Heartgard for Dogs: Dosage and Side Effects

How much do you love your dog?

If there was a condition that was common, easy to catch, and deadly, whilst treatment against it could put your dog’s life in danger, wouldn’t you move heaven and earth to protect your dog against it?

This is exactly what Heartgard for dogs can do for the canine companion in your life.

Heartgard is a preventative treatment to protect dogs against a deadly blood-borne parasite called Dirofilaria immitis, perhaps better known as Heartworm.

Heartgard for dogs is highly efficient and yet even so there are still rules to play by to keep your pet safe, such as the nature of heartworm.

But to appreciate these rules and to understand how best to use Heartgard, let’s get a measure of heartworm and the devastating damage it can cause your pet.

1. The Life and Times of Heartworm

The heartbreaker behind the whole story is a tiny worm, called Dirofilaria immitis.

This unpleasant parasite is relatively unusual in that it matures slowly, the practical upshot of which is that dogs may not show signs of infection for many months or years.

Indeed, the average age for a dog to become sick with heartworm is two to eight years of age.

cute puppy

But back to the beginning and how heartworm infects your dog.

Dirofilaria travels between hosts (in this case dogs) in the digestive tract of a mosquito.

When that mosquito bites it introduces the infectious larval form of Dirofilaria into your dog.

For a golden period of around two months, those larvae can be killed by a preventative, such as Heartgard.

If however, you miss that step through moment then the consequences are serious…and costly.

After two months those larvae are now young adult worms, and these youngsters are like mini Super worms as they are invulnerable to both preventatives and drugs that kill adults.

They survive and grow for around five months, by the end of which time they are adults.

The adult worms cause major heartache by plugging the heart and major blood vessels, in the same way, that grease plugs a drain.

The heart can’t work properly, the dog shows signs of fatigue, and will eventually go into heart failure.

sad dig

The glimmer of light is that adult worms can be killed by a drug called melarsomine (Immiticide) but the bad news is those dying worms cause a whole new raft of problems including pneumonia, and so the dog isn’t out of the woods yet.

Looking at things logically, the best way to keep your dog healthy is to use a preventative, such as Heartgard for dogs, on a regular basis.

2. What is Heartgard?

Heartgard is a tasty tablet that the dog takes once a month.

It is a prescription product which means it is either supplied direct by your veterinarian or they can write a script for you to order Heartgard from a reputable online pharmacy.

It is so important that your dog takes Heartgard that it’s important to be sure he swallowed the whole tablet and doesn’t spit any out.

With this in mind, encourage your dog to chew the tablet (it improves the release of the active ingredient) and watch him like a hawk to make sure the whole dose disappears down his throat.

The active ingredient is ivermectin, and Heartgard Plus contains the addition of pyrantel as a wormer.

However, it should be borne in mind that despite Heartgard is not a broad spectrum wormer and is not effective against whipworms and tapeworms.

This means that although you use Heartgard conscientiously once a month, you do need to use a top up product against a full spectrum of worm infections.

puppy bone in mouth on dogstruggles

Heartgard is designed to give once a month, although whether to give this preventative all year round is debated by some.

Some people argue that the dog should be given a break from medication during the non-mosquito season, whilst others say this puts the dog at risk should the weather be unseasonably warm.

Indeed, it should be noted that unless Heartgard is given for 12-months of the year, some suppliers refuse to guarantee its effectiveness.

3. Can all Dogs take Heartgard?

Heartgard for dogs has been widely tested and passed as safe for puppies over the age of 6 weeks, pregnant females, and those nursing puppies.

Heartgard only kills the larvae associated with early infection.

Leave the dog unprotected for two months and then apply Heartgard, and it won’t be able to take out those young adult worms.

The next window of opportunity to get your dog healthy again is in five months’ time when the Dirofilaria are fully fledged adults.

Owners of Collie breeds may feel twitchy at the mention of ivermectin.

This is because Collie breeds are more vulnerable to the side effects of ivermectin, which can result in coma and death.

bored pomchi on dogstruggles

However, they can breathe easy where Heartgard is concerned. When you stick within the recommended dose range there is no problem.

Safety trials show that regular dogs can tolerate up to 16 times the recommended dose, whilst for Collies this is still a respectable 10 times.

4. How Else is Heartgard Used?

Heartworm has a complex life cycle, but if your dog is diagnosed with heartworm, the vet will prescribe Heartgard for at least two months.

This is used to kill larvae which could develop into adults further down the line and to prevent reinfection whilst the original problem is being sorted out.

5. What are the Side Effects of Heartgard?

The side effects of Heartgard can either be related to the drug itself or the effects of larvae dying in the blood.

In truth, ivermectin given in accordance with the label is very safe, with occasionally the odd dog getting mild diarrhea.


When used as a preventative in a continuous manner, the burden of larvae in the bloodstream is kept under control and so side effects from this are less likely.

It is again a rare reaction, but if Heartgard is used to kill larvae in a Heartworm positive dog, there is a chance of dying larvae causing ill-effects such as allergic reactions and blood infections.

6. What are the Signs my Dog has Heartworm?

If you haven’t been as conscientious with your Heartgard as perhaps you wish, then be alert for signs that could indicate your dog has a heartworm infection.

Remember, for several months your dog has no symptoms, even if infected, so clinical signs indicate an infestation with mature worms of at least seven months duration.

  • Lack of energy
  • Coughing
  • Heavy breathing
  • Rapid breathing
  • Difficulty exercising
  • Fainting
  • Fluid in the belly
  • Heart failure
  • Death

7. How does the Vet Diagnose Heartworm?

Heartworm is diagnosed by either looking for the presence of worms in the blood or on a blood test.

In the event of a positive test, the vet will look to confirm the diagnosis by running a second type of test.

This is treatment is long, arduous, and expensive, and sometimes false-positive results do occur.

8. How is Heartworm Treated?

The treatment for adult heartworms is difficult and dangerous, so the vet takes time to get the dog into the best physical shape possible first.

This involves strict rest and treating with Heartgard for dogs to nip recent infections in the bud.

If the dog is showing signs of heart disease, the vet will prescribe medication to support the heart, so that it is better able to cope with the rigors of dying worms passing through it.

pain in dog

A long course of antibiotics may be prescribed against bacteria the heartworm carry, and after two months the dog may be given steroids to take to make the effects of next stage of treatment less problematic.

Ultimately, to kill the adult heartworms three injections of Immiticide are given over a six week period.

Complications such as pneumonia are common, so the dog needs careful monitoring and supervision by a vet.

9. How to use Heartgard?

Crucial to avoid the trauma of putting your dog and you through treatment for heartworm is the regular use of a preventative such as Heartgard.

Give it to your dog monthly from a young age (eight weeks) and continue for life.

Remember, any gap in dosing exposes your dog and puts them at risk of the deadly infection that is heartworm. Read our comprehensive guide on Dog Medication here. 

Dr Sarah Robinson attended veterinary school at Oklahoma State University receiving a D.V.M. in 2008. Sarah’s longtime interest is to help people to better communicate with their pet companions, and in doing so, to help them to strengthen their relationships with their dogs and cats. Sarah has published numerous articles on canine feeding in pet related magazines, veterinary journals and leading natural health web sites.

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