# NexGard and Heartgard Without a Vet Prescription You cannot legally buy NexGard for dogs without vet prescription in the United States. NexGard and Heartgard are FDA-approved prescription drugs, so a licensed veterinarian must examine your dog and authorize the purchase before any pharmacy can dispense them. Buying these products from an unverified seller puts your dog at risk and may break federal law. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - NexGard (afoxolaner) and Heartgard (ivermectin plus pyrantel) are both prescription-only in the U.S. You need a current veterinarian-client-patient relationship to get either one. - A veterinarian must examine your dog, confirm the dose for its weight, and rule out health problems or medications that make these drugs unsafe. - Legitimate online pharmacies and telehealth services can fill these prescriptions, but only after a licensed veterinarian approves them. - Products sold without any prescription, often from overseas sites or social media sellers, may be counterfeit, expired, wrongly dosed, or not the drug on the label. - Giving the wrong parasite preventive can cause serious side effects, and some dog breeds carry a genetic risk that makes certain drugs dangerous. ## What NexGard and Heartgard Are and What Veterinarians Use Them For NexGard is the brand name for afoxolaner, an oral chewable tablet in the isoxazoline class. Veterinarians prescribe it mainly to kill fleas and ticks on dogs and to keep new infestations from taking hold. Research keeps expanding what this drug can do. A single oral dose of afoxolaner was effective against natural infestations of the chewing louse Heterodoxus spiniger in dogs under field conditions [1]. It has also shown 100% larvicidal efficacy against Dermatobia hominis larvae, the fly that causes furuncular myiasis, with all larvae found dead within 24 hours of treatment [2]. A related product, NexGard PLUS, combines afoxolaner with moxidectin and pyrantel pamoate and has demonstrated effectiveness against ear mites (Otodectes cynotis) in dogs [3]. Heartgard is a monthly soft chew that prevents heartworm disease and treats and controls roundworms and hookworms. It contains ivermectin and pyrantel pamoate. Heartworm prevention matters because heartworm disease is spread by mosquitoes, is serious, and is far harder and more expensive to treat than to prevent. Both drugs are classified as prescription-only for good reasons. The dose depends on your dog's exact weight, the products interact with other medications, and some dogs should never receive them. Your veterinarian is the person who sorts all of that out. If you want to understand what NexGard does in more detail, see [Nexgard For Dogs](/knowledge/veterinary-medicine/clinical-methods/nexgard-for-dogs) and [Nexgard Flea Treatment For Dogs](/knowledge/veterinary-medicine/parasitic-diseases/nexgard-flea-treatment-for-dogs). ## How These Medications Work Afoxolaner, the active ingredient in NexGard, is an isoxazoline. It works by blocking certain chloride channels in the nervous system of fleas, ticks, lice, and some mites. Insects and arachnids are far more sensitive to this effect than mammals are, which is why the drug can kill parasites at doses that dogs tolerate. After your dog swallows the chew, the drug is absorbed into the bloodstream. Fleas and ticks that bite your dog take in the drug and die. Because it works through the blood, it kills parasites after they feed, which is why it is not a repellent. Ivermectin, one of the two active ingredients in Heartgard, works through a different mechanism. It interferes with nerve signals in parasites, causing paralysis and death. Pyrantel pamoate acts on the worms in the gut. Together they clear roundworms and hookworms and stop heartworm larvae from developing into adult worms in the heart and lungs. Neither drug prevents fleas or mosquitoes from getting on your dog. They kill or stop the parasites after exposure. That is why consistent monthly dosing matters. ## Dosing Basics ### Who decides the dose Your veterinarian does. The dose is calculated from your dog's body weight and health status, and the product comes in weight bands. A dog that gains or loses a significant amount of weight may need a different size chew. When a specific dose appears in the veterinary literature, it is a research dose, not a dose for you to copy. For example, one study treated lizards with 2.5 mg of afoxolaner per kilogram of body weight orally [4], and another used a single oral dose of 2.5 mg/kg in dogs with myiasis [2]. Those numbers describe study protocols. Your dog's actual dose is a decision your veterinarian makes. ### How it is given NexGard is a chewable tablet given by mouth, usually with or without food according to the label directions. Heartgard is a soft chew given monthly. Most dogs take both willingly because they are flavored. If your dog spits the chew out or vomits soon after taking it, call your veterinarian for advice, because the dose may not have been absorbed. ### What to do about a missed dose For a monthly product, give the missed dose as soon as you remember, then restart the monthly schedule from that date. Do not double up to catch up unless your veterinarian tells you to. If you are not sure how late the dose is or whether your dog is protected, call your veterinary clinic. Heartworm prevention works best when given on a steady monthly schedule, and gaps leave your dog unprotected. ## Side Effects: What to Watch For Most dogs tolerate NexGard and Heartgard well. When side effects happen, they are usually mild and short lived. The table below covers the common ones and the ones that need a phone call. | Side effect | How common or serious | What to do | |---|---|---| | Vomiting, diarrhea, drooling, decreased appetite, lethargy | Common and usually mild | Watch for 24 hours. Call your veterinarian if it continues or your dog seems unwell | | Itching, hives, facial swelling | Uncommon but can be serious | Call your veterinarian right away. Facial swelling or trouble breathing is an emergency | | Tremors, wobbliness, seizures | Rare but serious | Stop the medication and contact a veterinarian immediately | | Signs of ivermectin sensitivity in dogs with the MDR1 gene mutation (stumbling, dilated pupils, tremors) | Rare but serious, seen in some herding breeds | Tell your veterinarian your dog's breed before starting Heartgard. Call immediately if these signs appear | | No visible side effects | Most common outcome | Keep giving the monthly dose on schedule | Some dogs with a specific genetic mutation, most often seen in collies and related herding breeds, are unusually sensitive to ivermectin. This is one of the main reasons Heartgard requires a prescription and a conversation with your veterinarian. Your vet may recommend a different heartworm preventive for these dogs. ## Who Should Not Take These Medications and What Interacts With Them Puppies below the labeled minimum age and weight should not receive these products. Pregnant or nursing dogs need a veterinarian's judgment before any parasite preventive is started. Dogs with a history of seizures or neurologic disease need extra caution with isoxazolines, and your veterinarian will weigh the benefits against the risks. Drug interactions matter too. Ivermectin can interact with other medications, and dogs already taking certain drugs may not be good candidates for Heartgard. Because heartworm prevention and flea and tick control are often combined, your veterinarian needs a full list of everything your dog takes, including supplements and any product you bought without a prescription. Giving two products that contain the same ingredient by accident is a real risk when owners buy from multiple sources. Dogs with a known allergy to any ingredient in the chew should not take that product. Tell your veterinarian about every reaction your dog has had to any medication, even a mild one. ## Overdose and When to Call a Veterinarian or Poison Control If your dog eats several chews at once, or eats another pet's dose, treat it as an emergency. Call your veterinarian or an animal poison control hotline right away. Have the product packaging with you so you can read the ingredient list and the strength of each chew. Do not wait for symptoms to appear before you call. Signs that need immediate attention after a possible overdose include vomiting, tremors, wobbliness, dilated pupils, excessive drooling, or seizures. These can appear within hours. Early treatment is easier and safer than waiting. If you are not sure whether your dog actually swallowed the chew, or you found a chewed wrapper, call anyway. Veterinary staff would rather answer a worried phone call than treat a preventable poisoning. ## Getting These Medications Legally: Telehealth and Online Pharmacies The good news is that you do not always need an in-person office visit. Many veterinarians now offer telehealth appointments, and online pharmacies can ship prescription parasite preventives directly to your door. The catch is that a licensed veterinarian still has to examine your dog (in person or by video, depending on your state's rules), establish a veterinarian-client-patient relationship, and write or approve the prescription. Here is how the legitimate path works: 1. Book a telehealth or in-person visit with a licensed veterinarian. 2. Have your dog's weight, health history, and current medications ready. 3. The veterinarian approves a prescription and sends it to a pharmacy of your choice. 4. The pharmacy verifies the prescription with your veterinarian before dispensing. 5. You receive the product with proper labeling and dosing instructions. This process protects your dog and protects you. It also means the pharmacy is accountable if something goes wrong. For a step-by-step look at the process, see [How to Get a Vet Prescription for Your Dog Without an Office Visit](/knowledge/veterinary-medicine/pet-medication-guide/get-vet-prescription-dog-without-office-visit). If your dog takes a preventive for a long-term condition and you want to avoid repeated visits, [How to Get a Vet Prescription for Your Dog's Chronic Condition Without Repeated Office Visits](/knowledge/veterinary-medicine/pet-medication-guide/dog-chronic-condition-prescription-refills) explains how refills usually work. ### Why buying without a prescription is risky When you buy NexGard or Heartgard from a seller who does not ask for a prescription, you are skipping every safety check. Common problems include: - Counterfeit or diluted product with no active ingredient, so your dog is not protected. - Wrong weight band, which means an underdose or an overdose. - Expired or improperly stored chews. - Products formulated for a different species or a different country's label. - No way to trace the product or report a problem if your dog gets sick. There is also a legal side. Prescription animal drugs are meant to be dispensed by or on the order of a licensed veterinarian. Sellers who ship them without a prescription are operating outside that system, and you have little recourse if the product harms your dog. ## Limitations and When to Contact a Veterinarian No article can tell you whether a specific product is right for your specific dog. That decision needs a veterinarian who knows your dog's weight, breed, health history, and other medications. Contact your veterinarian the same day if: - Your dog vomits repeatedly or refuses food after a dose. - You see hives, facial swelling, or intense itching. - Your dog seems weak, wobbly, or unusually tired. - You gave the wrong size chew or are unsure what your dog swallowed. Go to an emergency clinic now if: - Your dog has a seizure, collapses, or has trouble breathing. - Your dog's gums look pale or blue. - Your dog ate multiple chews or another pet's medication. Schedule a routine visit if: - Your dog is due for its annual heartworm test. - Your dog's weight has changed since the last prescription. - You want to switch products or add a second parasite preventive. - You are traveling to an area with different parasite risks. Heartworm testing before starting or continuing prevention is standard practice, because giving a preventive to a dog that already has adult heartworms can cause problems. Your veterinarian will explain the timing. ## Frequently Asked Questions ### Can I buy NexGard without a prescription in the United States? No. NexGard is a prescription-only medication, and no legitimate U.S. pharmacy will sell it without a veterinarian's authorization. Any seller offering it with no prescription is operating outside the regulated supply chain, and the product may not be what the label says. ### Is Heartgard available over the counter? No. Heartgard requires a prescription for the same reasons NexGard does. Your veterinarian needs to confirm your dog's weight, test for heartworm infection, and check for the genetic sensitivity that makes ivermectin risky in some dogs. ### Can an online pharmacy fill these prescriptions? Yes, as long as it has a valid prescription from your veterinarian. Reputable online pharmacies contact your vet to verify the prescription before shipping. If a site lets you check out with no prescription and no vet contact, that is a warning sign. ### Do I need an office visit, or can I use telehealth? Many veterinarians offer telehealth visits for preventive care, and some states allow a prescription after a video exam. Rules vary by state, and your veterinarian may still require an in-person exam or a heartworm test before approving the prescription. ### What happens if I give my dog the wrong dose? Call your veterinarian or an animal poison control hotline right away, even if your dog seems fine. Bring the packaging so staff can see the strength of the chew and the ingredients. Signs like vomiting, tremors, or wobbliness can start within hours and are easier to treat when caught early. ### Are these products safe to give together? Many dogs take a flea and tick preventive and a heartworm preventive at the same time, and some products combine both. Whether that is right for your dog depends on the specific products, your dog's weight, and its health history. Ask your veterinarian before combining anything, including products you already have at home. ## Related Articles - [Nexgard For Dogs](/knowledge/veterinary-medicine/clinical-methods/nexgard-for-dogs) - [Nexgard Flea Treatment For Dogs](/knowledge/veterinary-medicine/parasitic-diseases/nexgard-flea-treatment-for-dogs) - [How to Get a Vet Prescription for Your Dog Without an Office Visit](/knowledge/veterinary-medicine/pet-medication-guide/get-vet-prescription-dog-without-office-visit) - [How To Treat Dog Ear Mites At Home Without Vet Visit](/knowledge/veterinary-medicine/parasitic-diseases/how-to-treat-dog-ear-mites-at-home-without) - [How to Get a Vet Prescription for Your Dog's Chronic Condition Without Repeated Office Visits](/knowledge/veterinary-medicine/pet-medication-guide/dog-chronic-condition-prescription-refills) - [How to Give Your Dog a Pill Without Food: Alternative Methods](/knowledge/veterinary-medicine/pet-medication-guide/give-dog-pill-without-food) - [Heartgard Side Effects in Dogs: What to Expect](/knowledge/veterinary-medicine/pet-medication-guide/heartgard-side-effects-dogs) - [Heartgard for Dogs: Ingredients and How It Works](/knowledge/veterinary-medicine/pet-medication-guide/heartgard-dogs-ingredients) - [Heartgard for Dogs with Seizures: Is It Safe?](/knowledge/veterinary-medicine/pet-medication-guide/heartgard-dogs-seizures-safety) - [Canine Heartworm Prevention and Flea Control: Combination Pill Options and Veterinary Guide](/knowledge/parasites/pet-parasites/canine-heartworm-prevention-flea-combination-pill) - [Combined Heartworm and Flea Prevention in Dogs: Efficacy and Safety of Oral Pills](/knowledge/parasites/pet-parasites/dog-heartworm-flea-pill) ## References 1. [Efficacy of oral afoxolaner (NexGard®) against natural infestations with Heterodoxus spiniger (Phthiraptera: Boopiidae) in dogs under field conditions](https://pubmed.ncbi.nlm.nih.gov/40803800/). Veterinary parasitology, regional studies and reports, 2025. 2. [Efficacy of Afoxolaner (NexGard®) in the treatment of furuncular myiasis caused by Dermatobia hominis fly (Diptera: Cuterebridae) in naturally infested dogs](https://pubmed.ncbi.nlm.nih.gov/39025552/). Veterinary parasitology, regional studies and reports, 2024. 3. [Field efficacy of a combination of afoxolaner, moxidectin and pyrantel pamoate against natural infestation with Otodectes cynotis in dogs](https://pubmed.ncbi.nlm.nih.gov/41854285/). Parasite (Paris, France), 2026. 4. [Efficacy and safety of afoxolaner (NexGard®) in a collection of captive-bred lizards for the treatment and control of mite infestation](https://pubmed.ncbi.nlm.nih.gov/40606263/). International journal for parasitology. Parasites and wildlife, 2025. ## Further Reading - [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: Dog Owners](https://www.merckvetmanual.com/dog-owners) - [Cornell Richard P. Riney Canine Health Center: Canine Health Information](https://www.vet.cornell.edu/departments-centers-and-institutes/riney-canine-health-center/canine-health-information)