Why Heartworm Preventative is Dangerous in Overdose
Heartworm preventatives contain drugs like ivermectin, milbemycin, or selamectin, which are very safe at the correct dose but can cause severe neurological signs in overdose.
Some dog breeds (e.g., Collies, Australian Shepherds) can have a genetic sensitivity making them prone to toxicity even at standard doses.
Accidental ingestion of multiple doses by a pet, or giving a dog's dose to a cat, are common causes of poisoning.
Why speed matters
Delaying care significantly worsens outcomes. Some signs appear hours or days later, so "looks fine right now" is not a reason to wait.
Signs to watch for
- Dilated pupils
- Drooling
- Loss of coordination and stumbling
- Muscle tremors or shaking
- Seizures
- Lethargy and weakness
What to do right now
- Call your vet or a pet poison helpline immediately.
- Have the product packaging ready to tell them the active ingredient and how much your pet may have ingested.
- Do NOT induce vomiting unless specifically instructed by a professional.
- Your vet may induce vomiting if the ingestion was very recent, or provide supportive care and medications to control seizures.
If your pet shows any of the signs above, go to the nearest emergency vet now and call ahead.
The step-by-step response, from the first minute to the vet visit, is in our Heartworm Preventative Overdose emergency guide.