Ivermectin is the active ingredient in Heartgard, a prescription heartworm preventative for dogs. It is also the ingredient in Ivomec, a large animal wormer available at feed stores over the counter.

This has led some people to try to save money on the purchase price, and to avoid paying for heartworm testing, by purchasing the large animal Ivomec for their dogs. There are three reasons why this is a bad idea.

1. This practice is the number one cause of ivermectin toxicity in dogs**. This is because Ivomec is much, much more concentrated than Heartgard. Even if you are trained in administering medicine, it is very easy to give your dog too much. Think how you would feel if you poisoned your dog trying to save a buck.

2. Federal law* prohibits vets from telling you to use Ivomec this way. This means, you are probably not getting annual HW testing done on your dog--and annual testing is a good idea, because no preventative is perfect, especially when imperfect (read: human) pet owners are administering it.

3. Heartgard comes with a warranty. If you are getting Heartgard from your vet, and you are annually testing, the manufacturer will pay for the costs of treatment if your dog develops heartworm (as of the time of this writing). There is no such warranty on Ivomec.

*AMDUCA, the Animal Medical Drug Use Clarification Act. This is a widely misunderstood law, even by vets, but it prohibits veterinarians from using an off label drug when the active ingredient already exists in a labelled drug for that purpose in that species. It's not widely enforced in small animal medicine, but it is federal law, nonetheless.

**Information obtained from the American Association of Veterinary Toxicologists.

(I am not a licensed veterinarian. I possess an Associate of Applied Science degree in Veterinary Technology)