"NexGard side effects" is one of the highest-traffic safety searches in veterinary medicine, and the results page is a mess of conflicting signals — a real FDA neurologic warning on one side, social-media claims that NexGard is uniquely dangerous on the other, and very little that explains how to actually read the numbers. This article is about that last part: what the label says, what the FDA's post-marketing reports show for afoxolaner (NexGard's active ingredient), and how to interpret those reports so you can have a useful conversation with your veterinarian instead of a frightened one.

This is a side-effects and adverse-event article. For what NexGard covers and the differences between the NexGard family of products, see NexGard Plus for dogs and NexGard Plus vs Simparica Trio. For the class-level neurologic warning, see our isoxazoline safety article, and for choosing between brands, see Bravecto vs NexGard.

Quick answer

NexGard (afoxolaner) is a monthly isoxazoline chew for fleas and ticks. In the field study supporting approval, the most common reactions were vomiting, dry/flaky skin, diarrhea, lethargy, and lack of appetite — mostly mild and self-limiting. As an isoxazoline it carries the FDA warning that it can cause neurologic adverse events (muscle tremors, ataxia, seizures) in a minority of dogs, including dogs with no prior seizure history, and the label says to use it with caution in dogs with a seizure or neurologic history.

In the public FDA adverse-event data, afoxolaner's report profile is similar to other isoxazolines: gastrointestinal signs dominate, neurologic signs are a clear minority (roughly 7% of reports mention seizures), and a large share of reports are actually lack-of-efficacy reports (the product not working), not side effects. NexGard has been on the U.S. market since late 2013 and has held a large market share, which is the main reason its raw report numbers are high — not evidence that it is more dangerous per dose than its peers.

What the label lists

The NexGard (afoxolaner) prescribing information lists the most frequently reported adverse reactions as vomiting, pruritus (itching), lethargy, diarrhea, and lack of appetite. In the U.S. field study (615 treated dogs, 415 on afoxolaner), vomiting was the most common reaction and was generally self-limiting and shorter with later doses. Notably, three dogs with a known seizure history were enrolled in the field study; two of those three had seizures during the 90-day trial.

The post-approval section lists reactions reported in real-world use, in decreasing order of reporting frequency: vomiting, pruritus, lethargy, diarrhea (with and without blood), anorexia, seizure, hyperactivity/restlessness, panting, erythema, ataxia, dermatitis, allergic reactions (including hives and swelling), and tremors. The label says to use with caution in dogs with a history of seizures or neurologic disorders, and notes that safe use in breeding, pregnant, or lactating dogs has not been evaluated.

The isoxazoline neurologic warning

In September 2018 the FDA alerted pet owners and veterinarians that the isoxazoline class — NexGard, Bravecto, Simparica, and later Credelio — has been associated with neurologic adverse reactions including muscle tremors, ataxia, and seizures. The agency stated that seizures may occur in animals without a prior history and, in the same communication, that the class "continues to be safe and effective for the majority of animals." The full reasoning, including the Project Jake owner survey, is in our isoxazoline safety article.

The practical takeaway: if your dog has a seizure disorder or neurologic disease, that history is the single most important thing to discuss before using any isoxazoline, NexGard included.

What the FDA adverse-event data show

VetMedGuide analyzed the public FDA CVM animal adverse-event database (extract dated June 8, 2026). Because brand names are masked in this dataset, we used afoxolaner as the dimension. To isolate the original NexGard chew specifically, we looked at single-ingredient afoxolaner reports in dogs — that is, reports where afoxolaner was the only listed drug, excluding combo products like NexGard Plus (which adds moxidectin and pyrantel). That gave us roughly 43,300 dog reports.

The reaction profile, with the share of reports mentioning each term:

• Vomiting — about 24% of reports
• Diarrhea — about 9.3%
• Lack of efficacy for fleas — about 9.1%
• Lethargy — about 8.7%
• Lack of efficacy for ticks — about 8.6%
• Seizure — about 6.8%
• Skin disorders — about 5.6%
• Pruritus — about 4.3%
• Itching — about 3.8%

A neurologic cluster (seizures, tremors, shaking, ataxia, paresis, muscle weakness, recumbency) appeared in about 10.6% of reports. The 6.8% seizure figure lines up closely with an earlier peer-reviewed analysis of FDA afoxolaner reports from 2013–2017, which reported seizures in 6.9% — a useful consistency check that the underlying data are being read correctly.

How to read these numbers — and how not to

The mistake almost every online summary makes is treating the raw report count as if it were a rate. It is not.

Reports are a numerator without a denominator. We know roughly how many reports were filed; we do not know how many NexGard doses were given to U.S. dogs over the same years, so we cannot compute the true frequency of any reaction. NexGard has been a market leader for more than a decade. A product given to many millions of dogs will accumulate a large raw number of reports even if the per-dose risk of any one reaction is very low.

Reports do not prove causation. A report records what happened after a dose — close enough in time to be worth noting — not what was caused by the dose. Many reported dogs were also on other medications or had other conditions. The FDA is explicit about this in its own post-approval language: "It is not always possible to reliably estimate the adverse event frequency or establish a causal relationship to product exposure using these data."

Reporting is biased. Litigation, news cycles, and social-media attention inflate reporting for some products and some outcomes. A spike in reports after a viral post tells you about reporting behavior, not necessarily about the drug. This is why regulators use disproportionality methods rather than raw counts to look for real signals.

So when you read that NexGard has tens of thousands of adverse-event reports, the correct reaction is not "therefore NexGard is dangerous" any more than it is "therefore the reports are meaningless." The correct reaction is: these reports are why the FDA monitors the class, why the label carries a neurologic warning, and why your individual dog's history matters more than the population total.

Lack-of-efficacy reports are not side effects

Roughly a third of afoxolaner reports involve lack of efficacy — fleas or ticks (or, notably, tick-borne infection with Borrelia, the Lyme disease agent) despite treatment. These feed into genuine surveillance questions about product performance and resistance, which we cover separately in our reported preventive-failure analysis. But they are not adverse reactions and should not be counted as harm when you are weighing side-effect risk.

Monthly duration: a real difference from Bravecto

NexGard is given monthly; Bravecto (fluralaner) lasts 12 weeks. For side-effect management, that difference is not trivial. Afoxolaner's half-life in dogs is much shorter than fluralaner's, so if a dog has a true drug-attributable reaction, the compound clears the body faster and the washout period is shorter. For most dogs this will never matter; for the small minority who react, a shorter-acting product can be easier to manage. There is no antidote for an isoxazoline reaction either way — care is supportive — but faster clearance is a legitimate factor in the risk conversation for a dog who has had a prior reaction to the class.

Most gastrointestinal reactions — the vomiting, diarrhea, and mild lethargy that dominate the report data — are self-limiting and short-lived, and they are often lessened by giving the chew with a full meal rather than on an empty stomach. If your dog vomits the chew, the practical rule is timing: if vomiting happens within about two hours of dosing, the medication may not have been absorbed and you should contact your veterinarian about redosing; if it happens later, the dose has likely been absorbed and the dog is protected. Repeated vomiting, vomiting with blood, or vomiting with lethargy is a different situation — call regardless of timing.

Who is at higher risk

• Dogs with a history of seizures or neurologic disorders — the label's core caution, and the group the FDA warning addresses most directly.
• Dogs who have reacted to a prior isoxazoline dose — a prior reaction is reason to avoid the class, not switch brands within it. See our flea and tick medicine poisoning in dogs article for recognition and response.
• Dogs with known allergy to afoxolaner.
• Breeding, pregnant, or lactating dogs — safe use has not been established.

What to ask your veterinarian

• Given my dog's history — especially any seizures or neurologic signs — is an isoxazoline appropriate, or should we use a different class?
• If my dog has had a reaction to a flea/tick product before, does that change your recommendation?
• What signs should I watch for in the 24–72 hours after the dose, and which ones mean I should call the same day?
• Should I give the chew with food, and what do I do if my dog vomits shortly after dosing?

When to call your veterinarian or an emergency clinic

Seek same-day care — or an emergency clinic after hours — for:

• Seizures, tremors, twitching, stumbling, or loss of coordination after a dose
• Persistent vomiting or diarrhea, especially with blood, lethargy, or not eating
• Facial swelling, hives, difficulty breathing, or collapse (possible allergic reaction)
• Any sudden, frightening change in the days after giving the chew

You do not need to be certain the drug is the cause before calling. Suspected adverse events can be reported to the FDA at fda.gov/reportanimalae or 1-888-FDA-VETS, or to the manufacturer.

Sources

• Boehringer Ingelheim. NexGard (afoxolaner) and NexGard family prescribing information. https://docs.boehringer-ingelheim.com/NexGard_Brand_Products_PI.pdf
• FDA. Fact Sheet for Pet Owners and Veterinarians about Potential Adverse Events Associated with Isoxazoline Flea and Tick Products. https://www.fda.gov/animal-veterinary/animal-health-literacy/fact-sheet-pet-owners-and-veterinarians-about-potential-adverse-events-associated-isoxazoline-flea
• FDA. Animal Drug Safety-Related Labeling Changes (isoxazoline class labeling). https://www.fda.gov/animal-veterinary/drug-labels/animal-drug-safety-related-labeling-changes
• FDA CVM animal adverse-event database, public extract dated June 8, 2026; analysis by VetMedGuide (single-ingredient afoxolaner dog reports).
• Halos L, et al. Survey of canine use and safety of isoxazoline parasiticides (FDA reportable adverse events, 2013–2017). PubMed Central. https://pmc.ncbi.nlm.nih.gov/articles/PMC7738705