Heartgard Plus Chewables for Dogs

Everything about HeartGard chewables

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Looking for information on HeartGard Plus? This page outlines all you need to know about whether this medication is right for your dog, including administration, side effects and the parasites and diseases treated.

What is Heartgard Plus?

Heartgard Plus is a monthly medication designed to prevent heartworm in dogs, as well as treat infestations of roundworm and hookworm. If you want to make sure that this medication is right for you and your dog, here’s what you should know about HeartGard Plus, the parasites it treats and the details of heartworm disease.

HeartGard protects dogs against

heartworm vector


intestinal worm vector

Roundworm & Hookworm

3 different weight ranges

Heartgard 6 chewables 11 kg Heartgard 6 chewables 12-22 kg Heartgard 6 chewables 23-45 kg
Dog Weight (lbs) Below 25 26-50 51-100
Dog Weight (kg) Below 11 12-22 23-45
Chewable size (mm) 25 32 38
Ivermectin (mcg/chew) 68 136 272
Pyrantel (mg/chew) 57 114 228
Each strength comes in convenient cartons of 6 and 12 chewables.

Active ingredients


Ivermectin is a broad-spectrum antiparasitic that treats or prevents both internal and external parasites in pets and livestock, depending on the dose and method of application. In the formulation for Heartgard Plus, it is used only for the prevention of heartworm in dogs.

Ivermectin is not effective in killing adult heartworm, but rather prevents heartworm disease by killing the heartworm at the tissue larval stage before it can develop. Ivermectin is part of a group of drugs known as macrocyclic lactones, which can cause adverse effects for dogs with the MDR-1 mutation, including Collies. The dose provided in Heartgard Plus has been established as safe for use with all dog breeds.


Pyrantel is an endoparasiticide, meaning is it used against internal parasites. In the case of Heartgard Plus, it is formulated for the treatment of roundworms and hookworms in dogs.

The drug works by acting directly on the nervous system of the worms, disrupting the transmission of signals between the nerves and muscles. As a result, the worms become paralysed. They either die quickly due to paralysis or become unable to attach themselves to the lining of the intestines, at which point they are expelled from the body.

Heartgard Plus vs Other Brands

There are plenty of parasites that your dog will come into contact with over the course of its life. And unfortunately, due to the different ways that parasites impact an animal, there is no single medication that will protect or treat all pests at once. However, with the right combination, you may only need 2 to 3 medications to provide your dog with full-spectrum protection.

Each medication comes with its pros and cons, which is why you should always be as informed as possible before deciding on the treatment that’s best for your dog.

Heartgard Plus logo Heartgard logo Sentinel logo Trifexis logo Nexgard logo
Type of Application Chewable Chewable Tablet Tablet Chewable
Prevents Heartworm ✔️ ✔️ ✔️ ✔️
Prevents Roundworms ✔️ ✔️ ✔️
Prevents Hookworms ✔️ ✔️ ✔️
Safe for Pregnant or Nursing Pets ✔️ ✔️ ✔️
Active Ingredient(s) Ivermectin, Pyrantel Pamoate Ivermectin Milbemycin Oxime, Lufenuron Spinosad, Milbemycin Oxime Afoxolaner
Dosage Monthly Monthly Monthly Monthly Monthly


Your dog is at risk from a range of parasites at any given time in its life, regardless of your lifestyle and location. The combination of ivermectin and pyrantel makes Heartgard Plus effective in preventing or treating heartworm, hookworm and roundworm.


(Dirofilaria immitis)


Heartworm is one of the most problematic parasites that can affect your dog, due to the fact that they are so hard to eliminate. The heartworms themselves are very easily transmitted and when heartworm disease develops it is not only life-threatening for your dog, but costly and difficult to treat. Prevention is the most effective way to make sure your dog remains safe from this deadly disease.

These deadly parasites were at one point only found in the southern United States, but have now spread to nearly all parts of the country and most continents. Unlike many other parasites, heartworm cannot be spread directly from host to host. Heartworm is only spread through the bite of an infected mosquito and as such, tropical and temperate climates offer a much higher risk. 

Heartworms affect not only dogs, but cats, wolves, coyotes and foxes among others. Locations that have a high population of any of these animals will greatly increase the risk of your dog contracting heartworm.

Heartworm disease is the name given to the condition in which the heartworms that are transmitted to your dog develop into adults, causing damage to the lungs, heart and arteries. The adult worms grow up to a foot long, and will continue to reproduce unless treated, with reported cases of up to 250 worms living inside a single dog.


(Toxocara canis, Toxascaris leonine)

toxocara canis roundworm
Just like fleas, roundworms are one of the sad realities of owning a dog. These intestinal worms are incredibly prevalent in all parts of the world, and almost all dogs will be affected at some point in their lives. The good news is that they are easily treated and for most dogs, roundworms do not cause serious issues.

Adult roundworms are found in your dog’s digestive tract where they live, feed and reproduce. They do not attach themselves to the wall of the intestines in the way that some other worms do, instead they move freely around, eating partially digested food.

Symptoms of roundworm

It’s not unusual for a healthy adult dog to be infected with roundworms and not show any symptoms. However, you may notice signs of the roundworm in your dog’s faeces or vomit, where they are will show as white or light brown and up to several inches long. 

Roundworms cause the most problems for puppies, but dogs with large number of roundworms can also be seriously affected by symptoms including:

  • Diarrhea
  • Vomiting
  • Weight loss
  • Pot-bellied appearance
  • Dull coat

If the roundworms have moved into the lungs, they can cause coughing and respiratory issues.

How roundworms are transmitted

One of the reasons roundworms are so common is that they are very easily spread, and can be transmitted in a great number of ways.

The first way that dogs might come into contact with these intestinal worms is actually before they are born – it’s possible for the mother to transmit the roundworms through the uterus to the puppies. If the mother is infected with roundworms, she can also pass the larvae through her milk as she is nursing. For this reason, it’s important that all breeding dogs are treated for roundworm.

The most common way that adult dogs contract roundworm is through their environment. As adult roundworms lay eggs in the intestines, they are passed out via the faeces and often into the soil. A dog licking grass or even sniffing at the faeces of another dog can then ingest the eggs where they go on to develop in the body. A dog that has walked on contaminated soil then licked at its paws also has the risk of contracting roundworms in this way.

Finally, roundworms may be passed on from another host. Small animals such as birds, rodents, cockroaches and worms can also pick up the eggs from contaminated soil. If you dog eats one of these infected animals, the roundworm eggs will pass into their system and continue their lifecycle.

Roundworms in humans

These intestinal worms are what’s known as a zoonotic parasite, meaning that they can be transmitted between different species, including humans. Roundworms can actually cause serious issues, particularly for young children and pregnant women. An untreated roundworm infestation in humans can cause problems with the eyes, lungs and heart or even neurological issues.


(Ancylostoma caninum, Uncinaria stenocephala, Ancylostoma braziliense)


This is the name of several species of intestinal worms that can affect dogs. They are named for the hook-like parts in their mouths, which they used to attach to the intestinal lining of their host. Here they suck blood and other nutrients, leading to some unpleasant issues for your dog, even proving fatal in some cases.

Hookworms are a worldwide risk for dogs, but some species are more prevalent in particular areas. The preferred habitat of Ancylostoma caninum is tropical climates and the species is mainly found in Southeast Asia. Uncinaria stenocephala is the most common hookworm in cooler climates, while Ancylostoma braziliense is widespread through North, Central and South America, South Africa and some parts of Asia.

Symptoms of hookworm

Due to the way that hookworms feed, most symptoms are associated with the blood loss that they cause. It’s particularly problematic for puppies, who don’t have the reserves of blood that adult dogs do, and may need blood transfusions to survive.

Symptoms of hookworm in dogs include:

  • Anaemia
  • Pale gums
  • Dark, sticky stools
  • Lethargy
  • Weight loss or failure to grow
  • Coughing

As hookworms can enter the host through the skin, dogs that have contracted the parasite in this way may have red, itchy lesions at the site of entry, usually on the paws.

How hookworms are transmitted

Just as with roundworms, hookworms can also be passed from a mother to her unborn puppies through the placenta. Puppies may also be infected after birth as larva pass through the mother’s milk as they are nursing.

One of the keys to hookworm transmission is that after the eggs are laid and expelled from the host, they develop into larvae and they can remain dormant for weeks or even months in the soil or water while waiting for a new host. Dogs licking or sniffing the soil or drinking contaminated water will be infected with the hookworm larvae which will then continue their lifecycle.

Hookworms can also be transmitted directly through the skin, as the dog walks on contaminated soil. The larvae burrow through the skin, usually in between the toes.

Hookworms in humans

Hookworms are a zoonotic parasite and can be transmitted to humans. The most common way that humans contract hookworms is by walking barefoot on contaminated soil. Hookworm infections are a common condition in some parts of the world, causing itching on the skin where the worms have burrowed in. More problematic, however, are the species of worm that burrow further into the body, where they can damage the eyes and internal organs.

Heartworm disease

heartworm disease

There are many parasites and diseases that dogs will come into contact with during the course of their life, but heartworms are one of the most problematic. There are a number of reasons why pet owners should be particularly aware of how this parasite can affect their dog.

The first is that it is very easily transmitted, meaning simply avoiding infection is not an option. The second is that unlike other worms or parasites, there is no medication that can easily eliminate the parasite from the body. The final and most important reason is that heartworm disease can cause incredible damage to the body and is usually fatal if not detected and treated early.

How heartworm is transmitted

The only way that heartworm can be transmitted is via the bite of an infected mosquito. The mosquito picks up the microfilariae of the heartworm when feeding on an animal with heartworm disease. These then develop into infective larvae within the mosquito’s body, which are then transmitted to the new host as the mosquito feeds. A quick bite is all that is required for transmission to occur.

Symptoms and stages of heartworm

The first stage of heartworm disease is when the larvae have entered the body of your dog, but have not yet matured into breeding adults. At this stage your dog will not usually display any signs or symptoms. At this stage it is still possible to eliminate the larvae using a heartworm preventative.

As the larvae mature and reproduce in the heart and lungs, they will begin to cause issues for your dog. You may notice signs such as a light cough or a reluctance to exercise. As the heartworms are now adults, they are detectable via a blood test. Heartworms that have reached the adult stage can no longer be eliminated via regular medication.

As the disease progresses and the heartworms reproduce, your dog will now display obvious signs of the disease. At this point the heartworms are large enough to be seen with an x-ray. The most common symptoms are:

  • Lethargy or reluctance to exercise
  • Fatigue after even moderate exercise
  • Breathing difficulties
  • Persistent coughing, often accompanied by blood

In the advanced stages of heartworm disease, the symptoms will be the same, but much more pronounced. At this point the heartworms will have caused damage to the organs including the lungs, heart and liver, which will remain even if the heartworms are eliminated. While treatment at this stage is possible, the damage already caused may cause debilitation or death.

How heartworm is treated

If a blood test reveals the presence of adult heartworms in your dog, you will need to begin a heartworm treatment program with your veterinarian.

The first step will be to conduct more tests to determine how many worms are present, what size they are and if any damage has been done to the organs. These tests may include x-rays or radiographs to check for any enlargement of the heart or obstructions to the arteries, or ultrasounds to examine the structure of the heart, lungs and liver. The next steps will depend on the severity of the disease and your dog’s condition.

As exercise speeds up the rate at which heartworms cause damage to the heart and lungs, your dog will need to be prevented from exercising during the course of their treatment. This can often be very difficult for active dogs. In advanced cases, dogs may have to be prevented from moving at all by placing them in a crate.

If your dog is experiencing serious symptoms, your vet will also need to stabilise these problems. This may include pain relief, antibiotics to kill bacteria that live alongside the heartworm, diuretics to remove fluid that may have built up in the lungs, and drugs to improve heart function. In severe cases the dog may need to be hospitalised until these immediate conditions are stabilised.

Treatment for adult heartworms does not usually occur until 60 days after the initial diagnosis. This allows for the health problems to be stabilised and for any preparatory medication to take effect. Your vet will also start your dog on a heartworm preventative to ensure no new infections occur once treatment has started.

In order to eliminate the worms themselves, your vet will administer a series of adulticide injections. For the first injection your dog may be required to stay overnight with the vet so that they can be monitored for any adverse reactions. Your vet may prescribe a steroid treatment to prevent inflammation. 

One month after the first injection, your dog will require two injections spaced 24 hours apart. Again, your dog may be required to stay in for observation. An interim assessment will be conducted one month later to determine if the treatment is working and to check the progress of any health problems. 

Even if your vet determines that the treatment is working correctly, your dog will still need to be restricted. As adult heartworms die, their bodies are pushed by the blood flow into the smaller branches of the vessels that send blood to the lungs. With any kind of exercise at all, there is the increased risk that these dead worms can block blood flow, leading to complications including death.

Six months after the first injections, your vet will do a blood test to check that all adult heartworms have been successfully eliminated from the body.

Parasite lifecycles

Understanding the lifecycles of heartworms and intestinal worms can provide insight into how best to prevent and treat these parasites and keep your dog safe and healthy.


heartworm lifecycle


Heartworms start their life as these microscopic offspring, which the females release into the bloodstream of the host. Over their five-year lifespan the adult can produce millions of microfilariae. As the mosquito feeds on the host, they take up the microfilariae with them in the blood.

Infective larvae

Heartworms are somewhat unusual in that they require an intermediate host to complete their lifecycle. Inside the body of the mosquito, the microfilariae develop into infective larvae, a process that takes around 2 weeks. These larvae are then transferred to the end host when the mosquito feeds.


This is the final stage of the lifecycle and takes place within the end host. As the infective larvae enter the bloodstream, they make their way through the tissue while developing into adults. They finally end up in the heart or lungs, where they are able to begin breeding within 6 to 7 months.


roundworm lifecycle


The male and female mate within the intestines, after which the female lay thousands of eggs per day. These are then passed out via the host’s faeces and into the environment. At this stage the eggs are not infective, but after 2 to 4 weeks, they will contain infective larvae within them.

Infective larvae

Once the eggs have been ingested by a suitable host, they hatch to release the infective larvae, usually within the intestines of the host. The larvae then make their way through the liver and into the lungs. From here they are coughed up and swallowed once again. In the case of in utero transmission from mother to pup, the infective larvae are released directly into the lungs.


After the larvae have come up through the lungs and been swallowed again, they end up in the digestive tract, which becomes their final home. Here they continue to develop into adult roundworms, reaching mating age within a week of their return – 4 to 5 weeks after the initial infection.


hookworm lifecycle


As with other intestinal worms, hookworms start life as eggs, which are released into the intestines of the host and passed out via the faeces and into the environment. With the right conditions (moisture, warmth and shade), these eggs will hatch into larvae within 1 to 2 days. At this stage they are not infective. After two stages of moulting that take around 5 to 10 days, the hookworms emerge as infective larvae.

Infective larvae

At this vital stage, they can remain in the soil or water for weeks even months while waiting for the right host. The infective larvae are either ingested directly or through a secondary host, or enter the host directly through the skin. Once inside the host they make their way through the tissue to the lungs. The irritation causes the host to cough up the larvae, at which point they are swallowed.


Once the infective larvae have been swallowed, they end up in the small intestine where they complete their maturity into adults. Once they have reached breeding age, the female will produce eggs and the lifecycle is complete. Some eggs may stay within the host and develop into larvae that become embedded in the tissue – these larvae can be passed via the milk in the case of nursing dogs.

Tips for preventing worms in dogs

Preventative medication

This is obviously the most valuable thing you can do for your dogs in terms of worms. In the case of heartworm, a preventative medication is the only way to ensure that any larvae they contract do not develop into heartworm disease. In the case of intestinal worms, this medication is not actually a preventative, but a treatment. If your dog has been infected with hookworms or roundworms, this medication will eliminate them from the body, disrupting the breeding cycle.

Regular check-ups

Even if you are administering medication such as Heartgard Plus on a monthly basis, your dog will still need regular check-ups. No medication can be 100% effective, and although rare, there have been reported cases of parasite resistance. The appointment will allow your vet to check for any signs of heartworm or intestinal worms.

Keep their home clean

In the case of hookworm and roundworm, one of the most common methods of transmission is via contaminated soil or water. Even in your own yard it is important that you clean up faeces as soon as possible. Be aware that containers such as buckets fill with rainwater and become a potential breeding ground for mosquitos, which are the vector for heartworms.

Eliminate pests

Another way that dogs can ingest hookworm or roundworm larvae is by eating small animals like mice or cockroaches. If you can eliminate these pests from your home, you will be helping to protect your dog against infection.

Make sure your dog is healthy

If your dog is unlucky enough to contract heartworm or intestinal worms, being in good health will give them the best chances or dealing with any symptoms, or in the case of heartworm, the best chance at recovering from the treatment. Make sure that your dog receives a balanced, nutritional diet, plenty of exercise and of course, lots of love.

Myths about heartworm

Heartworm has something of a complicated lifecycle and disease path, which can make it difficult for owners to understand. As such, many myths have emerged about the disease and how it works, which can lead to dangerous misunderstandings. Here are some common myths and truths about heartworm.

Heartworm disease is only a risk in summer

False. While it is true that mosquitoes are more prevalent during the warmer months or in warmer climates, your dog is at risk wherever there is even a single mosquito. Even in cold climates, mosquitos have been known to survive the winter months by taking shelter indoors, or in sheds or kennels.

Heartworm disease is rarely fatal

False. Heartworm disease is a debilitating disease that is often fatal. Even if the worms are eliminated with treatment, the damage that they cause to the dog’s body is often permanent. To give your dog the best chance at survival, the disease needs to be detected and treated early.

Dogs taking heartworm preventatives don’t require annual testing

False. Even dogs taking a regular preventative such as Heartgard Plus will still require annuals testing from the veterinarian. There are a number of reasons for this. The most common is human error, as even the most dedicated of owners can be late with medication or miss a dose, which reduces the effectiveness of the treatment.

Heartworms are contagious

False. The only way that heartworm can be transmitted to a dog is via the bite of an infected mosquito. There is no way that an infected dog can directly transmit the worms to another dog or pet.

Mosquitos need to stay attached to transmit heartworm

False. Unlike ticks, which often need to be attached for hours to your dog in order to transmit disease, a mosquito can transmit heartworm with a quick bite. As it is impossible to prevent your from dog being bitten by mosquitos, a heartworm preventative such as Heartgard Plus is the only way to protect them from this parasite.

It’s easy to tell if a dog has heartworm disease

False. In the early stages of this condition, it is very common for dogs to display no symptoms at all. By the time that begin to show the diseases typical symptoms such as coughing, the disease is often established and treatment may be difficult. This is why it is so important to continue with annual testing even if you have been administering a preventative medication.

Side effects

Side effects following the administration of Heartgard Plus are rare, with vomiting and diarrhea observed in just 1.1% of cases studies. Less common side effects include lethargy, loss of appetite, dilated pupils, loss of coordination, staggering, drooling and convulsions.

Overdose can occur, especially when giving a small dog a dose that is intended for larger dogs. Signs of overdose include vomiting, diarrhea, staggering, dilated pupils and lethargy. If you suspect your dog has had an overdose, please contact your veterinarian immediately.


Dogs must be tested and cleared for existing heartworm before you begin administration of Heartgard Plus. If your dog is found to be infected, it will require treatment to remove the heartworm before starting use of Heartgard Plus.

When storing, administering or disposing of Heartgard Plus, it is important that you:

  • Keep this and all medication out of reach of children and other pets
  • Do not remove the chewable from the blister pack until you are ready to administer
  • Remove only one chewable at a time
  • In the case of accidental ingestion by humans, contact a doctor immediately
  • Store between 68°F – 77°F (20°C – 25°C) away from light
  • Dispose of empty packaging or unused chewables responsibly – ivermectin may be toxic to marine life

Our review

The original Heartgard is a trusted medication that provides protection against heartworm disease in dogs. With the addition of the active ingredient pyrantel, Heartgard Plus now also treats infections of roundworm and hookworm in dogs.

Package contents

Heartgard Plus is available in three different doses that will relate to your dog’s body weight. These will have a different colour band on the front of the box to help distinguish them, which is useful if you have dogs of different sizes, or if your dog requires a combination of chews. You can see this box has a blue band to denote it is for dogs weighing up to 25lbs. Heartgard Plus is available in packages of 6 or 12 doses.

Inside the box there is a blister pack containing the chews. The chews are a rectangular shape, regardless of the dosage. On the blister pack you will also see here there are heart-shaped stickers – these are used to stick over the monthly spaces on the back of the box which serve as a reminder for the next dose.

heartgard review packaging

Front packaging and blister pack

heartgard review wrapping

Wrapped nicely

heartgard review chewable

Tasty chewable

heartgard review feeding

Your dog will love it!


The chewables are designed to give to your dog like you would a treat. They have a beef smell and flavour that most dogs should find appealing, and a slightly crumbly texture to make it easy for the dog to chew. It doesn’t need to be given with food.

After giving Heartgard Plus to your dog, you should monitor them to make sure that they don’t drop, spit out or vomit any of the chewable. If this happens, you’re advised to redose.
Heartgard Plus needs to be chewed for maximum effectiveness. If your dog likes to swallow food whole without chewing, you should break up the chew into smaller pieces and administer them this way.


  • Month long protection against heartworm, roundworm and hookworm
  • Oral formula means no messy application
  • Chewable design is great for dogs that won’t swallow tablets


  • Does not protect against fleas or ticks
  • Active ingredient Ivermectin may cause sensitivities in some dogs


Can I use Heartgard Plus with any breed of dog?

Yes, Heartgard Plus is well tolerated and has been tested as safe for use in all breeds of dogs, including Collies.

Can I give Heartgard Plus to pregnant or nursing dogs?

Yes, Heartgard Plus has been evaluated as safe for use with breeding, pregnant or lactating dogs.

Does Heartgard Plus need to be given with food?

No, Heartgard Plus is designed to be given as a treat and can be given at any time of day, with or without food.

My dog weighs more than 100 lb (45 kg), what dose of Heartgard Plus do they need?

For dogs weighing more than the maximum weight listed on the package, they will need the appropriate combination of chewables. For example, a dog weighing 120 lbs (54 kg), will need one chewable for dogs weighing 51-100 lb, plus one chewable for dogs weighing up to 25 lbs.

Can I use Heartgard Plus with other medications?

In clinical studies, Heartgard Plus has been successfully used with other medications including flea collars, worming medications, medicated shampoos and antibiotics. If you have any concerns about using Heartgard Plus with other medication, please contact your veterinarian.

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