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Is Heartgard Safe for Dogs?

Dog eating monthly heartworm preventive
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Heartworm disease affects dogs of all ages and is seen in all 50 states [1]. Heartworm is carried by mosquitoes. Just one bite from an infected mosquito can transmit this potentially deadly disease to your dog. 

Heartgard and Heartgard Plus are two of the top vet-recommended heartworm disease preventives for dogs. But what are the active ingredients and is Heartgard safe for dogs? 

Here’s what you need to know regarding the safety of Heartgard and Heartgard Plus so that you can make informed decisions for your dog. 

What Is Heartgard for Dogs?

Heartgard is a medication that is used to prevent heartworm disease in dogs. Heartgard contains ivermectin and only prevents heartworm disease. Heartgard Plus contains both ivermectin and pyrantel, and is intended to prevent heartworm disease, as well as treat and control hookworms and roundworms. 

Dogs are infected with heartworm larvae from the bite of an infected mosquito. Ivermectin prevents heartworm disease by targeting and killing any immature heartworm larvae that may have been transmitted to your dog via mosquito bites. 

Pyrantel, the other active ingredient in Heartgard Plus, is an intestinal dewormer that kills hookworms and roundworms. Most pet parents and veterinarians prefer using Heartgard Plus vs. Heartgard because of the additional protection against intestinal parasites that are also contagious to humans.

Is Heartgard Safe for Dogs?

Heartgard Plus for dogs

Heartgard, when appropriately prescribed through a veterinarian, is safe for most dogs. Your veterinarian will take several factors into account when prescribing heartworm prevention to your dog, including your dog’s breed, concurrent health conditions, and which parasites your dog needs protection from. If your veterinarian has any concerns about the safety of Heartgard in your dog, they may want to run some blood tests before prescribing it. Dogs with seizures and dogs with kidney disease must be evaluated by a veterinarian before they are prescribed any heartworm prevention, including Heartgard Plus. Your veterinarian is best suited to determine the ideal heartworm prevention for your dog. If your dog has liver disease, kidney disease, or a seizure disorder, talk with your veterinarian about options for heartworm prevention.

Heartgard Plus has shown to be very safe when the recommended dose is administered to most dogs, including pregnant dogs, breeding dogs, stud dogs, and puppies who are older than 6 weeks of age.

As mentioned earlier, ivermectin is one of two active ingredients in Heartgard Plus. Ivermectin is safe for most dogs when it is used at the correct dosage and under the supervision of a licensed veterinarian. However, not all dogs can take ivermectin. Some herding dog breeds are more sensitive to ivermectin due to a mutation in their MDR1 gene. This can include:

  • Collie
  • Border Collie
  • Australian Shepherd
  • Mini Aussie
  • Old English Sheepdog
  • Sheltie
  • Mixed breed with herding dog

In dogs who have the MDR1 mutation, ivermectin can cross the blood-brain barrier and cause severe brain damage that can be lethal. There is a simple genetic test available to check if dogs have the mutation that causes sensitivity to ivermectin. In general, the dosage of ivermectin contained in heartworm prevention is far below the toxic dose and considered safe even in sensitive breeds. However, if you have concerns, talk to your veterinarian. 

Heartgard for Dogs: Precautions to Consider

Heartgard Plus has been prescribed to millions of dogs and used safely without any side effects. The vast majority of dogs who receive Heartgard Plus do not experience side effects. 

Can Heartgard Plus cause diarrhea in dogs? Clinical trials testing the safety of Heartgard in dogs showed that about 1 percent of dogs experienced vomiting or diarrhea after administration [2]. Pet parents have reported some adverse effects after giving Heartgard Plus to their dog. Heartgard Plus for dogs side effects reported in some dogs include:

  • Low energy
  • Vomiting
  • Diarrhea
  • Dilated pupils
  • Incoordination
  • Confusion
  • Seizures
  • Drooling

If you notice any side effects after giving Heartgard Plus to your dog, consult with your local veterinary, local emergency clinic, or the Pet Poison Hotline as soon as possible. Most side effects will resolve within a day or two, but some may require emergency treatment.

Heartgard should only be used in dogs who have tested negative for heartworm disease. Heartgard products are not effective against adult heartworms. Therefore, dogs who are infected must be treated to remove adult heartworms and microfilariae that are circulating in the blood before using Heartgard as monthly heartworm prevention [3]. 

The safety of Heartgard Plus has also been tested and shown to be safe in dogs when given at the same time as many treatments, including:

  • Flea collars
  • Medicated and unmedicated shampoos
  • Antibiotics
  • Vaccines
  • Steroids 
  • Parasite medication

The ingredients in Heartgard can, however, have negative interactions with certain medications and herbs, so be sure to tell your veterinarian about anything you are giving your dog before they are prescribed Heartgard, including vitamins, herbs, and supplements. 

Some dogs who gobble their treats whole can be at risk for choking on the chewable tablets. If this is the case for your dog, break up the tablet before administering it.

It is possible to overdose a dog on Heartgard Plus, so never double up doses to “catch up” or give your dog more than the prescribed amount. If your dog accidentally overdoses on Heartgard Plus, call your local veterinarian, the emergency clinic, or the Pet Poison Hotline immediately for support. If you miss a dose of Heartgard Plus and it is longer than 30 days since the last dose was given, call your veterinarian for advice.

In general, Heartgard Plus is safe and well tolerated by most dogs, and since its introduction has saved countless lives from heartworm disease. In addition, Heartgard Plus treats and controls intestinal parasites in dogs that are also public health concerns for humans. Heartgard Plus should not be used in all dogs, however, and should only be prescribed and administered under the supervision of a licensed veterinarian. 


  1. Heartworm Incidence Map. American Heartworm Society. 2022. Retrieved from: https://www.heartwormsociety.org/pet-owner-resources/incidence-maps
  2. Heartgard Plus prescribing information. Retrieved from: https://docs.boehringer-ingelheim.com/AH/HEARTGARD_Plus_PI.pdf
  3. Heartworm Life Cycle illustration. American Heartworm Society. Retrieved from https://www.heartwormsociety.org/veterinary-resources/practice-tools/heartworm-life-cycle