What is zonisamide used for in dogs?

In this article:

• Zonisamide for dogs
• Is zonisamide safe for dogs?
• Dosage
• Alternatives
• Summary

In this article

• Zonisamide for dogs
• Is zonisamide safe for dogs?
• Dosage
• Alternatives
• Summary

Zonisamide is an antiseizure drug also known as Zonegran, the brand-name version of the drug. Veterinarians usually use it as an add-on drug when other anticonvulsants aren’t succeeding, but they may also prescribe it as their first choice. It’s a safe drug for daily use. Side effects are generally mild, but there are rare reports of dogs having serious liver problems when on this drug.

What is zonisamide used for in dogs?

Zonisamide is a daily maintenance treatment for dogs with seizure disorders. It helps reduce the incidence of seizures. Veterinarians can prescribe it as an initial standalone treatment, but they are likely to use it as an add-on drug when other drug therapies are not adequately controlling seizures.

Seizure disorders

Seizures are the result of simultaneous overexcitement and overactivity of nerves in the brain. These are often short-lived episodes with symptoms ranging from “blanking out” to convulsions. When seizures happen in a dog more than once in a six-month period, veterinarians diagnose the dog with epilepsy, a condition that affects about 0.75% of dogs in the U.S. 

Veterinarians use two types of drugs called anticonvulsants to treat epileptic dogs. Some drugs are used to stop active seizures. They are used only when a dog is seizing. Maintenance drugs are taken daily when the dog is not seizing. They help to reduce the incidence of seizures. Maintenance drugs are usually used only after a dog has been diagnosed with epilepsy. 

One-time-only seizures usually don’t warrant maintenance treatment unless the dog has had a severe or long-lasting seizure or has taken a long time to recover. If seizures are spaced farther than six months apart, the veterinarian may start maintenance treatment if seizures are becoming longer, more severe, or require longer recovery periods. Having more than one seizure in a day—called a cluster seizure—is also a reason to start maintenance treatment.

Veterinarians usually begin maintenance treatment with only one anticonvulsant. The goal is to reduce the incidence of seizures and spare the dog too many side effects from multiple drugs. Using just one drug to treat a medical condition is called “monotherapy.” 

As first-line seizure control monotherapies, vets most commonly start with either phenobarbital or potassium bromide, but zonisamide and levetiracetam can also be used. Zonisamide is more effective with some types of seizures than others, so the type of seizure disorder will determine if zonisamide monotherapy is appropriate. However, veterinarians will also consider seizure frequency and severity when choosing the most appropriate monotherapy.

Veterinarians would prefer to treat seizures with just one drug. It’s easier on the dog. If one drug is not enough to control seizures, a veterinarian will add a second anticonvulsant. Adding drugs to a primary drug treatment is called adjunctive drug therapy. The list of possible adjunct anticonvulsants is long, but zonisamide is a preferred add-on treatment. 

All antiseizure medications work by slowing nerve signals and making the nervous system less excitable. Zonisamide works slightly differently. While it can make dogs tired or slow-moving, the drug primarily stabilizes nerve activity. Scientists aren’t sure of its exact mechanism of action. They believe zonisamide reduces the electrical conductivity of nerves by decreasing the movement of electrolytes into the nerve. Electrolytes like sodium and calcium are responsible for the electrical firing of nerves. By doing this, zonisamide prevents large clusters of nerve cells from firing at the same time, which is what a seizure is.

Zonisamide lasts a long time in people, so once-daily doses are enough. Its half-life is much shorter in dogs, so they need two doses each day for the medicine to work. 

Is zonisamide safe for dogs?

When used as directed for seizure control, zonisamide therapy is safe for dogs. Based on veterinarians’ clinical experience, side effects are minimal. Unfortunately, side effects and other problems have not really been studied in veterinary medicine. A veterinarian can tell pet parents about reported side effects but may need to discuss possible side effects based on human studies.

Veterinarians never use zonisamide in dogs who:

• Have had an allergic reaction to zonisamide
• Have a known allergy to sulfonamide drugs

Zonisamide can cause heart defects in unborn puppies. Pet parents should be aware of this risk before starting a pregnant dog on zonisamide.

Veterinarians use zonisamide cautiously in dogs with liver (hepatic) problems.

Side effects of zonisamide in dogs

The most commonly reported side effects of zonisamide in dogs are:

• Sleepiness
• Vomiting
• Inappetence
• Listlessness
• Poor coordination (ataxia)

The most serious side effect reported in at least two case reports has been liver damage. Call the veterinarian if you notice signs of liver damage (hepatopathies) including:

• Complete loss of appetite
• Yellowing of the eyes or gums
• Dark or smelly urine
• Decreased energy

Although not reported, zonisamide could theoretically cause other adverse reactions such as high acid levels in the blood or electrolyte abnormalities. Talk to the veterinarian about any possible signs to watch for.

Zonisamide could reduce thyroid function.

Interactions of zonisamide with other pet meds

Before giving a dog zonisamide, pet owners should tell the prescribing veterinary professional about all the drugs, remedies, and supplements being given to the dog. 

Zonisamide’s most serious drug interaction is with phenobarbital. Both drugs are frequently prescribed together as epilepsy maintenance treatments. Phenobarbital speeds up the metabolism of zonisamide, making it less effective. When the two drugs are combined, veterinarians will often double the dose of zonisamide to 10 mg/kg per day.

In people, zonisamide can cause problems when combined with:

• Drugs that cause sleepiness, like narcotic pain relievers, benzodiazepines, sedatives, and CBD
• Drugs called carbonic anhydrase inhibitors, such as topiramate, or drugs that treat glaucoma.

Zonisamide dosage for dogs

The standard veterinary zonisamide dosage for dogs is 5–10 mg/kg, given by mouth every 12 hours. The higher dose is used when the drug is combined with phenobarbital.

Zonisamide is not FDA-approved for use in dogs, but the veterinarian can prescribe veterinary formulations of zonisamide capsules. Unless specially prepared, the standard dosage strengths of both veterinary and human versions of zonisamide are 25, 50, and 100 mg per capsule. 

People can also use a brand-name oral suspension (liquid) of zonisamide if they can’t take capsules. As a brand-name drug, it carries a premium price. If a liquid version is needed, the veterinarian can prescribe a generic oil-based liquid version of zonisamide prepared by a compounding pharmacy.

Can dogs overdose on zonisamide?

Dogs can overdose on zonisamide. A large enough overdose can be fatal.

In people, large overdoses can cause slow heartbeats, low blood pressure, and slow or shallow breathing.

If you see or believe a dog has overdosed on zonisamide, immediately call an animal poison control center or veterinarian. The dog may need to be treated in a veterinary hospital for several days.

Can I give my dog zonisamide every day?

Zonisamide is a long-term treatment that requires daily doses to be taken for months or years. It will take about one or two weeks of constant dosing for the drug to reach steady concentrations within zonisamide’s therapeutic range. Do not miss doses. 

How to give your dog zonisamide

Doses of zonisamide are given twice a day, twelve hours apart. Zonisamide helps prevent seizures, so do not miss doses. If necessary, get an app or set an alarm as a reminder.

• Follow all the instructions given by the veterinarian.
• Do not give more than prescribed or give zonisamide capsules more often than prescribed.
• Do not stop giving the dog zonisamide doses until you’ve talked to the veterinarian. The vet will have to gradually decrease the dose so that the dog doesn’t experience rebound seizures.
• Zonisamide capsules and liquid can be given to a dog with or without food. If the dog vomits or has other gastrointestinal issues after taking a dose, give future doses with food.
• The dog should swallow the capsule whole. If you do not know how to give a capsule to a dog, have a veterinary professional show you the best procedure. 
• Zonisamide liquid may need to be shaken before measuring a dose.
• Always use the calibrated oral syringe or dropper that comes with the liquid to measure accurate doses.
• Give a missed dose when remembered unless it’s almost time for the next dose. In that case, don’t give the missed dose. Give the next dose at its regular time. Do not give the dog two doses to compensate for a missed dose.
• The veterinarian will require follow-up visits and blood tests to monitor the effectiveness of treatment. Please keep all appointments. Keep a seizure diary for the dog and share it with the vet at these visits.
• Keep zonisamide capsules or liquid securely out of the reach of children and pets.
• Store zonisamide capsules at room temperature in a dry place protected from light. 
• Read and follow the storage instructions for zonisamide liquid.

Zonisamide alternatives for dogs

Veterinarians use zonisamide either as a first-choice drug for canine seizures or as an add-on treatment.

If zonisamide alone doesn’t control seizures, the veterinarian may choose to add another drug or switch to another first-line drug, such as phenobarbital, potassium bromide, or levetiracetam.

For refractory seizures—those that can’t be controlled with monotherapy—a veterinarian has a long list of possible drugs to choose from, including: 

• Gabapentin
• Clonazepam
• Valproic acid
• Clorazepate
• Pregabalin
• Felbamate
• Topiramate
• Mephenytoin
• Imepitoin

Summary

Zonisamide treats dogs with seizure disorders. Veterinarians use it alone or in combination with other drugs. When given under the supervision of a veterinarian, zonisamide is safe as a long-term, daily treatment. Adverse effects are minimal and infrequent. Zonisamide can, on rare occasions, cause liver problems, so pet parents should watch for symptoms. Zonisamide could cause heart defects in puppies if given to pregnant dogs.