NexGard is marketed as a monthly flea and tick prevention that comes in the form of a convenient chewable treat. One would assume that a product put on the market with the intent to protect dogs would never harm them, but NexGard doesn't quite fit that profile. Ever since it got approved for veterinary use, it has been catching heat from dog owners who have seen horrific adverse effects in their dogs — including death.
Most dog owners only become aware of NexGard's dark side after it's already too late for their dogs. If you are hearing about this product for the first time or swear by it as your go-to flea and tick prevention, this article sheds light on how NexGard can harm your dog and why you should feel comfortable saying no if your veterinarian suggests it.
The active ingredient in NexGard is called afoxolaner — a pesticide from a class of chemicals called isoxazolines. Other FDA-approved products in the isoxazoline group that target ticks and fleas in pets are Bravecto and Bravecto Plus (tablets and topical solutions for cats and dogs), Credelio (tablets for cats and dogs), Simparica and Simparica Trio (tablets for dogs), and Revolution Plus (topical solutions for cats).
A Pesticide That Targets the Central Nervous System
Isoxazolines are neurotoxic; they work by targeting the insect's central nervous system, which quickly leads to death. NexGard's package insert emphasizes “selective toxicity” of isoxazolines, stating they are only toxic to insects and acarine, not mammals. And yet, almost a decade of reported adverse effects by dog owners and veterinary professionals shows a different truth. Isoxazolines are neurotoxic to more than just insects; they also affect mammals. There is a reason why so many dogs experience seizures, ataxia, and tremors after taking NexGard — the pesticide compromises the dog's central nervous system. Once this happens, there is often no way back.
NexGard's cross-species neurotoxicity creates a dangerous gamble for dog owners who opt to give their dogs this product. One dog owner I spoke to compared it to Russian roulette. Her Sheltie, previously a healthy agility dog, started seizing after NexGard. “I talked with his breeder, and no others in his lines have had epilepsy and none since. His sire is a silver grand champion, dam grand champion, and numerous others in his lines are also champions.
So, many dogs were bred throughout his lines with no epilepsy. In fact, I know lots of dogs and owners in those lines all across the country. So, why my dog? […] Well, after reading the dangers and understanding how Nexgard works killing fleas, it made sense how it could have the same effect on my dog. From what I know from others, it's a bit like Russian roulette. It's fine until it's not, and you won't know till it's too late and there is no antidote to reverse it. The damage is done.” — Melanie and her dog Ely
What Does NexGard's Package Insert Say?
NexGard came on the market in 2013 and 2014. In 2018, due to consistent reports of serious adverse effects, all companies that manufacture flea and tick prevention products based on isoxazoline had to change their labels and include a clear warning about potentially irreversible side effects to the nervous system.
The package insert for NexGard says: “Afoxolaner 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. Therefore, use with caution in dogs with a history of seizures or neurologic disorders. The safe use of NexGard in breeding, pregnant or lactating dogs has not been evaluated.”
There are two sections of adverse reactions listed on the package insert. The first are the adverse reactions observed in a 90-day pre-approval study (conducted by NexGard, of course), which include vomiting (with and without blood), dry and flaky skin, diarrhea (with and without blood), lethargy, and anorexia. In addition, two dogs experienced seizures, both with a previous history of seizures.
The second section lists adverse effects most commonly reported directly by dog owners in the 4-year period after approval. “The following adverse events reported for dogs are listed in decreasing order of reporting frequency for NexGard: Vomiting, pruritus, lethargy, diarrhea (with and without blood), anorexia, seizure, hyperactivity/restlessness, panting, erythema, ataxia, dermatitis (including rash, papules), allergic reactions (including hives, swelling) and tremors.”
Notice how high on the list seizures are. They are the 6th most reported side effect!!! The most-reported one is vomiting, which is very telling. Vomiting is a natural way of purging the substance that's disrupting your dog's stomach. Throwing up the pesticide could actually protect him from the adverse effects if the pesticide is completely eliminated. The packaging insert encourages dog owners to redose the dog with a full dose if he throws up within two hours of getting NexGard — I implore you to trust the dog's natural defense system instead and never give him another dose.
What about protection from tick-borne diseases?
Don't the potential benefits of NexGard outweigh the risks? Well, according to the official product information: “Parasites need to start feeding on the host to become exposed to afoxolaner; therefore, the risk of the transmission of parasite borne diseases cannot be excluded.” NexGard can only kill fleas and ticks once they've already bitten your dog, so if a tick happens to carry a disease, it can still pass it on to your dog. NexGard doesn't keep fleas and ticks away from your dog — it just kills them once they're already feeding on your dog. It's not a repellent; it's a pesticide for existing infestation.
Adverse Effects in Official FDA Numbers
The FDA (Food and Drug Administration) and EMA (European Medicines Agency) collect reports of adverse reactions to veterinary drugs and products they approved. The FDA issued a public warning to dog owners and veterinarians who consider using isoxazoline products: “The FDA is alerting pet owners and veterinarians of the potential for neurologic adverse events in dogs and cats when treated with drugs that are in the isoxazoline class. […] 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.”
Since NexGard doesn't include the numbers on their label, and the FDA doesn't include them directly in their warning, I think it's important to share them here, so you can understand how many dogs are hurt by these chewables. These side effects are not isolated events — they are a pattern. Between January 2013 and January 2017, the FDA found 1,728 reported seizures and 801 reported deaths caused by isoxazoline products. NexGard, specifically, was responsible for 341 of those deaths.
Vomiting was the most reported isoxazoline side effect, experienced in 13,251 dogs. It was followed by behavioral issues (9,266), skin issues (7,502), and anorexia (4,639). These numbers only cover cases in the USA that have actually been reported in the first 4 years after most isoxazoline products hit the market. Today those reported numbers are even higher. The FDA makes it very difficult to find the exact numbers, and it's only possible to find them for active ingredients in veterinary drugs, not for individual brand names.
According to their records, afoxolaner (the active ingredient in NexGard) has caused 1,261 deaths by the time of writing this article in April 2022. NexGard Spectra contains milbemycin oxime in addition to afoxolaner — this pairing is responsible for 6,455 reported deaths to date. The EMA's numbers are equally horrifying. Between January 2013 and January 2019, they recorded 6,272 reported seizures and 5,556 reported deaths caused by isoxazoline products.
Here's my question for the FDA and EMA: how many dogs have to die for a product to be considered unsafe? If thousands of deaths are not enough, and the number is consistently rising with each passing year, I shudder to think of how many dogs have yet to lose their life because of this. How can thousands of deaths directly related to a product that advertises protection result only in adding a small warning to the label? A label that most people don't even get to read!
From Adverse Effects to Broken Hearts
Thousands of dead dogs means thousands of broken hearts, thousands of traumatized owners whose trust in veterinarians is forever compromised, thousands of people mourning losses that never should have happened! Sue, a dog owner who lost her Cavalier King Charles to seizures after NexGard, shared her story. “I was distraught,” she says. “Phoned vets to get him seen to. I called my partner as I was a wreck. By 4:15, he started fitting again in my arms and continued fitting for 20 minutes — he was dying in my arms. He stopped fitting for 15 minutes and started fitting again in the surgery, and died in my arms. That was 6 years ago; his death has scarred me, and I still mourn his loss. […] I didn't know any better than to trust what I was being told by my vets, and I didn't know then what those pills cause. It was a couple of years later that the pieces of the puzzle came together — I gave my boy that 1 dose of poison that killed him.”
Brenda's Golden Doodle started seizing after NexGard when he was only 14 weeks old, and he passed away 7 months later. She tells me about the experience: “We tried everything, from detox to every anti-seizure medication and nothing helped. He continued to decline until he had more bad days than good. 7 months of not sleeping and hardly eating while I tried desperately to save my dog left me with what I believe is PTSD. That was almost two years ago, and I still have insomnia and anxiety. It was horrific and the worst thing I have ever gone through in my life!!”
EMA's reports also show that over 55% of seizures and 60% of deaths occurred in dogs over the age of 5, which means that isoxazoline flea treatments can be risky even for dogs who haven't reacted to them before, but could have a reaction as the years progress. “My Brody is one of the lucky ones,” Ginger tells me about her dog, who was on NexGard his whole life before it started causing problems at 7 years old. “Once I connected his seizures to Nexgard after basically one whole year, I stopped the meds. He has been seizure-free now for 1 1/2 years.”
Independent Study Shows Majority of NexGard Users Experience Adverse Effects
The most recent independent study into the adverse effects of isoxazoline-based flea and tick chewables was published in 2020. The results analyze survey answers from veterinarians, pet owners, and other pet caregivers. 1,325 people reported using isoxazoline products — of those, 911 people used Bravecto (fluralaner), 342 used NexGard, and the rest used Simparica (sarolaner). 87% of those who reported using Bravecto, 69% of those who used NexGard, and 61% of Simparica users experienced an adverse reaction in their dog. This shows, once again, that the adverse effects of isoxazoline products are not a rare exception. In fact, if you are reading this article as an avid user of these brands and haven't seen a reaction in your dog yet, chances are you are the exception.
From most common one to least common one, the reported side effects of NexGard were: lethargy and depression, loss of appetite, vomiting, diarrhea, itching, restlessness and anxiety, panting, weakness, shaking and trembling, ataxia (loss of coordination and muscle control), seizures, death, abnormal stools, excessive drinking, weight loss, hair loss, decreased water intake, excessive urination, and flatulence. Note how seizures and death, again, rank very high. 12% of surveyed NexGard users lost their dogs! It is also interesting to see a broader range of side effects observed by dog owners than are disclosed on the package insert or even in the official FDA reports.
If NexGard Is Toxic, Why Do Vets Recommend It?
NexGard and other isoxazoline products cannot be purchased without a vet's prescription, and the first dose is often given directly by the veterinarian. Most people don't question veterinarians when they prescribe a product, and why would they? A certain level of trust in their expertise is implied just by being there. I want to emphasize that I'm not attempting to take a dig at veterinarians with this commentary — they have a lot of knowledge and a ridiculously tough job that is physically and emotionally taxing.
What I am trying to do here is highlight a chink in the armor! Companies that sell isoxazoline flea and tick products have to market to veterinarians before they can market to dog owners. They need to convince vets to prescribe and recommend their product over all others. When a new product hits the market, the only information about it comes directly from the manufacturer — usually in the form of a carefully curated, elegantly worded, questionably researched, and informationally depleted package insert.
A larger conversation is desperately needed about how much independent research veterinarians should do into the products they are routinely prescribing. One of the most significant pieces of reliable information veterinarians can get on any product is client feedback! There needs to be a level of empathy and understanding for their clients' intuition regarding their dogs. The majority of dog owners who shared their NexGard stories with me were met with doubt and denial by their veterinarians.
Nadine's Frenchie was 6 months old when he started seizing after NexGard. “Vet said it was idiopathic. Spent a lot of money running tests to see what was wrong with him. […] I asked the vet if Nexgard could be the cause and I was reassured, NO. We gave him the 2nd dose, and he had a 2-hour long seizure; it was so bad we couldn't get him in the car to get to the vet. When we arrived, he was brain dead, couldn't walk, blind and deaf. I don't think he heard us say goodbye. […] He was a five thousand dollar dog that had zero health issues, not even worms. His DNA tests were beautiful. He was the sweetest boy on the planet. I was unaware of the risks, and I trusted our vet. They lied to us, and I lost my puppy. After he died, the vet dropped us when I asked why they didn't err on the side of caution.”
“It's been hard for him and us,” says Dimitri, whose American Cocker Spaniel is currently battling severe seizures because of NexGard. “Apart from the actual physical impact … which is a real battle, you also need to “fight” most vets, specialists — heck, even friends and family — who just keep on denying the fact these kinds of pesticides can really harm our and their companions. Reactions like “there must have been something wrong with the dog anyway, send him back, put him out of his misery,” etc. have been so common. I've no words anymore for what we've seen, heard in the last year.”
A few dog owners did tell me their veterinarians were sympathetic and reported the adverse reactions. Official FDA numbers show 33,182 veterinarians reported adverse effects of afoxolaner + milbemycin oxime (NexGard Spectra), and 8,625 veterinarians reported them for afoxolaner (NexGard). The majority of reports remain filed directly by the owners, though, which is why veterinarians must start listening to their clients, take them seriously, trust in the owners' knowledge of their dogs, diligently report all adverse effects, and stop gambling with these dangerous products.
A public Facebook group with over 34 thousand members is a community of people sharing their heartbreaking experiences with isoxazoline products and discussing their natural alternatives. The group was created by Carol, whose Boxer Finny got paralyzed after his first dose of NexGard when he was 7 years old. He never fully recovered and unfortunately passed away from seizures 3 years later. This experience prompted Carol to create the group. In her own words: “I couldn't save my boy. This is why I started this FB group, hoping to help save others and their furry family members from going through the same horrors our boy endured.”
On this note, I want to thank the dog owners who entrusted me with their heartbreaking stories. I wish I could highlight all of them in this one article; I never expected to receive so many. I dedicate this article to you and your dogs — both the warriors and the angels.
Safe Solutions to Flea and Tick Prevention
The silver lining lies in the fact that there are so many natural ways to prevent fleas and ticks, ways that work with the dog's immune system rather than against it. 100% natural flea and tick repelling shampoos and sprays can be purchased in many pet stores or online, but you can easily make your own products as well. This article shares exactly how you can protect your dog from parasites without ever having to worry about pesticides again.
Palmieri, Valerie. Dodds, W. Jean. Morgan, Judy. Carney, Elizabeth. Fritsche, A. Herbert. Jeffrey, Jaclyn. Bullock, Rowan. Kimball, P. Jon. “Survey of canine use and safety of isoxazoline parasiticides.” Veterinary Medicine and Science, 02/06/2020.
Luna’s passion for learning about canine psychology and behavior began when she adopted a severely reactive puppy from a local shelter. She is now a big advocate for positive reinforcement and compassionate training. As a writer, she strives to spotlight the topics that fly under the radar and be the voice for all who cannot speak for themselves.