If your dog ate more than one NexGard chewable, the most likely outcome is mild vomiting or diarrhea that resolves on its own. NexGard has a wide safety margin, and clinical studies show dogs tolerated up to five times the maximum recommended dose without serious health effects. That said, the active ingredient belongs to a drug class the FDA has linked to rare neurological reactions, so an overdose is worth taking seriously, especially in large amounts or in dogs with a seizure history.
How Much Is Too Much
NexGard’s active ingredient is dosed at a minimum of 2.5 mg per kilogram of body weight once a month. In safety studies, dogs as young as 8 weeks old received the drug at one, three, and five times the maximum exposure dose repeatedly. Even at five times the dose, researchers found no changes in growth, bloodwork, or organ tissue on examination. Vomiting and diarrhea occurred sporadically across all groups, including dogs that received no drug at all.
So if your 30-pound dog grabbed a second chewable meant for a similar-sized dog, that’s roughly a 2x dose, well within the tested safety range. The risk increases when a small dog eats a chewable meant for a much larger dog, or when multiple chewables are consumed at once, pushing the dose to levels that haven’t been formally studied.
Digestive Symptoms to Expect
The most common reaction to an overdose is gastrointestinal upset. In safety trials at three and five times the normal dose, dogs experienced mild diarrhea (sometimes with blood) and vomiting, typically within 48 hours of ingestion. These episodes resolved without treatment in every case. In field studies at the standard dose, about 6.7% of dogs had diarrhea and 4.5% vomited, so some stomach upset is possible even at normal levels. A larger dose simply makes it more likely.
The Neurological Risk
NexGard belongs to a class of drugs called isoxazolines, which kill fleas and ticks by overstimulating their nervous systems. In rare cases, these drugs can affect a dog’s nervous system too. The FDA has issued a safety alert noting that isoxazoline products have been associated with muscle tremors, loss of coordination (ataxia), and seizures in some dogs. Most dogs never experience these reactions, but they can occur even in animals with no prior seizure history.
An overdose raises the concern because the dog’s nervous system is exposed to more of the drug than intended. If your dog shows any twitching, trembling, wobbliness, or has a seizure after eating extra NexGard, that’s a veterinary emergency.
What to Do Right Away
Call your veterinarian, the ASPCA Animal Poison Control Center (888-426-4435), or the Pet Poison Helpline (855-764-7661) as soon as you realize your dog has eaten extra NexGard. Try to note how many chewables were consumed and what size they were, since the milligram dose per chewable varies by weight bracket. This helps your vet calculate the actual overdose level.
There is no antidote for isoxazoline toxicity. Treatment is supportive, meaning the vet addresses symptoms as they appear. For mild cases, your dog may be monitored at home with guidance from your vet. If symptoms are more serious, hospitalization could involve IV fluids, anti-nausea medication, muscle relaxants, or seizure medication. Your vet may also monitor your dog’s temperature, blood sugar, and kidney function.
How Long the Drug Stays in Your Dog’s System
One reason an overdose warrants monitoring is that NexGard doesn’t clear the body quickly. The drug’s half-life (the time it takes for half the dose to be eliminated) is approximately two weeks in most dogs. In some breeds, particularly Collies, the half-life can stretch much longer, up to 47 days in one study. This means if your dog does develop symptoms, they could persist for days rather than hours, and your vet needs to know the timeline they’re working with.
This long half-life also means that if your dog recently received their regular monthly dose and then ate an extra chewable, the drug levels in their body will stack. Mention the date of their last dose when you call your vet.
Dogs at Higher Risk
While the safety data is reassuring for most dogs, certain animals deserve extra caution. Dogs with a history of seizures or epilepsy face a higher baseline risk for neurological reactions to isoxazoline drugs, even at standard doses. Very small dogs who eat a chewable sized for a larger dog can receive a proportionally massive dose relative to their body weight. And breeds known to be sensitive to certain antiparasitic drugs, such as Collies and related herding breeds, may process the drug more slowly, keeping it active in their systems for weeks longer than average.