NexGard is a beef-flavored soft chew given once a month, and dogs take it like a treat. That is the whole problem. There is no blister pack left over to count, no daily routine to check it against, and nothing about handing a dog a chew that feels like giving medication — so one adult does it on the way past the cupboard, the other does it that evening, and now there are two in the dog.
Search that and the reassurance is immediate and unanimous. Huge safety margin. Five times the dose was proven safe. Watch for a bit of vomiting. Almost every page on the first screen is a question-and-answer thread, and they are all quoting the same study — a real one, from the drug's own label. The trouble is that it answers a different question from the one you're asking at 9pm with an empty packet in your hand.
Key takeaways
- Never give two on purpose, and call if it already happened. VCA: "Do not give your dog two doses at once," and "if you suspect an overdose or an adverse reaction... call your veterinary office immediately."²
- A doubled chew lands well below the 3X level that was tested. One accidental double delivers roughly 5–12.6 mg/kg once; the safety study's 3X group received 18.9 mg/kg six times, with no treatment-related findings.³
- That study measured organ toxicity, not seizures. Seizure, ataxia and tremors appear in the label's post-approval adverse-event list — the signal that only surfaced after nationwide use.¹
- Vomiting is not the expected sign. In the label's field study, vomiting occurred in 4.1% of 415 treated dogs and 12.5% of 200 controls.¹
Can I give my dog two doses of NexGard?
No. VCA's afoxolaner handout says it in one line, right after its missed-dose advice: "Do not give your dog two doses at once."² That covers the deliberate version too — the fleas are still there, so surely one more chew would finish them off.
If the second chew is already down, VCA's instruction for that moment is not to sit and watch: "If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately."²
Here's the part the reassuring pages get right, worth saying plainly before anything else: as accidental doublings go, this is one of the more forgiving ones. A doubled chew is not a doubled insulin dose or a doubled NSAID. What follows is why that's true — and why it still isn't a reason to skip the call.
| Which situation are you in? | What it means | What to do |
|---|---|---|
| Two chews given the same day by two people, otherwise healthy adult dog | The commonest version, and the one that lands well inside the tested exposure range³ | Call your clinic today. Don't give any further dose until they tell you to |
| You can't remember whether this month's chew was given | Unknown — possibly a full dose already absorbed and still on board weeks later⁴ | Don't give another. Call and ask; uncertainty is not a reason to top up |
| Your dog has a history of seizures or a neurologic disorder | The group the label singles out: "use with caution in dogs with a history of seizures or neurologic disorders"¹ | Call immediately and say so first |
| Your dog got into the box and ate several chews | Unbounded exposure — the amount is the whole question and you don't have it | Emergency. Clinic or ASPCA APCC (888) 426-4435 now⁸ |
| A chew meant for a much heavier dog, given to a small one | A different arithmetic problem: the per-kilogram dose, not the count | Call with both weights and the exact chew strength |
| Muscle tremors, wobbliness, or a seizure at any point | The neurologic reactions the FDA alerted on in 2018⁵ | Emergency — go in now |
From the FDA-approved NexGard label, VCA Animal Hospitals' afoxolaner handout, and the FDA's isoxazoline safety communication. Every row ends in a phone call — they differ in how fast you dial, not whether.
What should you do in the first hour?
Call, with four facts ready, and don't try to make your dog vomit on your own initiative. The Merck Veterinary Manual is direct about what treatment looks like: "There is no specific antidote for afoxolaner poisoning in pets. Treatment rests with preventing further exposure and supportive measures. Gastric lavage and administration of activated charcoal are advised if the exposure is via accidental ingestion."⁶ Those are clinic procedures, decided by a vet who knows how long ago it happened.
What the clinic needs from you is arithmetic you can do faster than they can:
- Your dog's current weight, not last year's. Every number in this drug's literature is per kilogram, and the chew you bought was picked from a weight band.
- Which chew, and how many. NexGard comes in four strengths matched to weight bands — 11.3, 28.3, 68 and 136 mg of afoxolaner per chew.¹ "Two chews" means little without which one.
- The clock time of both doses. Peak levels arrive 2 to 6 hours after a chew, so how long ago it happened changes what's useful now.⁴
- Whether your dog has ever had a seizure, a tremor, or any neurologic diagnosis — including one from a different clinic. This is the question that changes the answer most often.
While you wait for a callback, keep your dog quiet and where you can see it. If you can't reach a clinic, the ASPCA Animal Poison Control Center takes calls on (888) 426-4435, 24 hours a day, 365 days a year; a consultation fee may apply.⁸
How does a doubled chew compare with the doses that were tested?
It lands well below the 3X level that was given six times over. NexGard's minimum label dose is 1.14 mg/lb — 2.5 mg/kg — once a month, and the chews are sized so that a dog at the light end of its weight band receives up to the maximum exposure dose of 6.3 mg/kg.¹³ Two chews at once therefore deliver somewhere between 5 and 12.6 mg/kg, in a single day.
Now put that against the study everyone quotes. In 2014, Drag and colleagues gave 32 Beagle puppies, 8 weeks old at enrollment, afoxolaner at 1X, 3X or 5X the maximum exposure dose — 6.3, 18.9 and 31.5 mg/kg — six times each. The first three treatments were a month apart; the last three only 14 days apart, deliberately stacking exposure. Two weeks after the final dose, every dog went to necropsy. The result: "No afoxolaner-related changes were observed in growth, physical variables, clinical pathology variables, or tissues examined histologically."³
So the comparison isn't close. One accidental double is a fraction of what the 3X group absorbed six times over. Vomiting and diarrhea did occur in that study, but the authors noted they "were observed sporadically across all groups including the controls."³ That is the real basis for the reassurance you've been reading, and it holds.
If 5X was safe, why call your vet at all?
Because that study was looking somewhere else. Its endpoints were growth, physical measurements, clinical pathology and tissues under a microscope³ — a search for organ damage. It found none, and that finding is real. But the reaction that worries veterinarians about this class isn't organ damage.
Look at where it does show up. The FDA-approved label's table of field-study adverse reactions lists vomiting, dry or flaky skin, diarrhea, lethargy and anorexia — no neurologic signs at all. (Seizures do appear in that study, narrated separately, in three dogs that already had a seizure history; more on those below.)¹ Then read the label's separate post-approval list, compiled from reports after the drug went into general use: "Vomiting, pruritus, lethargy, diarrhea (with and without blood), anorexia, seizure, hyperactivity/restlessness, panting, erythema, ataxia, dermatitis... allergic reactions... and tremors."¹
Seizure, ataxia and tremors appear only in the post-approval column. That's the whole point: it took nationwide use to surface them. On 20 September 2018 the FDA issued a class-wide alert about "the potential for neurologic adverse events in dogs and cats when treated with drugs that are in the isoxazoline class,"⁵ and the label now carries the precaution: "Seizures have been reported in dogs receiving isoxazoline class drugs, even in dogs without a history of seizures."¹
So "5X was proven safe" is true, and it is being used to answer a question it cannot reach. A reaction that appears unpredictably in a minority of dogs is not something eight puppies per group can rule out, and it isn't governed by the chart above the way organ toxicity is. Both things are true at once: the dose you gave sits low on the ladder, and the ladder isn't what the phone call is about.
None of this makes NexGard a bad drug. The FDA "considers products in the isoxazoline class to be safe and effective for dogs and cats," and published its fact sheet "so that pet owners and veterinarians can take it into consideration."⁷ Fleas and ticks carry real disease. The point is narrower: a vet who knows your dog is better placed than a search result.
What should you watch for, and for how long?
Two windows, not one, because this drug arrives quickly and leaves slowly. Afoxolaner reaches maximum plasma concentration 2 to 6 hours after an oral dose — but its terminal plasma half-life averages 15.5 ± 7.8 days, with oral bioavailability of 73.9%.⁴ Tonight is when levels peak. The following fortnight is when the extra amount is still measurably there.
What you're watching for is neurologic, not digestive. Here the label quietly contradicts the most repeated line on the internet. In the field study behind NexGard's approval, vomiting occurred in 4.1% of 415 treated dogs and in 12.5% of 200 control dogs — three times more often in the dogs given no afoxolaner at all. Anorexia ran the same way: 1.2% treated against 4.5% of controls.¹ Those are normal-dose figures and say nothing directly about a doubled one. But "expect some vomiting" is not what the label shows.
| What you're seeing | What it may point at | Urgency |
|---|---|---|
| Nothing at all, hours later | The commonest outcome, and consistent with the safety study³ | Still call the same day to agree the next dosing date |
| One episode of vomiting, or loose stool² | Listed side effect — though in the field study it was more common in controls¹ | Call today; mention it, and keep water available |
| Itching, redness, hives or facial swelling¹ | Post-approval list includes pruritus, erythema and allergic reactions¹ | Call now — swelling of the face or muzzle is urgent |
| Wobbliness, stumbling, or a drunk-looking walk (ataxia)¹ | A neurologic sign from the post-approval list and the FDA alert⁵ | Emergency — go in now |
| Muscle tremors or twitching¹ | Named in the label's isoxazoline precaution¹ | Emergency — go in now |
| A seizure, of any length, in a dog that has never had one¹ | FDA: seizures "may occur in animals without a prior history"⁷ | Emergency — go in now |
Signs as listed in the FDA-approved NexGard label, VCA Animal Hospitals' afoxolaner handout, and the FDA's isoxazoline fact sheet. This is a guide to urgency, not a diagnosis — describe what you're seeing to a veterinarian rather than matching it to a row yourself.
Does a history of seizures change the answer?
Yes, and it is the first thing to say on the phone. The label's precaution names the group without hedging: "Use with caution in dogs with a history of seizures or neurologic disorders."¹ VCA repeats it in its owner-facing handout — "USE WITH CAUTION in: Dogs with a history of seizures."² If your dog is epileptic, or on phenobarbital or levetiracetam, that fact outranks everything else about the incident.
The label doesn't leave that abstract. Its account of the pivotal US field study follows three enrolled dogs that already had a seizure history. One "experienced a seizure on the same day after receiving the first dose and on the same day after receiving the second dose," then "a third seizure one week after receiving the third dose." Another "had a seizure 19 days after the third dose." A third had none.¹ All three finished the study. Three dogs cannot establish cause, and the label doesn't claim they do — it's there because it's the kind of detail a veterinarian weighs, and you can't.
It helps to know why this class can touch the nervous system at all. Isoxazolines block chloride channels in nerve and muscle cells, stopping nerve signals getting through — and the reason a flea dies while your dog doesn't is selectivity, not absence of effect. The Merck Veterinary Manual: "binding of isoxazolines to these receptors is much more selective in fleas, ticks, and mites than in mammals, including humans."⁶ Afoxolaner blocks insect GABA-gated chloride channels "with nanomolar potency."⁹ Selectivity is very high. It is not infinite.
If your dog is already on anti-seizure medication, the timing question matters as much as the drug question, and it is a conversation worth having with the clinic that prescribed it — the same logic that governs strict seizure medication intervals for pets.
What if you missed a dose, or your dog spat the chew out?
Then the stakes are lower and the rule is written down twice. The label: "If a dose is missed, administer NexGard® and resume a monthly dosing schedule."¹ VCA, for owners: "If you miss a dose, give it as soon as you remember and start a new monthly dosing schedule."² Note what that says — the schedule moves, it doesn't get caught up. And the sentence VCA puts next is the one that matters most here: "Do not give your dog two doses at once."²
There is one situation where a second chew is correct rather than a mistake, and it's a common source of the confusion. The label's rule is broader than most owners realize: "If it is suspected that any of the dose has been lost or if vomiting occurs within two hours of administration, redose with another full dose."¹ A chew spat out, dropped, or vomited back up inside two hours is the intended case for a replacement.² A dose you're merely unsure about hours later is not — that one is a phone call.
A missed month is worth reporting rather than quietly fixing. The whole point of the monthly interval is that plasma levels stay above the concentration that kills fleas and ticks between doses,⁹ so a gap matters clinically — and your vet may want to know one happened before tick season. That's a calmer conversation than the other one.
One distinction worth keeping straight if your dog takes both monthly chews: this reasoning does not carry over to the heartworm preventive. That drug works backwards rather than forwards, clearing larvae already picked up, so as our guide to giving a dog two doses of Heartgard sets out, the doubled chew there is close to a non-event while the missed one is the half that needs a phone call and a test.
How do you stop this happening again?
By moving the record of the chew out of two people's heads and into one place both can see. Flea and tick preventives are unusually easy to double, and the reason is structural rather than careless — it is the same failure behind a doubled dose of a topical spot-on like Frontline, where the product sits on the coat instead of inside the dog and the animal most at risk may be the cat who sleeps against him.
Three things make this drug different from a twice-daily tablet:
- It happens once a month, so nobody has a habit around it. A daily medication builds a routine a second person bumps into. A monthly one doesn't — the last time either of you thought about it was four weeks ago.
- It doesn't feel like medication. It's a flavored chew handed over like a treat, often at the door. That's not a moment either person files as "I dosed the dog."
- There is nothing left to count. A blister strip tells you where you are; a chew from a box does not. The label also asks you to "keep NexGard® in a secure location out of reach of dogs, cats, and other animals to prevent accidental ingestion or overdose."¹
What closes the gap is a written record made at the moment of giving, not afterwards — and the date, product and strength, since the drug name alone won't tell the clinic what it needs. Keep monthly preventives on the same list as daily medications: the occasional drug is the one nobody has a habit around, which is exactly why it needs the record more. That's the failure described in why pets miss their medication doses, running in reverse, and the systems in how to avoid double dosing your pet's medication work the same for a chew as for a tablet.
Veterinary medical disclaimer: SteadyTails is a logging and coordination tool, not a substitute for veterinary care. Nothing here is a dosing instruction. Chew strengths are matched to weight bands, and the right response to an extra dose is specific to your dog — only your veterinarian can tell you what to do about a doubled, missed or delayed dose, or when the next one should be given. If your dog has received extra NexGard, or shows muscle tremors, incoordination, a seizure, facial swelling, or repeated vomiting, contact your veterinarian or an emergency animal hospital immediately. The ASPCA Animal Poison Control Center is available on (888) 426-4435, 24 hours a day, 365 days a year; a consultation fee may apply.⁸
A monthly chew is the easiest dose in the house to give twice
No blister pack, no daily routine, and it looks like a treat. SteadyTails records every dose with a name, a timestamp and the amount given, on a timeline the whole household can see — and clears the reminder on everyone's phone the moment somebody logs it. Coming soon to iOS and Android.
Join the SteadyTails waitlist →References
- NexGard® (afoxolaner) Chewables — FDA-approved prescribing information, via DailyMed (US National Library of Medicine). Dosage: "NexGard® is given orally once a month, at the minimum dosage of 1.14 mg/lb (2.5 mg/kg)"; dosing schedule by body weight — 4 to 10 lbs, 11.3 mg per chewable; 10.1 to 24 lbs, 28.3 mg; 24.1 to 60 lbs, 68 mg; 60.1 to 121 lbs, 136 mg; "If it is suspected that any of the dose has been lost or if vomiting occurs within two hours of administration, redose with another full dose. If a dose is missed, administer NexGard® and resume a monthly dosing schedule." Warnings: "Keep NexGard® in a secure location out of reach of dogs, cats, and other animals to prevent accidental ingestion or overdose." Precautions: "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. Use with caution in dogs with a history of seizures or neurologic disorders." Field study adverse reactions, afoxolaner (n=415) vs control (n=200): vomiting 4.1% (17 dogs) vs 12.5% (25 dogs); dry/flaky skin 3.1% vs 1.0%; diarrhea 3.1% vs 3.5%; lethargy 1.7% vs 2.0%; anorexia 1.2% vs 4.5%. Field study seizure narrative: "In the US field study, one dog with a history of seizures experienced a seizure on the same day after receiving the first dose and on the same day after receiving the second dose of NexGard®. This dog experienced a third seizure one week after receiving the third dose. The dog remained enrolled and completed the study. Another dog with a history of seizures had a seizure 19 days after the third dose of NexGard®. The dog remained enrolled and completed the study. A third dog with a history of seizures received NexGard® and experienced no seizures throughout the study." Animal safety: "In a margin of safety study, NexGard® was administered orally to 8 to 9-week-old Beagle puppies at 1, 3, and 5 times the maximum exposure dose (6.3 mg/kg) for three treatments every 28 days, followed by three treatments every 14 days, for a total of six treatments"; "There were no clinically-relevant effects related to treatment on physical examination, body weight, food consumption, clinical pathology (hematology, clinical chemistries, or coagulation tests), gross pathology, histopathology or organ weights"; "Vomiting occurred throughout the study, with a similar incidence in the treated and control groups." Post-approval experience (July 2018), in decreasing order of reporting frequency: "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." dailymed.nlm.nih.gov Retrieved 2026-08-30.
- VCA Animal Hospitals. Afoxolaner — "Afoxolaner is given by mouth in the form of a chewable tablet"; "If you miss a dose, give it as soon as you remember and start a new monthly dosing schedule... Do not give your dog two doses at once"; "If your dog vomits within two hours of dosing, give another full dose"; side effects including "Vomiting, Dry/flaky skin, Diarrhea, Lethargy, Lack of appetite, Neurological signs, such as incoordination, muscle tremors, or seizures"; "There is no specific monitoring required for dogs receiving afoxolaner"; "USE WITH CAUTION in: Dogs with a history of seizures"; "DO NOT USE afoxolaner in: Pets that are allergic to it; Dogs less than eight weeks of age... Dogs that weigh less than 4 pounds"; "There are no documented drug interactions for afoxolaner"; "If you suspect an overdose or an adverse reaction to the medication, call your veterinary office immediately." vcahospitals.com Retrieved 2026-08-30.
- Drag M, Saik J, Harriman J, Larsen D. Safety evaluation of orally administered afoxolaner in 8-week-old dogs. Veterinary Parasitology, 2014;201(3–4):198–203. "The safety profile of afoxolaner... was evaluated... when administered six times orally in a soft chewable formulation at a dose of at least 1×, 3× or 5× the maximum exposure dose (6.3 mg/kg) in 8-week-old Beagle dogs"; "Thirty-two healthy puppies (16 males and 16 females) were enrolled and allocated randomly to one of four treatment groups"; "Treatments were administered at three, one-month dose intervals (Days 0, 28 and 56) followed by three, 2-week dose intervals (Days 84, 98 and 112)"; groups received "at least 6.3 mg/kg (1×)", "at least 18.9 mg/kg (3×)" and "at least 31.5 mg/kg (5×)"; "On Day 126, 2 weeks following the last treatment, all dogs were humanely euthanized prior to the conduction of a full necropsy with tissue collection"; "No afoxolaner-related changes were observed in growth, physical variables, clinical pathology variables, or tissues examined histologically"; "Vomiting and diarrhea were observed sporadically across all groups including the controls." PubMed Retrieved 2026-08-30.
- Letendre L, Huang R, Kvaternick V, Harriman J, Drag M, Soll M. The intravenous and oral pharmacokinetics of afoxolaner used as a monthly chewable antiparasitic for dogs. Veterinary Parasitology, 2014;201(3–4):190–197. "afoxolaner was absorbed quickly following oral administration of the minimum effective dose of 2.5 mg/kg, with maximum plasma concentrations (Cmax) of 1,655 ± 332 ng/mL observed 2–6 h (Tmax) after treatment"; "The terminal plasma half-life was 15.5 ± 7.8 days, and oral bioavailability was 73.9%"; "Plasma concentration-versus-time curves fit a 2-compartment model"; "The pharmacokinetic properties of afoxolaner are suited for a monthly administration product because the fast absorption and long terminal half-life support a rapid onset of action while ensuring month-long efficacy." PubMed Retrieved 2026-08-30.
- U.S. Food and Drug Administration. Animal Drug Safety Communication: FDA Alerts Pet Owners and Veterinarians About Potential for Neurologic Adverse Events Associated with Certain Flea and Tick Products, 20 September 2018 — "The U.S. Food and Drug Administration is alerting pet owners and veterinarians to be aware of the potential for neurologic adverse events in dogs and cats when treated with drugs that are in the isoxazoline class"; products named include "Bravecto (fluralaner)", "Nexgard (afoxalaner)", "Simparica (sarolaner)" and "Credelio (lotilaner)"; "Some animals receiving [these products] have experienced adverse events such as muscle tremors, ataxia, and seizures"; "The FDA is working with manufacturers of isoxazoline products to include new label information to highlight neurologic events." fda.gov Retrieved 2026-08-30.
- Merck Veterinary Manual. Isoxazoline Toxicosis in Animals — "Isoxazolines bind to the chloride channels in nerve and muscle cells, which blocks the transmission of neuronal signals"; "Binding of isoxazolines to these receptors is much more selective in fleas, ticks, and mites than in mammals, including humans"; "isoxazolines, including afoxolaner, fluralaner, sarolaner, and lotilaner, can have neurologic effects in some dogs, such as muscle tremors, ataxia, and seizures"; "There is no specific antidote for afoxolaner poisoning in pets. Treatment rests with preventing further exposure and supportive measures. Gastric lavage and administration of activated charcoal are advised if the exposure is via accidental ingestion." merckvetmanual.com Retrieved 2026-08-30.
- U.S. Food and Drug Administration. Fact Sheet for Pet Owners and Veterinarians about Potential Adverse Events Associated with Isoxazoline Flea and Tick Products (last updated 08/08/2023) — "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"; "Pet owners should consult with their veterinarian to review their patients' medical histories and determine whether a product in the isoxazoline class is appropriate for their pet"; "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." fda.gov Retrieved 2026-08-30.
- ASPCA. Animal Poison Control Center — "(888) 426-4435"; "We are available 24/7, 365 days a year, so you can get expert guidance the moment a poisoning concern arises"; "A consultation fee may apply." aspca.org Retrieved 2026-08-30.
- Shoop WL, Hartline EJ, Gould BR, et al. Discovery and mode of action of afoxolaner, a new isoxazoline parasiticide for dogs. Veterinary Parasitology, 2014;201(3–4):179–189. "In vitro membrane feeding assay data and in vivo pharmacokinetic studies in dogs established an afoxolaner blood concentration of 0.1–0.2 μg/ml to be effective against both fleas (Ctenocephalides felis) and ticks (Dermacentor variabilis)"; "Pharmacokinetic profiles in dogs following a 2.5 mg/kg oral dosage demonstrated uniform and predictable afoxolaner plasma concentrations above threshold levels required for efficacy for more than one month"; "Mode of action studies showed that afoxolaner blocked native and expressed insect GABA-gated chloride channels with nanomolar potency." PubMed Retrieved 2026-08-30.