Ivermectin Toxicity in Dogs

See your vet immediately if you suspect ivermectin toxicity. Ivermectin is a medication used in dogs for parasite control, most commonly in heartworm prevention at very low doses. At labeled preventive doses, FDA-approved canine products are generally considered safe, including for many dogs with MDR1 sensitivity. Problems are more likely when a dog receives too much medication, gets into livestock ivermectin products, or is given higher extra-label doses for conditions that require careful veterinary supervision.

Ivermectin toxicity mainly affects the nervous system. In healthy dogs, a protective blood-brain barrier helps keep the drug out of the brain. In dogs with an MDR1, also called ABCB1, gene mutation, that protection is weaker, so toxic effects can happen at much lower doses. Herding breeds and related mixes are overrepresented, including Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, and some mixed-breed dogs.

Clinical signs often begin within a few hours, though they can be delayed up to about 24 hours depending on the dose and formulation. Signs range from drooling, vomiting, and dilated pupils to stumbling, tremors, seizures, coma, and breathing problems. Because there is no direct antidote, early veterinary care matters. Fast treatment can reduce absorption, support breathing and body temperature, and improve the chance of recovery.

Your vet usually diagnoses ivermectin toxicity based on history and exam findings rather than a single confirmatory test. The most helpful clues are what product was involved, how much may have been given or eaten, when exposure happened, your dog’s weight, and whether your dog belongs to a breed with known MDR1 risk. Bringing the package, tube, syringe, or box to the clinic can make a big difference.

On examination, your vet will look closely at neurologic status, pupil size, heart rate, temperature, breathing effort, and hydration. Bloodwork and urinalysis may be recommended to rule out other causes of neurologic illness, check organ function, and guide supportive care. If the diagnosis is uncertain, your vet may also consider other toxicities, head trauma, metabolic disease, or seizure disorders.

In some dogs, MDR1 genetic testing is part of the bigger picture, especially if future medication planning is important. That test does not diagnose the current emergency, but it can help explain why toxicity happened at a relatively low dose. In urgent cases, treatment often begins before every test result is back, because timing matters more than waiting for perfect certainty.

The most common cause of ivermectin toxicity in dogs is overdose. This may happen when a dog gets into flavored chewable medication, receives the wrong weight-based dose, or is given a concentrated livestock product meant for cattle, horses, or sheep. Extra-label use of large-animal ivermectin products is a well-known source of severe poisoning in dogs because the concentration can be much higher than pet parents realize.

Genetics are another major risk factor. Dogs with the MDR1 or ABCB1 mutation have reduced P-glycoprotein function, which allows ivermectin to cross into the brain more easily. Collies are the classic example, but other herding breeds and mixes can also be affected. Merck notes that dogs with this mutation can develop toxicosis at doses used for demodicosis treatment, while standard heartworm preventive doses are much lower and have been tested for safety in MDR1-affected dogs when used exactly as labeled.

Age and health status may also matter. Very young puppies should not receive ivermectin products outside labeled guidance, and dogs with liver or kidney disease may clear some medications more slowly. Risk also rises when ivermectin is combined with other drugs that interact with P-glycoprotein or the nervous system. That is one reason your vet may ask for a full medication and supplement list during the visit.

The best prevention is careful medication handling. Give parasite preventives exactly as your vet prescribes, based on your dog’s current weight, and never substitute livestock ivermectin products for canine medications. Store all pet and farm medications in secure cabinets, because many flavored products are easy for dogs to chew through if left within reach.

If your dog is a Collie, Australian Shepherd, Sheltie, Old English Sheepdog, or related mix, ask your vet whether MDR1 testing makes sense. Knowing your dog’s status can help guide future medication choices, especially if higher-dose antiparasitic treatment is ever being considered. Even so, FDA-approved heartworm preventives that contain ivermectin are generally considered safe when used exactly as labeled.

Prevention also means acting fast after mistakes. If you think your dog got the wrong dose, do not wait for symptoms. Call your vet, an emergency clinic, the ASPCA Animal Poison Control Center, or Pet Poison Helpline right away. Having the product name, strength, amount missing, and your dog’s weight ready can help your vet make a faster plan.

Prognosis depends on the dose, how quickly treatment starts, whether the dog has MDR1 sensitivity, and how severe the neurologic signs become. Dogs treated early, before major nervous system signs develop, often do well. Once a dog is comatose, having repeated seizures, or needs breathing support, recovery becomes more guarded and the cost range rises because care is more intensive.

One challenging part of ivermectin toxicity is that recovery can be slow. Because the drug is fat-soluble and affects the brain, some dogs need several days of hospitalization and then additional time at home before they are fully normal. Nursing care matters. Dogs may need help staying warm, eating safely, turning in bed, and avoiding pressure sores or aspiration while they recover.

Many dogs survive with prompt supportive care, but not every case has the same outcome. Your vet can give the most useful prognosis after seeing how your dog responds in the first 24 to 48 hours. Follow-up may include medication review, possible MDR1 testing, and a safer long-term parasite prevention plan.