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By hannah hollinger

Published: 05/22/2020, edited: 08/10/2021

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Heartgard is primarily a heartworm preventative. This medication comes in two types: Heartgard and Heartgard Plus. Both forms protect dogs from heartworm disease. Heartgard Plus also protects dogs from roundworms, hookworms, and other parasites.

Puppies as young as 6 weeks old can begin taking Heartgard. Keep reading to find out the recommended dosage, efficacy, and side effects of Heartgard.


Heartgard and Heartgard Plus are available in the following three dosage strengths based on the dog’s weight [1]. Each strength contains different amounts of ivermectin, the active ingredient in Heartgard. Heartgard Plus also contains pyrantel, which treats intestinal worms.

  • Up to 25 lbs. ‚ÄĒ¬†68 mcg ivermectin/57 mg pyrantel¬†¬†
  • 26 to 50 lbs. ‚ÄĒ¬†136 mcg ivermectin/114 mg pyrantel
  • 51 to 100 lbs. ‚ÄĒ¬†272 mcg ivermectin/227 mg pyrantel

The recommended dose is 2.72 mcg of ivermectin and 2.27 mg of pyrantel per pound. If your dog weighs more than 100 lbs., consult your veterinarian for advice on dosage instructions.

Dosage Instructions

Administer one Heartgard chewable once per month in the appropriate dosage strength based on your dog’s weight. Follow all package instructions.

Heartgard is available in flavored and unflavored chewable form. The flavored chewable tends to be more popular and easier to administer. For the best results, feed by hand or add to a small portion of dog food. Supervise the dog to ensure they chew the tablet instead of swallowing it whole.


Heartgard is 95.6% effective in preventing heartworm disease in dogs [2].

One¬†study¬†found that two dogs treated with Heartgard Plus and tested positive for heartworms both lived in the Mississippi Delta [3]. Heartworm resistance is thought to be more common in this region ‚ÄĒ which covers Arkansas, Tennessee, Mississippi, and Louisiana ‚ÄĒ due to an increased prevalence of disease-carrying mosquitoes. One theory is that an ivermectin-resistant strain of heartworms is endemic to this region [4].

However, it is important to note that research on this phenomenon is ongoing. Most dogs living in the region that tested positive for heartworms despite receiving preventative treatment did not develop any symptoms associated with heartworm disease.

A separate study found that the majority of dogs who tested positive for heartworms despite receiving preventative treatment experienced a lapse in treatment and/or testing [5]. In other words, owners of the affected dogs did not always comply with the dosage instructions or testing requirements. Following all instructions regarding dosages and testing is imperative for preventing heartworm disease.

Side effects of Heartgard

The following side effects of Heartgard typically appear within 12 hours to 3 days after ingestion:

  • Nausea
  • Vomiting
  • Fatigue
  • Drooling
  • Depression
  • Staggering
  • Weight loss
  • Convulsions
  • Dilated pupils
  • Loss of appetite
  • Loss of muscle control and coordination

Rare side effects include vomiting and diarrhea within 24 hours of administering the medication. Certain breeds with ivermectin sensitivity may also experience trembling, seizures, slowed heart rate, foaming at the mouth, and onset blindness.


Dogs should be tested and/or treated for heartworm disease before taking Heartgard. A negative heartworm disease test is required to receive a prescription for Heartgard.

Drug interactions

Ivermectin and pyrantel, the main ingredients in Heartgard and Heartgard Plus, may interact with the following drugs [6, 7]:

  • Anti-fungal medications (ketoconazole, itraconazole)
  • Immunosuppressive medications (cyclosporine)
  • Antibiotics (erythromycin)
  • Blood pressure medications (amlodipine besylate, nifedipine)
  • Intestinal deworming medications (levamisole, morantel, piperazine)

Let your veterinarian know if your dog is taking any other medications, prescribed or otherwise, before starting Heartgard.

Allergic reactions and sensitivity

It is possible for dogs to be allergic to ivermectin and pyrantel. Signs of an allergic reaction include, but are not limited to, swelling, hives, and excessive itching.

Due to a genetic mutation, certain breeds are sensitive to ivermectin, the primary ingredient in many heartworm preventatives, including Heartgard and Heartgard Plus. These include:

  • Australian Shepherds
  • Collie breeds
  • Sheepdog breeds¬†
  • German Shepherds¬†
  • Longhaired Whippets
  • Silken Windhounds
  • Skye Terriers
  • Hybrids of the above breeds

Dogs affected by the mutation who take ivermectin have an increased risk of developing severe, potentially fatal neurological symptoms [8]. Owners of herding breeds can schedule a DNA swab to test for the mutation.

It’s also important to note that herding breeds can also develop symptoms after ingesting the manure of livestock treated with ivermectin. If you have any questions or concerns, consult your trusted veterinarian.

Frequently asked questions

How do dogs develop heartworms?

Several species of mosquitoes carry heartworm larvae, which enter the dog’s bloodstream through mosquito bites. The disease cannot spread from dog to dog. Heartworm larvae only mature into adults once inside the dog’s heart and surrounding arteries.

What are the signs and risk factors of heartworm disease?

Clinical signs of heartworm disease include:

  • Fever
  • Depression
  • Difficulty breathing
  • Weakness and fatigue
  • Dry cough/coughing up blood

Unneutered males between 4 and 7 years old and large breeds in the hound group have a higher risk of developing heartworm disease. Spayed or neutered dogs, small breeds, and dogs less than 2 years old have the lowest risk [9].

Is heartworm disease treatable?

Yes, heartworm disease is treatable. However, the prognosis depends on the severity of the disease. Infected dogs are treated with a combination of injections and cage rest. Owners should prohibit exercise according to their veterinarian’s instructions. While most dogs diagnosed and treated promptly make a full recovery, severe cases may also require antibiotics, pain relief and/or heart medications, and dietary changes. Heartworm disease can lead to heart failure, which is one reason why preventative treatment is essential.

Does Heartgard treat existing heartworms?

Heartgard is a preventative drug. While it is effective in treating hookworms and roundworms, it does not treat adult heartworms. Maturation of heartworm larvae takes up to 7 months. By the time symptoms develop, the disease has usually damaged vital organs significantly.

Do I need a prescription for Heartgard?

Yes. To receive a prescription for Heartgard, your dog must test negative for heartworm disease. Annual testing for heartworm disease is an important and routine part of every dog’s veterinary care plan.

What’s in Heartgard?

Ivermectin is the main ingredient in Heartgard. Heartgard Plus also contains pyrantel. Ivermectin treats a wide range of parasites in addition to heartworms, including ear mites and capillaria. Pyrantel, also known as pyrantel pamoate, treats intestinal worms.

Should I choose Heartgard or Heartgard Plus?

While both versions treat heartworms, Heartgard Plus also protects against intestinal worms and other parasites. If your dog is taking other medications for hookworms and/or roundworms, do not administer Heartgard Plus, as these medications can interact.

What should I do if I miss a dose of Heartgard?

Although dogs are still protected from heartworm disease up to 2 weeks after the first missed dose, its efficacy is reduced and your dog’s risk of developing heartworm disease is increased. Administer Heartgard immediately and return to the previous dosage schedule. Your dog will not usually need to be retested for heartworm disease in this case.

If more than 2 weeks have passed, talk to your veterinarian, administer the dose immediately, and return to the previous dosage schedule. Your dog will need to be tested again 6 months after the missed dose.


[1] Boehringer Ingelheim Animal Health USA Inc. (2018) Heartgard Plus Prescribing Information [Leaflet]. https://heartgard.com/sites/heartgard_global/files/2020-HG-PI.pdf

[2] Blagburn, B., Dillon A.R., Arther R.G. et al. (2011). Comparative efficacy of four commercially available heartworm preventive products against the MP3 laboratory strain of Dirofilaria immitis. Vet Parasitol, 176(2-3):189-94. doi: 10.1016/j.vetpar.2010.12.049

[3] Kryda, K., Six, R.H., Walsh, K.F. et al. (2019) Laboratory and field studies to investigate the efficacy of a novel, orally administered combination product containing moxidectin, sarolaner and pyrantel for the prevention of heartworm disease (Dirofilaria immitis) in dogs. Parasites Vectors 12(445). https://doi.org/10.1186/s13071-019-3702-6

[4] Cima, G. (2013) Heartworm preventative resistance is real. American Veterinary Medical Association. https://www.avma.org/javma-news/2013-11-01/heartworm-preventive-resistance-real

[5] Atkins, C.E., Murray, M.J., et al. (2014) Heartworm ‚Äėlack of effectiveness‚Äô claims in the Mississippi delta: Computerized analysis of owner compliance ‚Äď 2004‚Äď2011. Veterinary Parisitology 206(1-2), 106-113.¬†https://doi.org/10.1016/j.vetpar.2014.08.013

[6] Gollanker, R. (2019) Ivermectin. VCA Hospitals. https://vcahospitals.com/know-your-pet/ivermectin

[7] Gollanker, R. (2018) Pyrantel Pamoate. VCA Hospitals. https://vcahospitals.com/know-your-pet/pyrantel-pamoate

[8] Paddock, A. (2015) Common Heartworm Medication Ivermectin Can Have Serious Side Effects for Some Breeds. American Kennel Club. https://www.akc.org/expert-advice/health/heartworm-medication-side-effects/

[9] Selby, L.A., Corwin, R.M., et al. (1980) Risk factors associated with canine heartworm infection. Journal of the American Veterinary Medical Association 

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