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The Dangers of Ivermectin Toxicity in Dogs

Don’t risk giving your dog a medication meant for a horse.

Using medications in ways other than how they are prescribed puts your pets at risk. By: Minda Haas Kuhlmann

We had a sweet little terrier come in to the hospital last week.

Miss Molly had eaten breakfast that morning and seemed OK, said the dog’s human, but gradually began to get disoriented and wobbly. By the time she reached our hospital, she had difficulty standing, was trembling uncontrollably and was apparently blind.

We asked Molly’s person if there had been any exposure to a poison or toxin. She recalled that she had given Miss Molly some horse medicine as a heartworm preventive — and maybe she gave a little more than usual.

Horse medicine? Yes, indeed — she did give too much.

Miss Molly was suffering from toxicity of ivermectin, the drug in the horse medication. Used in tiny doses in canine products such as Heartgard to prevent canine heartworm, ivermectin is safe. When given in large animal doses, however, ivermectin causes neurologic signs like ataxia (wobbly walking), respiratory depression, blindness and coma.

Happy Ending

I won’t leave you in suspense: Miss Molly left the hospital later that evening on the mend and her sight was returning. But this is a cautionary tale.

The only reason folks give a large animal product to a dog rather than a canine heartworm product is to save money. This is a dangerous practice.

If you accidentally overdose your dog by giving the bovine or equine medication, a trip to the veterinary ER is going to cost you a whole lot more than the price of proper heartworm medication — not to mention that you’re endangering your dog’s life.

Ivermectin toxicity can cause wobbly walking and apparent blindness in dogs. By: J. Todd Poling

Use the Proper Medication

Many dogs take monthly heartworm prevention as an important part of a wellness program. This is essential to prevent transmission of heartworm disease from mosquitoes. Most people are familiar with names like Heartgard and Interceptor.

The drugs used in heartworm preventives like ivermectin, known as MLs (macrocyclic lactones), have revolutionized veterinary medicine. They are a safe and effective way to both prevent and treat many parasitic infections in animals, small and large.

But like so many other medications, an accidental overdose can be dangerous and perhaps even fatal. The canine products are marketed by weight to make sure your dog gets the proper amount of drug. A toy breed takes the 0-25 pound size, while a Lab takes the over-50 pound Heartgard.

Trying to save money by giving a small dog medication that is formulated for a horse is a tall order. Equine ivermectin paste or bovine ivermectin liquid is concentrated for a 1,500-pound animal. When trying to titrate this product down to a proper dose for a 15-pound terrier, you can overdose your dog not by a little but by a lot.

Ivermectin Toxicity and Treatment

Overdoses discovered quickly have the best prognosis. Early signs include:

Treatment depends on how long ago the ingestion occurred and the severity of neurologic signs at the time of presentation.

I hesitated to write about Miss Molly and the guardian-induced ivermectin toxicity in case it gave anyone the idea to use these large animal products to save money. But I sincerely hope the suffering Miss Molly went through will impress upon everyone the danger of using equine or bovine ivermectin for dogs.

Watch this pup go from the lows of ivermectin toxicity to a full recovery:

YouTube player

Use as Prescribed

The canine heartworm preventives are safe, prevent heartworm and also protect against gastrointestinal parasites, a protection not supplied in the large animal products.

When I first started practicing, I would see many dogs suffering from heartworm and untreated GI parasites.  They were skinny, anorexic, unthrifty and had vomiting and uncontrolled diarrhea.

Today, those cases are few and far between thanks to the preventive powers of once-a-month canine heartworm medications. Don’t take chances — please protect your dog with the proper medication.


This pet health content was written by a veterinarian, Dr. Debora Lichtenberg, VMD. It was last reviewed March 15, 2017.