Zonisamide for dogs is an anticonvulsant (i.e. medication to control seizures) that veterinarians may use by itself or in combination with another anticonvulsant. Learn more about this medication, which is gaining popularity in veterinary medicine. Integrative veterinarian Dr. Julie Buzby explains the mechanism of action, uses, side effects, contraindications, and drug interactions for zonisamide for dogs.
Watching your dog have a seizure can be terrifying and heartbreaking. Time and time again, I have heard dog parents say, “I hope I never see another seizure.” I wish I could assure them that would be the case. But unfortunately, many dogs will go on to have more seizures. This doesn’t mean you should lose hope, though.
While anticonvulsant medications may not always be able to ensure that your dog never has another seizure, they can go far in decreasing the frequency, length, and severity of the seizures.
Some of the most common seizure-management medications are names you might have heard before— phenobarbital, potassium bromide, and levetiracetam (Keppra®). But your veterinarian may also prescribe other anti-seizure medications like zonisamide for your dog.
What is zonisamide for dogs?
Zonisamide is the generic name for a human prescription anticonvulsant medication (i.e. medication to control seizures), which veterinarians sometimes use off-label to treat seizures in dogs and cats. It is sold under multiple brand names, the most common of which is Zonegran®.
Interestingly, zonisamide is part of the sulfonamide drug class (i.e. sulfa drugs). Most sulfonamide drugs have anti-microbial properties and are commonly prescribed for pyoderma in dogs (i.e. skin infections) or UTIs in dogs. Zonisamide, however, varies slightly in its chemical structure. Therefore, even though zonisamide is a “sulfa” drug, it is used to control seizures, not infections.
Scientists are still unsure exactly how zonisamide works. But they believe it suppresses the action of the sodium channels and calcium channels that help transmit nerve impulses. As a result, the dog is less likely to have abnormal or repetitive firing of large groups of neurons, which is what is thought to contribute to seizure activity.
Plus, zonisamide may also help facilitate the transport of neurotransmitters like dopamine and serotonin. Both of these signaling molecules help with relaxation.
What is zonisamide used for in dogs?
If your dog has seizures, the vet may use zonisamide in one of two ways. Sometimes he or she may prescribe zonisamide as a sole agent (i.e. the only anticonvulsant medication your dog is taking). While other anticonvulsants like phenobarbital tend to be the mainstay for seizure control, not all dogs do well on them or can safely take them. In those situations, zonisamide may be a great choice as the primary anticonvulsant.
Alternatively, the vet may add zonisamide to your dog’s existing anti-seizure drug regimen to obtain better seizure control. For example, if a dog was already taking phenobarbital but still had too many seizures, the vet may decide to combine zonisamide and phenobarbital. Or the vet may use zonisamide and Keppra together, or zonisamide and a different anti-seizure medication.
What is the zonisamide dosage for dogs?
If your vet decides to prescribe zonisamide for your dog, he or she will discuss the dosage instructions with you. Like many medications, there is a dosage range for zonisamide and a maximum dose above which side effects are more likely. The vet will calculate your dog’s dose based on his or her weight. And he or she will also take into account whether your dog will take zonisamide alone, or in combination with another anticonvulsant.
Zonisamide is available in 25 mg, 50 mg, and 100 mg capsules. If your dog’s dosage doesn’t fit well with those capsule sizes, or you need liquid zonisamide because your dog won’t take pills, it is possible to have zonisamide compounded into the appropriate size or form. However, the amount of zonisamide in the compounded forms is variable compared to the commercially available options.
Typically, zonisamide works best when given every 12 hours. But sometimes once-a-day dosing can also effectively treat seizures. Your dog can take the zonisamide with or without food—whichever is best for him or her.
Caution: Do not stop giving zonisamide without talking to your vet. Rapidly stopping the zonisamide can lead to rebound seizures. Thus, your vet may recommend weaning your dog off the zonisamide.
How effective is zonisamide for dogs?
Since zonisamide is relatively new on the veterinary scene for treating seizures in dogs, there aren’t a lot of large studies looking at efficacy. However, in a study of 10 dogs with idiopathic epilepsy who were treated with zonisamide, 60% (six dogs) showed at least a 50% decrease in the monthly number of seizures. Of the remaining four dogs, 20% (two dogs) had an increase in the number of seizures and two dogs had no change in the number of seizures.
This is such a small sample size that it is hard to draw concrete conclusions. However, anecdotally, it does seem that zonisamide is a safe and effective option for many dogs with seizures. Hopefully, studies with a larger sample size in the future will be able to give us more information about efficacy.
Note of interest: Remember what I said at the beginning about how the goal is to reduce the severity, duration, and frequency of the seizures? The “success threshold” in this study (at least a 50% decrease in seizures) is a good example of how effectively controlling seizures doesn’t always mean your dog will never have another seizure again. In some cases, it may mean your dog doesn’t have them as often or they don’t last as long.
How long does zonisamide take to work in dogs?
The good news for dogs with seizures is that zonisamide can start working within the first two days of administration. However, blood levels can greatly fluctuate in the beginning. It takes about one to two weeks before zonisamide achieves a “steady state” (i.e. consistent levels) in your dog’s body.
Therefore, it is generally a good idea to wait at least two weeks after starting zonisamide or after changing the dose before deciding how effectively it is managing your dog’s seizures. Prior to that point, your dog may continue to experience some breakthrough seizures due to the zonisamide level fluctuations.
However, if your dog is having multiple seizures in a day, severe seizures, or seizures that last more than five minutes, please check in with your veterinarian. Also, contact your vet if you think you might be seeing any side effects of zonisamide.
What are the side effects of zonisamide?
Thankfully, zonisamide is generally safe for dogs, and its side effects are typically mild. The most common side effects include:
- Mild sedation or drowsiness
- Loss of appetite
- Vomiting (Giving zonisamide with food can decrease GI upset)
However, rarely dogs can have more significant side effects such as:
- Drug-induced hepatopathy (i.e. liver disease in dogs)
- Renal tubular acidosis (a rare kidney-related condition)
- Thyroid gland suppression leading to hypothyroidism in dogs (most commonly associated with high doses of zonisamide)
- Bone marrow disease
- Kidney stones
- Aggression
If you see any of these side effects, please contact your veterinarian immediately for advice.
At the typical therapeutic doses, zonisamide is safe for most dogs. But at higher doses, dogs may start to develop more adverse reactions. Signs of an overdose of zonisamide include:
- Vomiting
- Neurological signs
- Stumbling
- Slow heart rate
- Slow breathing
- Tremors
- Coma
If you suspect your dog received an overdose of zonisamide, or the wrong dog ate the zonisamide, please contact your veterinarian or ASPCA Animal Poison Control Center immediately for advice.
Which dogs should not take zonisamide?
While we have just established that most dogs tolerate appropriate doses of zonisamide well, there are some dogs who should not take zonisamide or should only take it with care. Zonisamide should not be prescribed for dogs who are sensitive to sulfonamides. The veterinarian must closely monitor dogs with liver disease or kidney failure in dogs when taking zonisamide. Additionally, pregnant dogs should not take zonisamide as it can cause severe heart defects in the unborn puppies.
Are there medication interactions with zonisamide?
Thankfully, there are very few medications that cannot be taken at the same time as zonisamide. When used with other nervous system depressants (e.g. sedatives or antihistamines like Benadryl for dogs), zonisamide can cause even more pronounced depression or sedation. And when combined with carbonic anhydrase inhibitors (i.e. medications that treat glaucoma in dogs), zonisamide may increase the risk of kidney stone formation.
As mentioned earlier, it is generally safe to combine zonisamide with other anticonvulsants like Keppra, phenobarbital, or potassium bromide. However, because phenobarbital increases the rate of removal of zonisamide from the body, dogs who take both drugs often need higher doses of zonisamide than dogs who only take zonisamide.
How should you monitor your dog who is taking zonisamide?
As with most medications that your dog will take long-term, monitoring is important. At home, you should keep a seizure log, which includes:
- Date and time of each seizure
- Duration of the seizure
- Short description of what you saw (e.g. paddling, loss of bladder control, eye movements, etc.)
- Any other observations about events or changes that may have contributed to the seizure
- Typical medication schedule and timing of seizures relative to medication administration
Bring this log to every recheck vet appointment. And contact your vet if your dog’s seizures are worsening or not responding.
Sometimes dogs will initially respond well to zonisamide and then become tolerant of it over time, leading to loss of seizure control. Others may need a higher dose to manage their seizures right from the start. And sometimes zonisamide just isn’t the right medication for a particular dog. All the information you collect about your dog’s seizures can help your vet determine how well the zonisamide is working.
In addition to evaluating your dog’s seizure log, your vet may also recommend performing bloodwork and a urinalysis on a regular basis. Typically, dogs on zonisamide should have a CBC and blood chemistry every six months or so. This allows the vet to look for elevated liver enzymes, bone marrow suppression, or other concerns.
Plus, the vet may also want to check your dog’s blood zonisamide levels periodically. For example, the vet may check blood levels several weeks after starting zonisamide or changing the dose. If this is the case, the vet will give you specific instructions about when he or she wants to draw the blood relative to giving the dose of zonisamide.
Can zonisamide help your dog?
If your dog just started having seizures or the seizures aren’t well-controlled, it’s easy to feel scared or helpless. However, the good news is that there are more and more options for controlling seizures in dogs all the time.
Zonisamide can be a great choice, either as the only anticonvulsant or in combination with another anticonvulsant. It is safe for most dogs, typically has minimal side effects, and is effective in many situations. Hopefully, zonisamide and a close partnership with your vet will be just the ticket to help get your dog’s seizures under control.
