The following sections will help you to determine whether Heartgard (ivermectin) is suitable for your dog. When using Heartgard Plus, add the contraindications and interactions of pyrantel on top of those associated with ivermectin, as Heartgard Plus contains contains both substances.


If any of the following applies to your dog, they could be unsuitable for Heartgard treatment:


Liver disease, kidney disease, pregnancy, nursing. Also contraindicated for dogs with an MDR1 gene mutation, which affects breeds such as:

  • Australian Shepherds
  • Collies
  • German Shepherds
  • English Sheepdog
  • Longhaired whippets
  • McNab Sheepdog
  • Mixed Breeds
  • Old English Sheepdog
  • Shetland Sheepdog
  • Silken Windhounds
  • Skye Terriers


Liver disease.

Drug Interactions

There are known drug interactions associated with ivermectin and pyrantel, be aware of the following:


Spinosad (Comfortis)


Diethylcarbamazine citrate, levamisole, morantel, organophosphates (azamethiphos, azinphos methyl, chlorpyrifos, diazinon, dichlorvos, fenitrothion, malathion, methyl parathion, parathion, phosmet, tetrachlorvinphos) and piperazine.

The mixture of spinosad with ivermectin can be very dangerous.

What Can I Do If I Gave Heartgard To An Unsuitable Dog?

The effects of ivermectin are irreversible. For that reason, it’s important that you call the vet as soon as you think there is anything wrong. The longer you leave it the worse the situation will be. If you catch negative reactions quickly, your vet may advise you to induce vomiting in your pet, and may give them activated charcoal.