# Nexgard for Cats: Side Effects and Allergic Reactions Most cats handle flea and tick products well, but a cat reaction to Nexgard or similar medicines can happen, and the signs range from mild and short-lived to serious. The biggest safety issue is that cats and dogs are not dosed the same way, so a product made for a dog can poison a cat. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - NexGard Combo is a topical flea, tick, and parasite product approved for cats in the United States. NexGard Chewables are labeled for dogs only and should never be given to a cat. - Mild signs after a topical product can include drooling, head shaking, or brief skin irritation at the application site. - Serious signs include wobbliness, dilated pupils, tongue protrusion, weakness, tremors, and seizures. These need emergency care right away. - If your cat is having a bad reaction, bathe it in dish soap such as Dawn to remove the product, then call your veterinarian and Pet Poison Helpline at (855) 764-7661 [1]. - Never apply a dog flea product to a cat. Pyrethrin and pyrethroid products made for dogs can be fatal to cats if untreated [1]. ## What Is NexGard and What Do Veterinarians Use It For? NexGard is a brand name that covers more than one product. The version most often discussed for cats is NexGard Combo, a topical combination of esafoxolaner, eprinomectin, and praziquantel. It is applied to the skin and is used to treat and prevent fleas and ticks and to treat certain internal parasites. Veterinarians also use these ingredients for less common parasitic problems. In one study, the topical combination of esafoxolaner, eprinomectin, and praziquantel was used to treat cats with a nasal nematode called Mammomonogamus ierei, a parasite that lives in the nasal cavities and nasopharynx of cats in some regions [2]. Another field study looked at the same topical combination against Lynxacarus radovskyi, a fur mite that causes itching, hair loss, and scaling. All 16 cats in that study were free of detectable mites by Day 30 after a single topical treatment [3]. There is also an oral form of afoxolaner (the NexGard chewable) that is labeled for dogs. It has been studied in dogs for flea and tick control over a full year [4]. It has also been used in at least one unusual case in a guinea pig with a mange mite infestation [5]. None of that makes it a cat product. The chewable is not approved for cats, and giving it to a cat is a dosing mistake. If your cat has itchy skin from fleas, the underlying problem may be more than fleas. Flea allergy dermatitis is a common cause of intense itching and hair loss in cats, and it is worth reading about [Cat Flea Allergy Dermatitis: Symptoms and Relief](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-flea-allergy-dermatitis) so you can recognize it. ## How Does NexGard Work? The active ingredients in NexGard Combo work in different ways at the same time. Esafoxolaner is an insecticide and acaricide. It targets the nervous system of fleas and ticks after they bite the cat. Eprinomectin is a macrocyclic lactone, a class of drug that kills certain worms and parasites. Praziquantel kills tapeworms. Because eprinomectin acts on the nervous system of parasites, it can also affect a cat's nervous system if too much of it reaches the brain. That risk is higher in cats with a specific genetic change. A case report described a 9-month-old Maine Coon cat that developed tongue protrusion, dilated pupils, weakness, and ataxia after routine application of an eprinomectin-containing product at label doses. Testing showed the cat was homozygous for a variant called ABCB11930_1931del TC, which means it had non-functional P-glycoprotein, a drug transporter that normally keeps certain drugs out of the brain. Cats with this change have an increased risk of neurotoxicity from macrocyclic lactones and other P-glycoprotein substrate drugs [6]. The cat recovered completely over 3 months with supportive care [6]. This is why some cats react badly even when the product was used correctly. It is also why you should tell your veterinarian about any previous reaction, and about the breed and health history of your cat. ## Dosing Basics for Cats Your veterinarian decides the dose. The dose depends on your cat's weight, age, health conditions, and the parasites being treated. Do not split a dog product, guess a dose, or use a product labeled for another species. NexGard Combo is applied topically. Part the hair at the application site on the back of the neck, place the liquid directly on the skin, and keep your cat from licking the area until it dries. Wash your hands after applying it. Keep the treated cat away from other pets until the product dries so they do not groom it off. If you miss a dose, do not double up. Call your veterinarian or the clinic that prescribed it and ask when to apply the next dose. For a monthly product, applying it late is usually better than applying two doses at once. If you are also managing allergies or other skin problems, ask your veterinarian how flea prevention fits with the rest of your cat's plan. Some owners find it helpful to review how other cat medicines are used, such as [Cat Allergy Medication Side Effects: What to Watch For](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-allergy-medication-side-effects) and [Cat Prednisolone for Allergies: Side Effects and Safe Use](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-prednisolone-allergies). ## Nexgard for Cats Side Effects: What to Expect Most reactions are mild and go away on their own. Some are serious and need treatment. The table below separates the two. | Side effect | How common or serious | What to do | |---|---|---| | Drooling, lip smacking, head shaking | Common and usually mild, often from tasting the product | Offer water, keep the cat calm, and watch for other signs. Call your vet if it continues. | | Mild redness, itching, or hair loss at the application site | Common and usually mild | Do not reapply. Note the spot and tell your vet at the next visit. | | Vomiting or diarrhea | Uncommon, usually mild | Withhold food briefly if your vet advises it, and call if it lasts more than a day. | | Lethargy or poor appetite | Uncommon | Call your veterinarian if it lasts more than 24 hours. | | Wobbliness, weakness, or ataxia | Serious | Call your veterinarian right away. | | Dilated pupils, tongue protrusion, or unusual eye movement | Serious | This can be a sign of neurologic toxicity. Seek care immediately [6]. | | Tremors or seizures | Emergency | Go to an emergency clinic now. | | Trouble breathing, facial swelling, hives | Emergency (allergic reaction) | Go to an emergency clinic now. | | Collapse or unresponsiveness | Emergency | Go to an emergency clinic now. | A cat allergic to Nexgard may show signs within minutes to hours of application. The most concerning allergic signs are swelling of the face or throat, hives, vomiting, and difficulty breathing. These are different from the neurologic signs caused by too much drug reaching the brain, but both need urgent care. If your cat was exposed to a pyrethrin or pyrethroid product, the signs of poisoning can include drooling, vomiting, tremors, and seizures. If that exposure is left untreated, death can occur [1]. Do not wait to see if it passes. ## Cat Reaction to NexGard Combo: What Owners Report Owners often describe a cat reaction to NexGard Combo in a few patterns. The first pattern is a taste reaction. The cat licks the application site, drools, shakes its head, and may vomit once. This usually settles within an hour or two. It happens because the product tastes bitter, not because the cat is poisoned. The second pattern is skin irritation. The application site may look greasy, red, or bald for a few days. This is usually mild and does not need treatment. The third pattern is neurologic. The cat becomes wobbly, weak, or unsteady, or its pupils look wide and fixed. This is the pattern seen in the Maine Coon case, where the cat developed tongue protrusion, mydriasis, generalized weakness, and ataxia after a label-dose application [6]. If you see this, treat it as an emergency. The fourth pattern is a true allergic reaction. This is less common but more dangerous. Swelling, hives, and breathing trouble mean an immediate trip to the vet. If your cat has a history of reactions to flea products, your veterinarian may choose a different product or a different class of drug. For cats with severe itching from allergies, other treatments may be part of the plan, and it helps to understand the options in [Cat Allergy Treatment](/knowledge/veterinary-medicine/clinical-methods/cat-allergy-treatment). ## Who Should Not Take NexGard and What Interacts With It Cats with a known allergy to any ingredient in the product should not receive it. Cats that have had a neurologic reaction to a macrocyclic lactone or to an eprinomectin-containing product should not receive it again without a full veterinary review. Cats with the ABCB11930_1931del TC variant are at higher risk of neurotoxicity from macrocyclic lactones and other P-glycoprotein substrate drugs [6]. If your cat had a reaction and you know its genetic status, share that information with every veterinarian who treats it. Kittens, pregnant cats, nursing cats, and cats with liver or kidney disease need a veterinarian's decision, not an owner's guess. The same is true for cats taking other medicines, especially other parasite preventives. Do not stack products that contain overlapping ingredients. Never use a dog product on a cat. This is the single most common and most dangerous mistake. Dog flea products, especially those containing pyrethrins or pyrethroids, can cause severe poisoning in cats, and untreated cases can be fatal [1]. ## Overdose and When to Call Poison Control If your cat licked off a full dose, was given a second dose by mistake, or was treated with a dog product, call for help right away. Pet Poison Helpline is available at (855) 764-7661 [1]. Call them and your veterinarian. If the product is on the skin, bathe your cat in dish soap such as Dawn to remove it, then head to the clinic [1]. Do not induce vomiting unless a veterinarian tells you to. Bring the product packaging with you. The clinic needs to know the active ingredients and the concentration to treat your cat correctly. ## Limitations and When to Contact a Veterinarian Call your veterinarian the same day if your cat is drooling heavily, vomiting, has diarrhea, is not eating, or seems tired after a dose. These signs are often mild, but they can be the first stage of something worse. Go to an emergency clinic now if your cat is wobbly, weak, unsteady, has dilated pupils, holds its tongue out, has tremors, has a seizure, collapses, or has trouble breathing. These are the signs that need treatment without delay [6]. Call your veterinarian before the next dose if your cat had any reaction at all, even a mild one. The next step may be a different product, a different dose, or a different schedule. Every cat is different. A reaction that is mild in one cat can be serious in another, so individual cases need a veterinarian who can examine your cat and review its history. ## Frequently Asked Questions ### Is NexGard safe for cats? NexGard Combo is approved for cats and is used by veterinarians for flea, tick, and parasite control. The chewable NexGard product is labeled for dogs and should not be given to cats. Safety depends on using the right product, the right dose, and the right cat. ### What does an allergic reaction to NexGard look like in a cat? An allergic reaction usually involves swelling of the face or throat, hives, vomiting, or difficulty breathing. These signs can appear within minutes to hours of application. They are different from the drooling and head shaking that happen when a cat tastes the product. ### How long do NexGard side effects last in cats? Mild taste reactions often settle within an hour or two. Skin irritation at the application site may last a few days. Neurologic signs can last much longer. In one reported case, a cat with a genetic risk factor took about 3 months to recover fully [6]. ### Can I use dog NexGard on my cat? No. Dog flea products, including those with pyrethrins or pyrethroids, can cause severe poisoning in cats. Signs include drooling, vomiting, tremors, and seizures, and untreated cases can be fatal [1]. Always check the label for the species before you apply anything. ### What should I do if my cat has a bad reaction to NexGard? Bathe your cat in dish soap such as Dawn to remove the product, then call Pet Poison Helpline at (855) 764-7661 and your veterinarian [1]. Take your cat to a clinic for evaluation. If it is wobbly, seizing, or having trouble breathing, go to an emergency clinic immediately. ## Related Articles - [Cat Allergy Medication Side Effects: What to Watch For](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-allergy-medication-side-effects) - [Cat Prednisolone for Allergies: Side Effects and Safe Use](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-prednisolone-allergies) - [Cat Allergy Treatment](/knowledge/veterinary-medicine/clinical-methods/cat-allergy-treatment) - [Cat Urinary Tract Infection Side Effects](/knowledge/veterinary-medicine/clinical-methods/cat-urinary-tract-infection-side-effects) - [Cat Flea Allergy Dermatitis: Symptoms and Relief](/knowledge/veterinary-medicine/skin-allergy-and-ear-care/cat-flea-allergy-dermatitis) - [Cat Ate a Bee or Wasp: Signs of Allergic Reaction and Emergency Steps](/knowledge/veterinary-medicine/vet-visit-decisions/cat-ate-bee-wasp-allergic-reaction) - [What If My Cat Licks NexGard? Safety and Risks](/knowledge/veterinary-medicine/pet-medication-guide/cat-licks-nexgard-safety) - [Credelio for Cats: Side Effects and Allergic Reactions](/knowledge/veterinary-medicine/pet-medication-guide/credelio-cats-side-effects) - [Cat Allergic to Antibiotics: Signs and What to Do](/knowledge/veterinary-medicine/pet-medication-guide/cat-allergic-to-antibiotics) ## References 1. [My Cat Had a Bad Reaction to Flea](https://www.petpoisonhelpline.com/uncategorized/my-cat-had-a-bad-reaction-to-flea-treatment/). Pet Poison Helpline. 2. [Efficacy of NexGard® combo (esafoxolaner-eprinomectin-praziquantel) against natural Mammomonogamus ierei infection in cats](https://pubmed.ncbi.nlm.nih.gov/42034956/). Veterinary parasitology, regional studies and reports, 2026. 3. [Pilot field study to assess efficacy of NexGard Combo®, a topical combination of esafoxolaner + eprinomectin + praziquantel, against natural Lynxacarus radovskyi infestation in cats](https://pubmed.ncbi.nlm.nih.gov/42735466/). Veterinary parasitology, 2026. 4. [Clinical efficacy and safety of a single administration of fluralaner injectable suspension (BRAVECTO® injectable) vs. monthly administration of oral afoxolaner (NexGard®) in dogs for tick and flea control over one year under European field conditions](https://pubmed.ncbi.nlm.nih.gov/39654069/). Parasites & vectors, 2024. 5. [Effective treatment with afoxolaner (NexGard) of Trixacarus caviae in a pet guinea pig](https://pubmed.ncbi.nlm.nih.gov/39239737/). Veterinary medicine and science, 2024. 6. [Inherent P-glycoprotein deficiency and macrocyclic lactone toxicity in an Australian Maine Coon cat homozygous for ABCB11930_1931del TC](https://pubmed.ncbi.nlm.nih.gov/41694419/). JFMS open reports, 2026. ## Further Reading - [Flea Medicine Side Effects on Cats (Pet Poison Helpline)](https://www.petpoisonhelpline.com/uncategorized/flea-medicine-side-effects-on-cats/) - [American Veterinary Medical Association: First Aid Tips for Pet Owners](https://www.avma.org/resources-tools/pet-owners/emergencycare/first-aid-tips-pet-owners) - [Merck Veterinary Manual: Cat Owners](https://www.merckvetmanual.com/cat-owners)