Brand: PROHEART ® 6

Company: ZOETIS

FORMULATION: sustained release injectable suspension for subcutaneous administration

ACTIVE INGREDIENT(S): MOXIDECTIN, 3.4 mg/mL (=0.34%) in the constituted drug

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


PARASITES CONTROLLED* (spectrum of activity):


  • 0.05 mL (= 5 mcL) of the constituted drug/kg bw, equivalent to 0.17 mg moxidectin/kg bw.

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


  • LD50 (acute oral) in rats: for the suspension: 1166 mg/kg bw (source: MSDS)
  • Estimated Hazard Class according to the WHO: not applicable for veterinary medicines

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

Never use on cats tablets approved only for use on dogs, and vice-versa. Never use on small dogs tablets approved for large dogs. 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 such as moxidectin 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**?

  • Moxidectin: GENERIC (introduced in the 1990s)

*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: USA, Canada
GENERIC BRANDS available? NO, not in this type of injectable formulation

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


PROHEART 6 is an original product from FORT DODGE, which was acquired by PFIZER ANIMAL HEALTH, now ZOETIS. It has a unique claim of six months heartworm prevention, in contrast with all other heartworm preventatives, which have a claim of only one month prevention. It is also one of the very few injectable antiparasitic drugs for dogs and cats. In this formulation, moxidectin is formulated in the form of microspheres that ensure a sustained slow release of the active ingredient once it is injected into the host's organism.

Moxidectin is a systemic macrocyclic lactone introduced in the 1990s (by AMERICA CYANAMID → FORT DODGE → PFIZER → ZOETIS) effective against mites, heartworms and roundworms. It has no effect on fleas. However, at the dose used in this injectable formulation it is only effective as a heartworm preventative and against hookworms, not against other internal or external parasites. Moxidectin is moderately used in pets and livestock but not in agriculture or household pesticides. 

Immature heartworms are transmitted to pets by infected 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 dose administered moxidectin kills the migrating microfilariae in the tissues of the pets, but normally not the adult heartworms in the blood vessels.

After its introduction in the early 2000, excessive adverse drug reactions (incl. numerous deaths) were reported and the product was withdrawn from the market in the US and other countries. The manufacturer found out that certain solvents were the main responsible for it. After a change in the manufacturing process the product was re-introduced in the US in 2008 with special safety restrictions, which were partially lifted in 2013.

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.