What does Heartgard for Dogs do?

Heartgard for dogs prevents heartworm infection and treats and controls roundworms and hookworms. Let’s take a closer look below at what this prescription preventative does for dogs. 

Heartgard for Heartworm Disease Prevention in Dogs. Heartgard Plus is one of the go-to heartworm preventative products. Heartworm is caused by a parasitic worm called Dirofilaria immitis. Heartworm infections in dogs are widespread and cause lung disease, heart failure, and internal organ damage. If left untreated, heartworm disease can be fatal, and even with the proper management, the prognosis is not always favorable. 

Heartgard to Control Roundworms in Dogs. Heartgard is effective against two species of roundworms, Toxocara canis and Toxocara leonina. Dogs get roundworms via feces from infected pups and contaminated surfaces, while puppies can get them from their mothers (via the placenta and milk). Roundworms cause the classical pot-bellied appearance, diarrhea, vomiting, weight loss, appetite changes, and dull hair.  

Heartgard to Control Hookworms in Dogs. Heartgard can be used against three species of hookworms or ascarids, Ancylostoma caninum, Ancylostoma braziliense, and Uncinaria stenocephala.  Dogs get hookworm when they ingest the larvae (contained soil, infected feces), and puppies can get them from their mothers. Hookworms cause anemia, weight loss, poor growth, overall weakness, bloody diarrhea, itchy paws, and pale gums. 

How does Heartgard for Dogs Work?


Heartgard for dogs works by removing the tissue stage of heartworm larvae after infected mosquito bites. It is also efficient in eliminating certain intestinal parasites. The mechanism of Heartgard Plus is the result of its two active ingredients, Ivermectin and Pyrantel Pamoate.  

Ivermectin as Active Ingredient. Ivermectin is a broad-spectrum antiparasitic medication, and it works by inflicting neurological damage to the parasite (mimics the work of a GABA agonist). The damage results in paralysis and the death of the parasite

Pyrantel Pamoate as Active Ingredient. Pyrantel Pamoate belongs to the class of anthelmintics. It is efficient against several intestinal parasites (two species of roundworms and three species of hookworms). This anthelminthic works by paralyzing adult intestinal worms, thus enabling their elimination through the dog’s stool. 

Why use Heartgard for Dogs?

You should use Heartgard for dogs because it has a broad spectrum of efficacy, a wide margin of safety, and effortless and convenient administration. 

Broad Spectrum of Efficacy. Heartgard ensures the efficient prevention of heartworm disease. Additionally, it treats and controls five different intestinal parasite species (of the roundworms and hookworms families).

Wide Margin of Safety. In the recommended dose levels, Heartgard is safe and rarely causes any side effects and adverse reactions. Plus, it is safe for stud dogs, pregnant and breeding female dogs, and puppies over six weeks of age

Effortless and Convenient Use. As a monthly treatment, Heartgard is straightforward to give. Plus, featuring a real beef flavor, the chewables are highly palatable and enticing, making the administration hassle-free. 

What are the Side Effects of Heartgard for Dogs?


Heartgard may cause digestive and neurological side effects in dogs. Although in clinical field trials, such side effects are rare, it is advisable to monitor dogs within the first few hours of dosing with Heartgard. 

The group of digestive side effects includes:

  • Vomiting
  • Diarrhea
  • Anorexia
  • Hypersalivation.  

The group of neurological side effects includes:

  • Depression or lethargy 
  • Mydriasis 
  • Ataxia
  • Staggering 
  • Convulsions

At the recommended dose level, Heartgard may trigger a mild hypersensitivity-type reaction because of the dying or dead microfilariae with transient diarrhea. Such adverse reaction has been observed in clinical trials after Heartgard use.

Additionally, Heartgard may cause an allergic reaction in dogs with beef protein sensitivities. This is because the chewable tablet is made with real beef for palatability. Such food allergies usually manifest with stomach upset. 

Finally, due to the increased risk of adverse reactions and drug interactions, Heartgard should not be given to dogs that have taken another heartworm preventive product within the last 30 days (one month). 

What Special Precautions are There for Heartgard for Dogs?

Before starting a Heartgard prevention program, dogs must have a current annual heartworm test result or be tested for existing heartworm infections. This is because Heartgard removes the tissue stage of heartworm larvae but is not effective against adult heartworms

In dogs that have tested positive for heartworm infection, the adult stages must be removed before initiating the Heartgard program. 

Extra caution is warranted when using Heartgard in herding dog breeds as they are sensitive to elevated dose levels of Ivermectin. Typically, the drug does not cross the blood-brain barrier in dogs. However, some breeds have a genetic mutation (MDR-1 gene) affecting the Ivermectin‘s pathways. The MDR-1 gene mutation enables the medication‘s blood-brain barrier crossing and causes neurotoxicity. 

Such Ivermectin sensitivities and MDR-1 gene mutations are likely to occur in the following dog breeds

  • Shetland Sheepdogs
  • Old English Sheepdogs
  • Australian Shepherds
  • German Shepherds
  • Skye Terriers
  • Silken Windhounds 
  • Long-Haired Whippets.  

Can Dogs Overdose Heartgard?

Clinical trials regarding the safety of Heartgard in dog breeds with sensitivities (Collies) showed no toxicity signs at dose levels ten times the recommended dosage (60 mcg/kg). 

However, at higher doses, toxicity is possible and usually triggers signs and symptoms like:

  • Mydriasis (dilated pupils)
  • Blindness 
  • Ataxia (loss of coordination)
  • Disorientation 
  • Dementia 
  • Vocalization 
  • Increased respiratory rate 
  • Profuse sweating 
  • Coma. 

In case of accidental Heartgard ingestion, it is best to consult with your trusted veterinarian. The sooner you act, the better the outcome. 

Heartgard for Dogs Dosage 

The recommended Heartgard dose depends on the dog’s body weight. Heartgard comes in three different strengths for different weight categories. Here is an explanation:

  • Blue Heartgard Plus Chewables (1 – 25 lbs) – 68 mcg Ivermectin 57 mg Pyrantel 
  • Green Heartgard Plus Chewables (26 – 50 lbs) – 136 mcg Ivermectin 114 mg Pyrantel 
  • Brown Heartgard Plus Chewables (51 – 100 lbs) – 272 mcg Ivermectin 227 mg Pyrantel

For dogs over 100 pounds of body weight, it is recommended to make a combination of these chewable tablets

If instead of Heartgard Plus, you are using Heartgard (only Ivermectin), the dosing is similar:

  • Dogs 1 – 25 lbs – Blue Heartgard Chews (68 mcg of Ivermectin)
  • Dogs 26 – 50 lbs – Green Heartgard Chews (135 mcg of Ivermectin)
  • Dogs 51 – 100 lbs – Brown Heartgard Chews (272 mcg of Ivermectin)
  • Dogs over 100 lbs – a combination of these chewable tablets.  

For optimal performance, Heartgard should be used as a monthly treatment and administered on about the same day of the month. Delaying the dose for over a month (more than 30 days) reduces the efficacy of Ivermectin

In case you forgot to dose your dog, the first step is immediate treatment with Heartgard, and the second is the resumption of the recommended dosing regimen. This will minimize the risk of the development of adult heartworms.  

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Heartgard Plus Chew for Dogs
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  • Heartgard Plus Chew for Dogs, 51-100 lbs. is a heartworm preventative that kills heartworms and also helps treat and control roundworms and hookworms. 
  • It comes in a delicious once-a-month, real-beef chew that dogs love and is safe for puppies 6 weeks and older.
  • When given every 30 days, HEARTGARD Plus can help keep your dog protected year-round.

Heartgard for Dogs Usage Guidelines 

The general rule is that Heartgard can be used in monthly intervals during the period of the year when vectors (mosquitoes) are active and potentially carrying infective heartworm larvae. Depending on where you live, all-year-round protection might be recommended (discuss this with your veterinarian). 

If the vet recommends seasonal use of Heartgard Plus, the initial dose is given within a month of the dog’s first exposure to mosquitoes. The final dose is administered within a month of the dog’s last exposure to active mosquitoes

If switching the dog from another preventive product to Heartgard, the first dose of Heartgard needs to be given within a month of the last dose of the former medication. The efficacy of Ivermectin will be reduced if the interval between the two preventive medications is greater than one month (30 days).   

Because of the real beef flavor, Heartgard is straightforward to give to dogs. It can be offered as a treat. Since the chewable tablet needs to be chewed and not swallowed, you can break the chew into smaller pieces if your dog tends to swallow its treats as a whole. 

You can also offer the chewable with a small amount of dog food. However, you need to take care the dog ingests the complete dose and monitor it for several minutes to make sure part of the dose is not rejected. 

To prevent reinfection after the use of Heartgard Plus, you need to:

  • Eliminate fleas on all pets in the household 
  • Avoid feeding your dog uncooked meat 
  • Practice daily cleanup of stools

How can I Store Heartgard for Dogs?

Once you dose your dog with Heartgard, return the foil-backed blister card with the remaining chewables to its original box and store it at room temperature (68°F – 77°F) away from heat and light. 

Please do not remove the chewables from the original wrapper beforehand (only when you are about to use them). Improper storage, the same as exposure to heat and light, decreases the efficacy of Ivermectin

Also, make sure the box is out of reach of children and dogs. In case of human ingestion, contact a physician or Poison Control Center. In case of accidental ingestion by pets, call your trusted veterinarian or an Animal Poison Control Helpline. 

Alternatives for Heartgard for Dogs


Heartgard for dogs is an excellent heartworm preventive. However, if for some reason you cannot use Heartgard, there are other products worth trying. Here are some of the best alternatives for Heartgard for dogs. 

Advantage Multi for Dogs. Advantage Multi features two active ingredients, Imidacloprid and Moxidectin. Together they offer protection against six common parasites, including heartworm, roundworms, whipworms, hookworms, fleas, and sarcoptic mange. The only disadvantage is that it comes in the form of spot-on treatment for topical application. 

Sentinel for Dogs. Sentinel contains two active ingredients Milbemycin Oxime and Lufenuron. It is available in the form of beef-flavored chews and offers protection against heartworm and intestinal parasites (roundworms, whipworms, and hookworms). Sentinel can also be used to control and treat fleas. 

Tri-Heart Plus for Dogs. Tri-Heart Plus is the most budget-friendly heartworm preventative option (it usually costs 20% less than Heartgard while featuring the same active ingredients). The Tri-Heart Plus chew is enticing, easy to use, and offers protection against heartworms, roundworms, and hookworms. 

Interceptor Plus for Dogs. Interceptor Plus is a monthly treatment in the form of tasty chicken-flavored chewable. The active ingredient in Interceptor Plus is Milbemycin Oxime is a powerful anthelminthic that works against several different worms, including roundworms, tapeworms, whipworms, and hookworms. 

ProHeart 12 for DogsProHeart 12 is the only FDA-approved heartworm preventive in injectable form. The downside is that you will need to make a trip to the vet’s office to have it applied, but the good news is ProHeart 12 offers up to one year of protection. Plus, ProHeart 12 is efficient against other worms (roundworms, hookworms, and whipworms) and sarcoptic mange. 

In theory, some natural remedies should decrease your pet‘s risk of infection with heartworm disease. However, they are not entirely efficient, and considering the severity of the infection, it is best not to experiment with such remedies and stick to the mainstream and prescription preventative formulas. 

Regardless of which formula you choose, make sure you talk with your veterinarian first. Please, keep in mind that the information in this article is purely educational and not a substitute for professional veterinary advice.