Heartgard with the active ingredient ivermectin is a medicine commonly used for preventing heartworm, a rare but dangerous parasite infection transmitted through mosquito bites. It works by stimulating an excessive release of neurotransmitters which paralyzes parasites or shuts down the parasitic digestive system leading to their death and removal from the body. Aside from preventing heartworm infection it can also be used to fight heartworm in dogs who are already infected, though the process can take a very long time. Dogs (apart from very young puppies) are immune to the dangerous effects of the drug as it does not cross the blood brain barrier.

Heartgard Plus?

There is another form of the product called Heartgard Plus which contains the additional ingredient pyrantel. This additional substance makes the drug effective against intestinal parasites. It is often recommended for use over the regular formulation as intestinal parasites are able to transmit from a dog to its owner(s), though it can increase the likelihood of vomiting.

Dosage For Dogs

Note: The misuse of Heartgard and any other ivermectin products can have serious consequences. Never give ivermectin to your dog unless you have permission from your vet.

The dosage depends on which type of parasite you are treating. For regular use in the prevention of heartworm the dosage is as follows:

Weight of dog

Box color


< 26 lbsBlueOne pill monthly
26 – 50.9 lbsGreenOne pill monthly
60 – 100.9 lbsBrownOne pill monthly

Always check the packaging carefully in case the colors and design have been changed. For treating dogs weighing 101 lbs or more administer an appropriate combination of tablets. You should never use the drug without consulting your vet first.

Dosage for treating mange

The dosage of ivermectin required for the treatment of mange is much higher than the amounts contained within Heartgard. For this reason you will need to use a generic form of the drug or a branded form containing higher amounts of active ingredient. The dosage of ivermectin for treating mange is around 136 mcg/lb body weight with doses most typically injected once every other week.

How long will it take for ivermectin to treat an already infected dog?

Depending on the extent of infection, treating a dog with heartworm can be a long and difficult process which usually involves hospitalization. The drug may be administered once a month to slowly rid the infection, but this can take up to two years. This method of treatment is thought of by many as being safer and less painful than treatment with Immiticide (melarsomine dihydrochloride).

Dog owners should be warned that using ivermectin to treat pets infected with larval stage heartworms could result in dangerous, even fatal side effects.

Is It Safe For Dogs?

Heartgard is often safe for use in dogs but you must always talk to the vet before beginning treatment, and should be aware of the contraindications and drug interactions associated with ivermectin. If using Heartgard Plus there are further interactions you should be aware of due to the inclusion of pyrantel. Certain breeds are at a higher risk of severe side effects, this includes:

  • Collies
  • English/Old English/McNab/Shetland Sheepdogs
  • Australian/German Shepherds
  • Longhaired Whippets
  • Silken Windhounds
  • Skye Terriers
  • Mixed breeds

Use should be avoided in pregnant and nursing dogs as puppies do not possess a blood brain barrier exposing them to the dangerous effects of ivermectin. Do not administer the drug to dogs under 6 weeks of age for this same reason.

Comfortis: Using this medicine alongside Comfortis is not recommended as ivermectin and spinosad (the active ingredient of Comfortis) interact. Usually this is not a major issue unless ivermectin is used in higher doses, but it’s still a good idea to avoid using the two medicines in conjunction.

Safety Guidelines

Always speak to your vet before you start giving your dog this medicine. You should talk to the vet about:

  • Any medical conditions your dog suffers with
  • Any other medicines your dog is taking
  • Whether ivermectin is safe for your dog’s breed

You should also talk about other medication on the market.

MDR1 mutation test:

Use of ivermectin is dangerous in dogs with a mutation of the MDR1 gene, which affects these breeds in particular. Doses over 20 mcg/lb can cause severe side effects in dogs with this mutation. If you have concerns, or if your dog is mixed breed, you may wish to have them tested for the MDR1 gene mutation before giving them any medicine containing ivermectin.

What Can It Be Used For?

Heartgard is often used for preventing heartworm or as a “slow-kill” option for the treatment of heartworm infection. It is sometimes preferred over the “fast-kill” Immiticide option.

The active ingredient can also be used for the treatment of mange, though the doses contained within Heartgard tablets are far too low for this purpose.

Side Effects

Side effects of these substances are common, however, serious reactions do not occur so frequently. Dogs who are taking immunosuppressive doses are more likely to suffer negative adverse effects. Expect the following:


  • Vomiting
  • Diarrhea
  • Lost appetite
  • Fatigue

Less common / rare

  • Raised heart rate
  • Tremors
  • Loss of coordination
  • Excess salivation
  • Slow heartbeat
  • Loss of sight
  • Paralysis
  • Weight loss
  • Shock
  • Coma

If your dog experiences serious side effects seek immediate veterinary attention. If you are using the medicine to treat an existing heartworm infection be extra careful and watch your dog closely for signs of shock such as hypothermia and vomiting which could become dangerous without prompt veterinary care. In cases of acute negative reactions your vet may ask you to induce vomiting in your dog using a substance such as hydrogen peroxide, and may administer activated charcoal.


Overdose of Heartgard is rare, this is because ivermectin is often safe in far higher doses and there are only a small number of tablets in each box. However, if you think you’ve given your dog an overdose contact your vet right away. Because the effects of ivermectin are irreversible, it is especially important that you avoid administering dangerous amounts and that you ensure the drug is suitable for your pet.