calculators-clinical-tools/mirtazapine-appetite-stimulant-dose-for-cats-and-dogs-guide) - [Cat Allergy Medication Side Effects: What to Watch For](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-allergy-medication-side-effects) - [Cat Urinary Tract Infection Side Effects](/knowledge/veterinary-medicine/clinical-methods/cat-urinary-tract-infection-side-effects) - [Gabapentin Cat Dose: Pain and Anxiety Dose and Monitoring](/knowledge/veterinary-medicine/calculators-clinical-tools/gabapentin-cat-dose-pain-and-anxiety-dose-and-monitoring) - [Zonisamide vs Phenobarbital or Keppra for Cats](/knowledge/veterinary-medicine/pet-medication-guide/zonisamide-vs-phenobarbital-cats) - [Transdermal and Liquid Meds for Cats: Zonisamide and Benazepril](/knowledge/veterinary-medicine/pet-medication-guide/transdermal-liquid-meds-cats-zonisamide) - [Metoclopramide for Cats: Uses, Doses, and Side Effects](/knowledge/veterinary-medicine/pet-medication-guide/metoclopramide-for-cats) ## References 1. [A retrospective study of the efficacy of zonisamide in controlling seizures in 57 cats](https://pubmed.ncbi.nlm.nih.gov/38240116/). Journal of veterinary internal medicine, 2024. 2. [Evaluation of antiseizure medications including zonisamide in feline idiopathic epilepsy at a referral hospital in Japan](https://pubmed.ncbi.nlm.nih.gov/37614732/). Open veterinary journal, 2023. 3. [The effect of oral zonisamide treatment on serum phenobarbital concentrations in epileptic dogs](https://pubmed.ncbi.nlm.nih.gov/38868500/). Frontiers in veterinary science, 2024. 4. [Clinical hypothyroidism in a cat associated with sulfonamide administration for the management of intracranial nocardiosis](https://pubmed.ncbi.nlm.nih.gov/39370263/). Australian veterinary journal, 2024. 5. [Iatrogenic Primary Hypothyroidism Associated With Sulfamethoxazole-Trimethoprim Treatment of Nocardiosis in a Cat](https://pubmed.ncbi.nlm.nih.gov/40072270/). Journal of veterinary internal medicine, 2025. ## Further Reading - [American Veterinary Medical Association: First Aid Tips for Pet Owners](https://www.avma.org/resources-tools/pet-owners/emergencycare/first-aid-tips-pet-owners) - [Merck Veterinary Manual: Cat Owners](https://www.merckvetmanual.com/cat-owners) - [Cornell Feline Health Center: Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information)