FORMULATION: «tablets» for oral administration; may be chewable, flavored, coated, etc, depending on the country


CHEMICAL CLASS of the active ingredient(s): MACROCYCLIC LACTONE


PARASITES CONTROLLED* (spectrum of activity):


USA and other countries

  • Dogs ≤25 lbs. ≈ 11 kg bw: 1 tablet with 68 mcg ivermectin (equivalent to ≥6.2 mcg/kg ivermectin)
  • Dogs 26 to 50 lbs. 12 to 22 kg bw: 1 tablet with 136 mcg ivermectin (equivalent to 11.3 to 6.2 mcg/kg ivermectin)
  • Dogs 51 to 100 lbs. 23 to 45 kg bw: 1 tablet with 272 mcg ivermectin (equivalent to 11.8 to 6.0 mcg/kg ivermectin)
  • Dogs >100 lbs. >45 kg bw: administer the appropriate combination of tablets
  • Cats <5 lbs <2.2 kg bw: 1 tablet with 55 mcg ivermectin (equivalent to ≥25 mcg/kg ivermectin)
  • Cats 5 to 15 lbs.  2.2 to 6.8 kg bw: 1 tablet with 165 mcg ivermectin (equivalent to 75.0 to 24.3 mcg/kg ivermectin)
  • Cats >15 lbs. >6.8 kg bw: administer the appropriate combination of tablets

* Can be slightly different in some countries: read the product label!


  • LD50 (acute oral) in rats: n.a. for the tablets. 37 mg/kg for the a.i. ivermectin (source: MSDS)
  • Estimated Hazard Class according to the WHO: not applicable for veterinary medicines

Suspected poisoning? Read the article on ivermectin safety in this site.

Never use on cats tablets approved only for use on dogs, and vice-versa. Never use on cats or small dogs tablets approved for large animals. Learn more about tablets and their safety.

WARNING 1! All heartworm preventatives contain macrocyclic lactones, (e.g. ivermectin, milbemycin oxime, moxidectin, selamectin) which must be handled very carefully on dogs. The reason is that dogs of some breeds do not tolerate macrocyclic lactones or other medicines (e.g. emodepside) that can cross the blood-brain barrier. They can suffer more or less serious adverse effects if treated at dose rates slightly higher than the recommended ones. Consequently dosing must be as accurate as possible. This is the case for Collies and related breeds, which have a mutation in the MDR-1 gene that affects the blood-brain barrier and makes it more permeable to such compounds than in dogs without this mutation. Besides Collies, other dog breeds have shown similar problems, although the MDR-1 mutation has not been confirmed in all of them. The breeds more affected by this mutation are (% frequency): Collie (70%), Long-haired Whippet (65%), Australian Shepherd (50%, also mini),  McNab (30%), Silken Windhound (30%), English Shepherd (15%), Shetland Sheepdog (15%), English Shepherd (15%), German Shepherd (10%), Herding Breed Cross (10%). Other less affected breeds are: Old English Sheepdog, Border Collie, Berger Blanc Suisse, Bobtail, Wäller. The only way to be sure that a dog breed is affected by this mutation or not, is to test for it. As more dogs are tested it is likely that the mutation is discovered in other breeds, or that the frequencies change.

WARNING 2! Heartworm preventatives stop development of microfilariae to adult worms but do not cure infections with adult worms. These preventative medicines are different from those curative anthelmintics that kill the adult worms. But preventatives may kill a few adult worms. If this happens, such dead worms may block lung vessels, which can be seriously harmful, even fatal for the pet. Consequently, heartworm preventatives are usually not administered to pets that are already infected with adult worms (hence the need for periodic diagnostic tests), because the risk of serious complications is real. The infection has first to be treated with adequate curative anthelmintics before preventative products are administered. This is however not trivial, and also risky for the same reason.

For these reasons, heartworm prevention should always be done under the supervision of a veterinary doctor.

Click here to learn more on heartworms.

You may be interested in the following articles in this site dealing with the general safety of veterinary products:


Risk of resistance development? YES

There are reports of resistance or tolerance of heartworm microfilariae (Dirofilaria spp) to ivermectin and other macrocyclic lactones in the USA (mainly in the South). This has happened after about 20 years of very intensive use of such compounds there. This may happen elsewhere as well. Currently there are no other once-a-month treatments for heartworm prevention other than those containing macrocyclic lactones.

Learn more about resistance and how it develops.


Are the active ingredients of this product ORIGINAL* or GENERICS**?

  • Ivermectin: GENERIC (introduced in the 1980s)

*Meaning that they are still patent protected and generics are not yet available
**Meaning that they have lost patent protection and may be acquired from manufacturers of generic active ingredients other than the holder of the original patent.

COUNTRIES where this product is marketed: worldwide in most countries where heartworms are endemic, including the USA, Australia, Spain, Italy, etc, but in some countries under other tradenames (e.g. CARDOMEC, CARDOTEK)
GENERIC BRANDS available? YES, in most countries

Click here to learn more about GENERIC vs. ORIGINAL drugs.


HEARTGARD was the first once-a-month heartworm preventative for dogs with ivermectin introduced in the early 1980s by MSD AGVET (→ MERIAL). Before, once-a-day pills were needed. Obviously a once-a-month pill was an enormous progress, and HEARTGARD soon dominated this lucrative market completely, until competitors launched comparable once-a-month products with other active ingredients.

Ivermectin is a broad-spectrum parasiticide effective against numerous parasites, both internal (mainly roundworms) and external (e.g. mites, lice). It is the most successfull parasiticide ever developed, nowadays used in thousands of brands for pets, livestock and also in agriculture.

Ivermectin was the first macrocyclic lactone introduced in the market in the early 1980s (by MSD Agvet, later MERIAL). It was a milestone and a tremendous progress that revolutionized the control of veterinary parasites. Nowadays there are thousands of brands with generic ivermectin worldwide. It is a broad-spectrum parasiticide effective against numerous parasites, both internal (e.g. heartworms, roundworms) and external (e.g. mites, lice) but not against tapeworms. It is the most successfull parasiticide ever developed, nowadays used in thousands of brands for pets, livestock and also in agriculture. At the administered dose in this product it is only effective as a heartworm preventative and for the control of hookworms (Ancylostoma tubaeformae and A. braziliense) in cats.

Immature heartworms are transmitted to pets by mosquitoes that inject microfilariae (i.e. immature heartworms) during their blood meal. These microfilariae then migrate through the pet's tissues towards the blood vessels. At the recommended dose ivermectin kills the migrating microfilariae in the tissues of the pets, but normally not the adult heartworms in the blood vessels.

In cats ivermectin at the recommended dose controls adult hookworms (Ancylostoma tubaeformae and A. braziliense) as well. But not in dogs, because for safety reasons the dose administered to dogs is lower than the dose administered to cats and insufficient to control other worms. This is why MERIAL introduced HEARTGARD PLUS later on, containing ivermectin and pyrantel, to add efficacy against other roundworm species in dogs.

In some countries HEARTGARD PLUS is marketed under other trade names, e.g. CARDOTEK, CARDOMEC.

For an overview and a list of the most popular pet wormers click here.


This article IS NOT A PRODUCT LABEL. It offers complementary information that may be useful to veterinary professionals and users that are not familiar with veterinary antiparasitics. 

Information offered in this article has been extracted from publications issued by manufacturers, government agencies (e.g. EMEA, FDA, USDA, etc.) or in the scientific literature. No guarantee is given on its accuracy, integrity, sufficiency, actuality and opportunity, and any liability is denied. Read the site's DISCLAIMER.

In case of doubt contact the manufacturer or a veterinary professional.