Ivermectin is safe for most dogs when used at the correct dose, and millions of dogs take it every month as part of their heartworm prevention. The dose used in heartworm preventatives (6 micrograms per kilogram of body weight) is extremely low, and even breeds known to be sensitive to the drug have a safety margin of at least 10 times that amount before toxicity signs appear. The real risks come from higher doses used for other conditions, certain breed-specific genetic factors, and drug interactions that can amplify ivermectin’s effects.
FDA-Approved Uses in Dogs
Ivermectin is FDA-approved for dogs as a heartworm preventative. It works by killing heartworm larvae during the first 30 days after a mosquito bite, before they can mature and cause damage. Several chewable tablet products combine ivermectin with other ingredients to also treat and control roundworms, hookworms, and tapeworms in a single monthly dose.
At the standard heartworm prevention dose of 6 micrograms per kilogram, ivermectin has an excellent safety record across all breeds. This is the dose found in commercially available heartworm chewables, and it’s far below the threshold that causes problems even in the most sensitive dogs.
Why Some Breeds Are at Higher Risk
Certain dogs carry a mutation in a gene called MDR1, which normally produces a protein that acts as a gatekeeper for the brain. This protein pumps drugs back out of the central nervous system before they can accumulate. Dogs with the MDR1 mutation have a faulty version of this gatekeeper, allowing ivermectin (and other drugs) to build up in the brain to dangerous levels.
Collies are the most commonly affected breed. Roughly 70% of Collies in the United States carry the mutation. Other breeds with significant rates include:
- Australian Shepherds (standard and mini): about 50%
- Long-haired Whippets and Silken Windhounds: about 30 to 50%
- McNabs: about 30%
- Chinooks: about 25%
- Shetland Sheepdogs and English Shepherds: about 15%
- German Shepherds: about 10%
- Old English Sheepdogs and Border Collies: under 5%
- Mixed-breed dogs: about 5%
Even for these breeds, the standard heartworm prevention dose is considered safe. Research published in JAVMA found that even ivermectin-sensitive Collies required a dose greater than 60 micrograms per kilogram to show any signs of toxicity, which is ten times the heartworm prevention dose. The danger increases sharply at higher doses: 30 to 40% of Collies develop toxicity signs at 150 to 200 micrograms per kilogram, while other Collies tolerate doses above 2,500 micrograms per kilogram without any problems.
Off-Label Uses and Higher Doses
Ivermectin is sometimes prescribed at much higher doses to treat skin conditions like mange. For sarcoptic mange (caused by burrowing mites), the typical dose is around 200 micrograms per kilogram, given every two weeks for two to four treatments. For demodectic mange, doses range from 300 to 600 micrograms per kilogram given daily, sometimes for weeks.
These doses are 30 to 100 times higher than the heartworm prevention dose, and this is where the risk becomes real, especially for MDR1-affected breeds. Vets who prescribe ivermectin at these levels typically start with a low dose and increase gradually while watching closely for any early signs of a reaction. For breeds known to carry the MDR1 mutation, alternative treatments are often preferred.
Signs of Ivermectin Toxicity
If a dog receives too much ivermectin, the drug accumulates in the brain and disrupts normal nervous system function. Symptoms can range from mild to life-threatening and typically include loss of coordination, exaggerated movements, disorientation, tremors, excessive drooling, dilated pupils, and heightened sensitivity to touch or sound. In severe cases, dogs can become blind, deeply depressed, or slip into a coma.
These signs can appear within hours of a large single dose or develop gradually over days with repeated high doses. If your dog shows any neurological symptoms after taking ivermectin, it’s a veterinary emergency. Treatment is primarily supportive, keeping the dog stable while the drug clears the body. In recent years, vets have used intravenous fat-based solutions that can help pull ivermectin out of brain tissue more quickly, since the drug dissolves readily in fat.
Drug Interactions to Watch For
Certain medications can make ivermectin more dangerous by interfering with the same brain-protective protein that the MDR1 gene produces. The most well-documented interaction is with spinosad, an oral flea prevention ingredient. The FDA has issued a warning that dogs receiving both spinosad and higher doses of ivermectin have shown increased rates of ivermectin toxicity.
Spinosad blocks the protein that normally keeps ivermectin out of the brain, effectively mimicking what happens in dogs with the MDR1 mutation. It also increases overall ivermectin levels in the bloodstream. If your dog takes a flea product containing spinosad, let your vet know before starting any ivermectin-based treatment, especially at doses above the standard heartworm prevention level.
Genetic Testing for MDR1
If you own a herding breed, a herding-breed mix, or any of the breeds listed above, genetic testing can tell you definitively whether your dog carries the MDR1 mutation. Washington State University, which pioneered much of the research on this mutation, offers testing for around $70. You can collect a sample at home using a simple cheek swab, or your vet can submit a blood sample.
The test tells you whether your dog has zero, one, or two copies of the mutant gene. Dogs with two copies are most sensitive, dogs with one copy have intermediate sensitivity, and dogs with no copies process ivermectin normally. Knowing your dog’s status is useful beyond ivermectin, since the MDR1 mutation also affects how dogs handle several other common medications, including some sedatives and anti-cancer drugs.
Keeping Ivermectin Use Safe
For the vast majority of dogs, monthly heartworm preventatives containing ivermectin are completely safe and have been used successfully for decades. The key points to keep in mind are straightforward: stick with commercially manufactured heartworm products rather than trying to dose ivermectin from livestock formulations, which are far more concentrated and easy to miscalculate. If your dog needs ivermectin at higher doses for a condition like mange, make sure your vet is aware of your dog’s breed background. And if you have any breed on the MDR1 risk list, a one-time genetic test can give you peace of mind and guide medication choices for the rest of your dog’s life.