Bravecto (fluralaner) is one of the most widely used flea and tick chews for dogs, and "Bravecto side effects" is one of the most searched safety questions in pet care. The concern usually comes down to three things: vomiting or diarrhea after a dose, the neurologic warning the FDA attached to the whole isoxazoline class, and the long 12-week duration that makes owners nervous if their dog does react. This article covers what the label lists, what the FDA's post-marketing data actually show, and how to read those reports without over- or under-reacting.

This is a side-effects and adverse-event article. For what Bravecto covers, dosing, and patient fit, see our Bravecto for dogs guide; for the class-level neurologic warning across Bravecto, NexGard, Simparica, and Credelio, see our isoxazoline safety article. If you are trying to choose between products, see Bravecto vs NexGard.

Quick answer

Bravecto is an isoxazoline-class parasiticide given as a chew every 12 weeks for fleas and ticks. In the controlled field study used for approval, the most common reactions were vomiting (7.1%), decreased appetite (6.7%), lethargy (5.4%), and diarrhea (4.9%) — all mild and self-limiting in most dogs.

Like every isoxazoline, Bravecto carries an FDA warning that it can cause neurologic adverse events — muscle tremors, ataxia, and seizures — in a minority of dogs, including dogs with no prior seizure history. In the public FDA adverse-event data, Bravecto's report profile is dominated by gastrointestinal signs, with neurologic signs in a smaller share. The reports are real and worth taking seriously; they are also not a measure of how often these things happen in the tens of millions of doses given each year.

What the label lists

The Bravecto (fluralaner) Chews prescribing information reports these reactions from the U.S. field study (224 dogs dosed every 12 weeks over 182 days), in decreasing frequency:

• Vomiting — 7.1%
• Decreased appetite — 6.7%
• Lethargy — 5.4%
• Diarrhea — 4.9%
• Polydipsia (increased drinking) — 1.8%
• Flatulence — 1.3%

The post-approval section adds reactions reported in real-world use, in decreasing order of reporting frequency: vomiting, lethargy, diarrhea (with and without blood), anorexia, pruritus, polydipsia, seizure, allergic reactions (including hives, swelling, erythema), dermatitis, tremors, and ataxia. The label also notes that in some cases birth defects, stillbirth, and abortion have been reported after treatment of breeding females — which is why Bravecto is not recommended for breeding, pregnant, or lactating dogs.

The isoxazoline neurologic warning

In September 2018, the FDA issued an alert across the isoxazoline class — Bravecto, NexGard, Simparica, and later Credelio — stating that these products have been associated with neurologic adverse reactions including muscle tremors, ataxia, and seizures in some dogs and cats. The agency was explicit that seizures may occur in animals without a prior history, and equally explicit that the class "continues to be safe and effective for the majority of animals." Manufacturers were required to add neurologic labeling.

That warning is the single most important context for any "Bravecto side effects" search, and it is covered in detail in our isoxazoline safety article. The key practical point: if your dog has a history of seizures or neurologic disease, that history changes the risk conversation, and the label says to use the class with caution in those dogs.

What the FDA adverse-event data show

VetMedGuide analyzed the public FDA CVM animal adverse-event database (extract dated June 8, 2026) for reports in dogs listing fluralaner. (Brand names are masked in this dataset, so the active ingredient is the reliable dimension.) We found roughly 64,000 dog reports involving fluralaner — a large number that reflects Bravecto's market position and its presence on the U.S. market since 2014, not a high rate of harm per dose given.

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

• Vomiting — about 31% of reports (the dominant term; "emesis" adds another ~8.5%)
• Lack of efficacy for fleas — about 20% (a product-effectiveness question, not a side effect — see below)
• Lethargy — about 7.6%
• Lack of efficacy for ticks — about 7.3%
• Diarrhea — about 6.7%
• Seizure — about 4.7%
• Shaking — about 1.5%
• Ataxia — about 1.5%

A neurologic cluster (seizures, tremors, shaking, ataxia, paresis, muscle weakness, recumbency) appeared in about 7.8% of reports. Roughly 27% of reports listed a second drug, which matters for interpretation.

These figures are directionally consistent with an earlier peer-reviewed analysis of FDA reports from 2013–2017, which tallied about 16,900 fluralaner reports with seizures in 2.8% and deaths in 2.5%. The total report volume has grown several-fold since then as use has expanded; the pattern — GI-dominant, with neurologic signs in a clear minority — has stayed the same.

Why you cannot read these reports as a rate

This is the part most summaries skip, and it is the difference between being informed and being frightened.

Adverse-event reports are a numerator without a denominator. We know how many reports were filed; we do not know how many doses of Bravecto were given across the entire dog population during the same period, so we cannot compute a true rate. A product that is given to millions of dogs will accumulate a large raw number of reports even if the per-dose risk is very low.

Reports also do not establish causation. They capture what happened after a dose — close enough in time to be worth recording — not what was caused by the dose. About a quarter of fluralaner reports in our analysis involved a concurrent medication, and many reported dogs have other conditions. Vomiting after a chew could be the drug, the flavor base, an empty stomach, motion sickness, or coincidence. The FDA is explicit about this limitation in its own post-approval language.

Finally, reporting is voluntary and biased. Attention, litigation, and social-media attention all inflate reporting for certain products and certain outcomes. That is why the FDA uses methods like disproportionality analysis rather than raw counts to judge signals — and why a high raw count of vomiting reports does not, by itself, mean Bravecto is unusually vomit-inducing.

"Lack of efficacy" is a different question

About one in five fluralaner reports in our analysis were lack-of-efficacy reports — fleas or ticks still present despite treatment. These are important (they feed into reported preventive-failure surveillance and resistance monitoring), but they are not side effects and should not be read as harm. When you see Bravecto's total report number, remember a large fraction of it is "the product did not work as expected," not "the product made the dog sick."

Why the 12-week duration matters for side effects

Fluralaner has a long half-life in dogs (on the order of 12 days), which is exactly why one chew lasts 12 weeks. That is a convenience benefit, but it has a side-effect implication the monthly products do not: if a dog has a true drug-attributable reaction, the compound is present in the body for weeks and cannot simply be "stopped." There is no specific antidote; care is supportive, and serious neurologic reactions are managed by a veterinarian (or emergency clinic) the same way seizures and tremors from any cause are managed.

This is not a reason to avoid Bravecto — for most dogs the duration is a major plus. It is a reason to (a) watch your dog for a few days after each dose, and (b) tell your veterinarian promptly if anything is off, because the long washout period makes early action more useful than waiting.

If your dog vomits the chew

Vomiting shortly after a dose is usually about the flavored chew, not toxicity, and there is a practical re-dosing rule. Per the manufacturer's guidance: if an otherwise healthy dog vomits within about 3 hours of receiving Bravecto, it is generally recommended to redose — wait about 24 hours and give another chew. If 4 or more hours passed before the vomiting, the medication has likely already been absorbed and your dog is protected, so you do not redose. Do not give a third chew if vomiting repeats, and call your veterinarian. This is distinct from repeated vomiting or vomiting with lethargy, which warrants a call regardless of timing.

Who is at higher risk

• Dogs with a history of seizures or neurologic disease — the label says use with caution, and this is the group the FDA warning most directly addresses.
• Dogs with known allergy to fluralaner or isoxazolines — a prior reaction is a reason not to use the class again. For recognized poisoning or overdose patterns, see our flea and tick medicine poisoning in dogs article.
• Breeding, pregnant, or lactating dogs — not recommended, given the reproductive-effect reports in breeding females.
• Puppies under 6 months for the standard 12-week chew (the label specifies age minimums; follow them).

What to ask your veterinarian

• Given my dog's history (including any seizures or neurologic signs), is an isoxazoline appropriate, or should we consider a different class?
• If my dog vomits the chew, do we re-dose, and how do we tell vomiting-the-chew from a reaction?
• What neurologic or allergic signs should I watch for in the days after the dose, and which mean I should call the same day?
• Because Bravecto lasts 12 weeks, what is our plan if a reaction does occur?

When to call your veterinarian or an emergency clinic

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

• Seizures, tremors, muscle twitching, stumbling, or loss of coordination that appears after a dose
• Persistent vomiting or diarrhea, especially with blood, lethargy, or refusal to eat
• Swelling of the face, 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 sure the drug is responsible before calling. Suspected adverse events can be reported to the FDA at fda.gov/reportanimalae or 1-888-FDA-VETS, or to Merck Animal Health at 1-800-224-5318.

Sources

• Merck Animal Health. Bravecto (fluralaner) Chews for Dogs product page and prescribing information. https://www.merck-animal-health-usa.com/products/bravecto-fluralaner-chews-for-dogs
• FDA. Animal Drug Safety-Related Labeling Changes — isoxazoline products (Simparica Trio and class). https://www.fda.gov/animal-veterinary/drug-labels/animal-drug-safety-related-labeling-changes
• 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 CVM animal adverse-event database, public extract dated June 8, 2026; analysis by VetMedGuide (dog reports listing fluralaner as an active ingredient).
• FDA. FOI Summary, NADA 141-599 — Bravecto Quantum (fluralaner extended-release injectable suspension), July 2025. https://animaldrugsatfda.fda.gov/
• 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