Do Popular Dog Tick Treatments Cause Seizures?

Updated December 8, 2022

There’s a new health scare in dogs and cats. Four common flea and tick treatments are being linked with nervous system disorders like muscle tremors, twitching, unsteadiness and even seizures.

Most of the discussion so far has been from the USA, but Australians have a unique perspective on this. The drugs we’re talking about are the best chance we’ve ever had to end the greatest fear of many pet owners: tick paralysis.

Tick paralysis? If like me, you live outside a tick area this might surprise you.

What Is Tick Paralysis?

Ticks are tiny bloodsucking parasites known to spread many blood-borne infections. Tick paralysis is caused by a toxin from the saliva of only certain species of Australian ticks. The map shows where they are found.

Adapted from Roberts FHS (1970) Australian Ticks. Yeerongpilly QLD by TAGS Inc, Bill Conroy & Norbert Fischer [Attribution], via Wikimedia Commons

Click here to learn how to recognise the signs of tick paralysis in dogs & cats.

Tick prevention options are found at our review of flea and tick treatments for dogs. However, it’s not as simple as it looks. The products that rely on being put on the outside of the animal (Frontline®, Advantix® and Seresto®) aren’t thought by most vets to be reliable enough, and are easily disturbed by bathing.

It’s the newer tablet-based Isoxazoline products that are the real game changers. Currently, these are Nexgard®, Bravecto® and Simparica®, and Credelio®. The regular use of these products is thought to give nearly 100% protection against ticks, although checking is still recommended.

But what, I hear you ask, about the side effects?

Simparica, Bravecto, NexGard & Seizures

Seizures in dogs are common, but it does appear that the risk increases when using Nexgard, Bravecto and Simparica. The only question is by how much. This is where life gets complicated.

The US Food and Drug Administration has issued the following public notice:

  • Isoxazoline products have been associated with neurologic adverse reactions, including muscle tremors, ataxia, and seizures in some dogs and cats
  • Although most dogs and cats haven’t had neurologic adverse reactions, seizures may occur in animals without a prior history
  • The FDA considers products in the isoxazoline class to be safe and effective for dogs and cats but is providing this information so that pet owners and veterinarians can take it into consideration when choosing flea and tick products for their pets

Personally, I have seen one dog develop intermittent twitches for two weeks after the second dose, and a puppy have a seizure after a first dose. Both dogs had other significant health issues, but I suspect the isoxazoline was at least partly responsible.

So when do we use these drugs? There are two groups of dogs to consider:

Dogs In Paralysis Tick Areas

For dogs with no history of seizures: use. The risk and consequences of tick paralysis are much higher than the risk of side effects. Other alternative drugs are likely to offer poorer protection.

For dogs with a history of seizures: consider carefully. Simparica, NexGard, Bravecto & Credelio may not increase the number of seizures, but the risk is real. Whether you use it will depend on how likely it is that your dog will get a tick and your confidence in other methods of prevention.

You can find the other methods listed in our page on canine ehrlichiosis. They generally work by preventing ticks attaching, and while less effective, are a lot better than nothing.

Dogs Not In Paralysis Tick Areas

For dogs with no history of seizures: consider carefully. Here you are using isoxazolines for the prevention of fleas and mites only. Personally, I consider it to be so far ahead of the rest that the low risk of seizures is easily justified by better results. I use it on both my dogs but you are welcome to do otherwise.

For dogs with a history of seizures: don’t use. Other products exist which can give nearly the same results. An exception might be for dogs with demodex mites, for which there is no good alternative to isoxazolines.

If a dog does have a seizure while on these drugs, it does not appear to be a disaster. These dogs are likely to be the ones who were always prone to seizures, and should go back to having none or very few once the medication is stopped.

The Cause vs Correlation Issue

We will never know how many of the dogs reported to have twitching or seizures would have started having them regardless. This is hard to say to an owner placed in such a situation, and of course I can never be certain. For a view of a similar debate with a different issue, visit our page called Does Bravecto Kill Dogs?

But the reality is that all drugs do have side effects. Vets are often guilty of not talking about them just because it’s such a can of worms. Without wanting to sound complacent, the reality is that everything has a potential downside. It’s up to vets and manufacturers to supply the information so you can make the best informed decision for your dog.

If you live in a tick area, or travel to one, please use these products if you can. Used correctly, they save countless lives. Vets in tick paralysis areas are seeing a big reduction in the number of cases being treated now that the oral treatments have arrived. That’s fewer prolonged stays in vet hospitals and fewer deaths.

Related: Treatment Of Seizures In Dogs | Causes Of Tremors & Twitching

Valuable advice was provided by Dr Jakki Yeomans, a vet working in a paralysis tick area.

Have something to add? Comments (if open) will appear within 24 hours.
By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. Meet his team here.

45 Replies to “Do Popular Dog Tick Treatments Cause Seizures?”

  1. My dog just turned 14 last week. I switched her to Credelio a few months ago. She’s had 3 doses and is due this weekend for her 4th. She has never had seizures or any related issues. She had 2 seizures last night in the span of 90 min and each were just barely under 2 min each. She is wobbly and off balance on her right side a bit today. She can’t get in to her vet til next week. Do you think the credelio could have caused her seizures? She’s a mixed breed. Australian cattle dog and chihuahua. No other major changes to meds or lifestyle.

    1. Hi Nicole. In your case I don’t believe that the Credelio was the cause of the seizures, as you had given plenty of doses with no problems. Given the age of your dog, there is probably another underlying reason and it’s definitely worth a check up. Of course stop the Credelio as well as we don’t like to give these drugs to dogs with a history of seizures regardless.

  2. I have a 10month Italian Greyhound I gave her first dose of Simparica chewable tick and flea treatment and she’s been shaking for the last week she’s had IV medication to help her and she is on valium to help her stop shaking after a $1400 worth of vet visits I’m extremely upset that this product does not carry any warning label!!!

    1. Hi Shannon. I’m sorry to hear that. Safety information is available for all such products, but you have to look inside the box in most cases and read the product leaflet or insert. This is what it says on the US version:
      Sarolaner is a member of the isoxazoline class. This class has been associated with neurologic adverse reactions including tremors, ataxia, and seizures. Seizures have been reported in dogs receiving isoxazoline class drugs, even in dogs without a history of seizures. Use with caution in dogs with a history of seizures or neurologic disorders.

      1. What can I do for her? She’s on valium and all her blood work is perfect, how long should tremors last for?

  3. My 9 year old male diabetic Labrador retriever received his 2nd dose of Simparica Trio yesterday evening. This afternoon he had what appeared to be a seizure. He kept falling, laid on the floor twitching, drooling, body was stiff, was incontinent of stool. No loss of consciousness. Maybe an hour later started falling again , had some twitching. Now 2 hours later seems fine, ate his dinner. I think it was due to the medication.

    1. Hi Eileen. These drugs do appear to increase the likelihood of seizures, but the dogs themselves seem to be the ones that have an underlying predisposition. Therefore you may see more seizures in the future even if you don’t use Simparica again.

  4. 48 hours after giving Simparica to our 18 month old dachshund he had two mini seizures. I found your post after doing my own research. My research found that hundreds of dogs have experienced seizures and neurological activity as a direct result from this medication. The manufacturer, Zoetis, even paid all of the vet bills for one dog who died from his seizures. We will no longer be using this medication. Hopefully, the manufacturer will do more to warn or even take the medicine off the market.

  5. I bought 80mg of Simparica, I divided it into two I gave the mother 40mg and divided the other 40mg into 6 and gave it to my 6 puppies , hope the mother and the puppies will be fine ?

    1. Hi Bianca. Definitely do not do this. Dividing pills will result in an even drug doses, creating a risk both of poor results or even overdosage. I also suspect that the puppies have been dosed before the first registered use of the drug when it is not known to be safe. For very young puppies, it is sufficient to treat the mother alone with many products but always check the label for specific allowed uses.

  6. Hi Andrew, we live in the Mount Pocono area of ​​Pennsylvania where there are a lot of ticks. I have already given my 14 month old mini poodle Vectra`3D (which seems safe so far) but am still not ready to use the Bravecto given to us by our veterinarian. Is there any information available on how the risk (and intensity) of possible convulsions varies with dose? With millions of doses already sold, at least hundreds of adverse events must be known, right? So there is enough statistical evidence to find out if lowering the dose leads to a lower risk of severe seizures. Maybe it makes sense to try with a small dose first? It could then be repeated “safely” several times until the desired dose was reached. Is there any point in trying?
    Thank you

    1. Hi Alex. I’m not aware of data that shows the prevalence of seizures but my experience tells me it’s uncommon with all drugs. Certainly I would not be using any medication at a dose below that recommended as it is likely to be sub-therapeutic.

  7. We have used Bravecto on my 11y old boxer and he had severe seizures. He is getting an epilepsy treatment now and he still has tremors and small short seizures (1-2sec). It has only been 5 days but I am devastated and still hope for a recovery despite his age. He had problems with his hips and now barely can get up on his own.
    Are there any ways to help this situation? He is receiving Pexion(Imepitoin) 3x 400mg daily. Thank you!

    1. Hi Johann. My only additional advice is that there are at least three other common seizure medications and that it’s often a case of trial and error to find the best drug or combination. The seizures may also settle down once the level of fluralaner in the body reduces.

    2. My puppy has 7 seizures in one night after taking Bravecto chew. He didn’t have any for a week, we thought he had made a full recovery and had clear blood test results yesterday. We were celebrating then last night he had another seizure. We still had some medicine from the last ones so gave him that but not sure if he would have had more. We are also devastated. I hope you dog has recovered.

  8. Hi Andrew. Thank you for this article, it is very helpful. I have an 8 year old kelpie x Labrador. She had her first grand mal seizure in August 2021 at age 7 and has just had her second one yesterday. This one appears to have been more severe with vomiting, urine and passing poo with blood in it. She also took longer to recover from it than she did with the first.
    I have given her nexguard all her life with no adverse reactions seen. I live in northwest aus and the Ehrlichiosis disease is prevalent in my area.
    My vet has done a seizure panel of bloods and everything is clear so we are aware that these seizures could be linked to a more sinister cause. Are there alternative flea and tick measures we can use for her given the risk of where we live?. Thank you.

  9. Hi Andrew,
    My labrador is just 4 and had a grand mal seizure 2 weeks ago. She’s been on Nexgard Spectra all her life. I’m very loathe to give her any preventative in the isoxazoline family after the seizure – though I know it quite possibly was not the cause. She has had and MRI and a spinal tap and nothing was found so IE is suspected. I’ve read and read and read for 2 weeks now and my head is spinning. I want her protected of course but what to use. Would Comfortis Plus be appropriate or should I do each area (heartworm, fleas and ticks, and worms) separately throughout the month. Your input would be invaluable!

    1. Hi Rosemary. The NexGard is unlikely to have caused the seizures (they are well known in labradors) but it is wise to avoid it from now on. Whether you can do so safely depends on the risk of tick paralysis in the area, which I’m afraid only your local vet can advise. Comfortis for example does not treat ticks. Other tick strategies such as Seresto may not be as affective depending on the risk in your area.

    2. My dog was in simparica for some time and I firmly believe it caused her recent grand mal seizures.i would not give a dog with seizures any of those drugs’s playing with fire. I use food grade diatomaceous earth rubbed into her fur now and avoid areas where ticks would be .it’s just not worth the risk . Zoetis who make simparica told me dogs who have been on the meds for ages can still develop seizures so they know

  10. Hi Andrew, my 10 week old Groodle recently suffered a seizure for the first time, and was taken to a vet. 46 hours prior to the seizure, I had given him Simparica trio. All the tests done after this were completely normal. He never showed any alarming signs in the lead up and is a healthy puppy, it seems like it was just a random episode. The vet can’t determine the cause of the seizure. Do you think it was the Simparica?

    1. Hi Nic. The best answer is: we all hope so. If this was my patient, that would be the most likely explanation if all the testing was normal.

  11. Hi Dr Andrew,

    My 5 yo kelpie has seizures (started at 3), and atm nothing will stop them (we’ve got midazolam for emergencies) but he goes into status for hours and presents to the emergency vet with a temp of 42.

    Just wondering if we should take Blue off bravecto to see if this aids in decreasing the severity of his seizures? He’s currently on zosinimide but just wondering would the toxicity from bravecto maybe caused his seizures to be this bad or just pure bad luck? What tick control would you recommend? Thank you


    1. Hi Ash. While it’s most likely that the seizures are unrelated, it’s also possible that isoxazolines like Bravecto may increase their frequency or severity. However, whether you stop it also depends on the risk from tick paralysis in your area. There are other treatments (see a list here) but none are as effective as this class of drugs. Therefore, I would talk to your vet about the risks of each approach. Good luck.

  12. Andrew, I have a curly question. My 5mth old labradoodle suffered from serious tremors, ataxia, fever for 7 days straight until he was put on steroids. He was unable to walk or control his body. He had the works, MRI, X-ray, CT, bloods and lymphosite? testing of some sort. All came back clear. It took 3-4 months of the steroids for him to recover and now, 9m later he’s probably back to 95%. Recently, he seemed to start to show the very early signs of the same thing again so we jumped on it with the steroids and he seems to be ok. One thing I did notice is that the night before his strange behavior happened the first time. He had hide second dose of bravecto. I did some research and decided I’d just be mindful and change him to simparica and see how he goes. He’s been on simparica for about 6m so o figured it was ok. But this second episode has occurred again the day after he had his simparica.
    Coincidence or something to be wary of?
    And if so, what are my other options for treating for heart worm, fleas and ticks?

    1. Hi Sara. There are two questions here. To the first one, yes these drugs do seem more likely to cause tremors or seizures in dogs with an underlying susceptibility. It also seems logical to assume that it’s more likely with 3monthly Bravecto than Simparica or NexGard but this is just conjecture. The fact is that all of them are likely to be linked to neurological signs in some dogs.
      As to your second question of what to do for such dogs in tick paralysis areas, the remaining options are the topical tick repellents. You can find these covered in my page on canine ehrlichiosis. They probably won’t work as well in preventing tick paralysis but they are a lot better than nothing. Good luck.

  13. I have a very small toy breed and one of my male dogs who weighs around 2 kilos has had very occasional seizures since having a near death experience with a toad…..the vet who treated him said he may be more prone to seizures following this. He had never had a seizure prior to the toad incident. Since then I have not treated him with any of the seizure suspect tick/flea treatments but when I gave my other dogs Simparica several weeks ago he had a seizure that day…it seems a huge coincidence, could it be linked as he is in close contact with the other dogs?

    1. Hi Deborah. It does seem like quite a coincidence but I think that’s all it is as I can’t see any way he could have been exposed to sufficient drug concentrations. Even in seizure-prone dogs, Simparica doesn’t always cause them to happen even if a whole dose is given.

  14. Hi Andrew
    I just wanted to say thank you for your unbiased and thorough information on flea, tick and worm treatments.

    I am concerned in moving from a topical treatment (which alienates my dog from me monthly when she sees me coming with it!) to an oral tablet. She was on Nexguard Spectra as recommended by the vet for her 3rd and 4th month, and then I switched her off it given she was vomiting after each treatment.

    Whilst I know there are no guarantees, your information has helped me feel a little better about making the switch and to watch out for symptoms after her first couple of treatments with Simparica Trio.

  15. Hi Andrew
    Can Simparica cause bloody diarrhoea and bloody vomit? My dog has had both for 24 hours.
    I swapped from Nexgard to Simparica for the first time yesterday but my dog was also on the anti inflammatory Previcox for his CCL he ruptured 2 days ago. We have stopped the Previcox in case it is causing the upset but now I am wondering about the Simparica.
    My dog is a gorgeous 3 year old Australian Terrier.
    Many thanks

    1. Hi Anna. Definitely, if it’s drug-related, the Previcox is the most likely culprit. NSAIDs are known for this and you are right to stop it.

  16. Hi Andrew, thank you your website is excellent and a great support for pet owners.
    Our Greyhound developed head tremors three days after a dose of Bravecto which he had been on for 3 years. It lasted 10 days and I haven’t seen it since (3 months). I am aware that Nexgard and Bravecto both contain isoxazolines but it’s been suggested that Nexgard may still be safe for him as he gets a smaller amount in each dose. Do you think a regular monthly dose might lessen his risk of reaction? Are there any internal medications that don’t contain isoxazolines? I can’t work that out from the table. I don’t want to use an external treatment that will rub off on humans.

    It’s been three weeks without flea protection now and I am concerned about the risk of fleas/tapeworm for my baby and children!

    Thank you

    1. Hi Rebecca. NexGard, Credelio and Simparica are all monthly isoxazolines. You are probably correct that these will have a reduced rate of adverse effects but of course, there’s no evidence for this. Comfortis is a nearly equally effective oral alternative (as long as you don’t need tick or mite control too). Its major adverse effect is vomiting, but only in a percentage of dogs.

  17. Hi Andrew can you please suggest what would be the most appropriate course of action since I want to avoid products containing isoxazolines as my dog has epilepsy and takes phenobarbital. Would Adavantix be appropriate and which wormer would pair well? Thank you for your time.
    Kind regards

    1. Hi Susana. It all depends on what you’re trying to prevent. For ticks, there’s clearly no equivalently effective product, so you need a balanced mix of preventative strategies and products like Advantix, Frontline or Seresto. For fleas and mites on the other hand you have plenty of options listed here.

  18. Hi Andrew,
    I have a four year old border collie, he has recently had two seizures within the last 6 months, both happening a day or two after giving him nexguard.
    He once happily took it but around a year or so ago he stopped wilfully taking it and we had to hide it in treats. The first was a horrible site to see we took him to the vet and he said to monitor it thinking another would occur within a day or two. He didn’t have another for 7 weeks, two days after his next dose, since then it’s been around 9 weeks, we haven’t given him another dose and he’s been seizure free.
    We live on the far south coast of nsw in heavy tick areas, I want him to be protected but am wary at doing that at the cost of more seizures. What are your recommendations?
    Thank you.

  19. Hi Andrew I bought some heart guard by Mistake. Can mix Heart guard with simparica for the monthly treatment of ticks and heart worms. Cheers Nino

  20. Thank you for these Andrew. I live in North Qld and visit regularly the Atherton Tablelands where paralysis ticks are extremely common. I give brevecto to my dog and have for 4 years without issues. So it is nice to read that he is unlikely to suddenly develop issues.

    The breeder I am looking at getting a pup from is heavily against the use of any tablet type tick prevention. But they are in a lower risk area and with the seresco collar have had a paralysis tick and almost lost their dog. You mentioned maybe aiming for something with a monthly dose instead of the 3 monthly. My vet favours brevecto, but likely due to the commonality of people forgetting to give monthlies. What products are monthly and would be most recommended to balance the breeders reluctance with my concern re: shaggy dog and ticks!

    1. Hi Lulu. The two monthly options are Simparica and Nexgard. Neither alas is likely to be favoured by your breeder, but then I’m not much good at breeding dogs either!

  21. Hi Andrew, thank you for your very informative article. I have a 3 & 1/2 year old Westie who had heart surgery at 6 months old and still has a serious heart murmur and is on 1/3 of 50mg tablet twice a day of Atenolol. We live in SA but travel at least twice a year to Qld and NSW with our dogs and so far have not used any prevention but we are vigilant with where they go. Each time we travel I worry, what is your opinion on using one of these 3 products on my boy to keep him safe? Thank you and regards, Sandy

    1. Hi Sandy. My opinion is that they are as safe as any other product, and much safer when in a tick area due to their increased efficacy. In the rare event of a seizure, I have never seen one cause problems in a dog with heart disease.

  22. Hi Andrew, just wanted to say thank you for your article which I found thorough and balanced. I see a FB article regarding Simparica and was concerned for my small one year old dog who uses the brand. We live on the back of the bush in a tick prone area so treatment is VIP. Quick question, in your opinion or experience…. if they have been using this and havent presented any adverse reactions is it likely that it wont develop in the future?? Ps. I had a lab with horrible seizures, just dont want to go back there…it was so awful for the poor dog. Thanks for your time

    1. Hi Angela. Our experience is that dogs that develop side-effects get them with the first or second dose. This is no doubt not universal but seems true in the majority of cases. My dogs are both on Simparica and I have very little concern as they have been taking it for some time.

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