Reviewed by Sophie, Veterinary Nurse at Dial A Vet
First — take a breath. I know it's stressful, especially when it happens in the evening and your regular vet is closed. The good news is that most common dewormers have a wide safety margin, and a double dose is unlikely to cause serious harm. But I can't give you a blanket "he'll be fine" without knowing which dewormer it was, because the risk level genuinely varies between products.
Here's what you need to know tonight, and what to watch for.
It depends on the active ingredient
Not all dewormers are the same, and the risk of a double dose varies significantly between them:
Pyrantel (Drontal, Cantel) — This is one of the safest dewormers on the market. It has a very wide safety margin, and a double dose in an otherwise healthy dog is unlikely to cause anything more than mild stomach upset. You might see a loose stool or two, but that's usually it.
Fenbendazole (Panacur) — Also has a generous safety margin. Fenbendazole is routinely prescribed at higher doses for extended periods (e.g. for Giardia treatment), so a single double dose is very low risk. Watch for mild vomiting or diarrhoea, but serious toxicity is rare.
Praziquantel (found in Drontal Plus, Milbemax) — Generally well-tolerated even at higher doses. You may see drooling, a brief bout of vomiting, or loose stools, but significant toxicity from a double dose is uncommon.
Milbemycin (Milbemax, Sentinel) — This is where you need to pay closer attention. Milbemycin belongs to the macrocyclic lactone family. At a standard double dose, most dogs will be fine. However, dogs with the MDR1 gene mutation (common in Collies, Shelties, Australian Shepherds, and their crosses) can be far more sensitive to this class of drug. If your dog is a herding breed or a herding-breed mix and they've had a double dose of milbemycin, contact your vet.
Ivermectin (Heartgard, Ivomec) — Another macrocyclic lactone, and the same MDR1 warning applies here, even more so. Ivermectin toxicity in MDR1-positive dogs can cause tremors, disorientation, blindness, and seizures. If your dog is a breed at risk and has received a double dose of an ivermectin-based product, call your vet immediately — don't wait for symptoms.
What to watch for in the next 24 hours
Regardless of the dewormer type, keep an eye on your dog for the following:
- Vomiting or diarrhoea — A single episode isn't usually cause for alarm. Persistent vomiting (more than 2–3 times) warrants a vet call.
- Excessive drooling or lip-licking — This often signals nausea and usually passes within a few hours.
- Lethargy — Some dogs may be a bit flat for half a day. If they're still unresponsive or unusually quiet after 12 hours, get them checked.
- Tremors, wobbliness, or disorientation — These are red flags, especially in herding breeds. This suggests possible neurotoxicity and requires urgent veterinary attention.
- Loss of appetite — Missing one meal after a double dose isn't unusual. Missing two or more meals is worth following up on.
When to call a vet immediately
Don't wait and see if your dog:
- Is a herding breed or herding-breed mix (Collie, Sheltie, Aussie, Border Collie, etc.) and received a macrocyclic lactone dewormer
- Is showing tremors, seizures, loss of coordination, or dilated pupils
- Is a puppy under 6 weeks old
- Has a pre-existing liver condition
- Is vomiting repeatedly and can't keep water down
How to prevent this happening again
The most common cause I see is two family members both dosing the dog without telling each other. A few simple habits can prevent it:
- Mark it on a shared family calendar or put the worming packet on the fridge after dosing
- Use a pet medication tracker app
- Write the date on the dewormer box when you administer it
- Designate one person in the household as responsible for medications
Frequently Asked Questions
Can a double dose of dewormer kill my dog?
It's extremely unlikely with most common dewormers like pyrantel, fenbendazole, or praziquantel — they have wide safety margins. The exception is macrocyclic lactones (ivermectin, milbemycin) in dogs with the MDR1 gene mutation, where toxicity can be serious. If you're unsure whether your dog carries this gene, your vet can order a simple cheek swab test.
My dog vomited after the double dose — should I redose?
No. Don't give any more dewormer. If they vomited within 30 minutes of the dose, some of the medication may not have been absorbed, but since they've already had a double dose, they've almost certainly received a full therapeutic dose. Wait for the vomiting to settle, and continue with your normal worming schedule at the next due date.
Can other medications interact with the dewormer?
Yes. Some medications can increase the risk of side effects when combined with certain dewormers. Spinosad (found in Comfortis) and ivermectin taken together can increase ivermectin blood levels significantly. Always tell your vet about everything your dog is taking, including flea and tick treatments, heartworm prevention, and supplements.
How long until the dewormer is out of my dog's system?
Most oral dewormers are metabolised and eliminated within 24–72 hours. If your dog is going to have an adverse reaction, you'll typically see symptoms within the first 12 hours. If 24 hours have passed and your dog is eating, drinking, and behaving normally, they're almost certainly in the clear.
If you're still worried or your dog is showing any symptoms you're unsure about, you can book a video consultation with our vet team right now — no app or account needed. We'll assess your dog and let you know whether they need to be seen in person.
— Sophie, Veterinary Nurse, Dial A Vet
Clinically Approved Standard by Dial A Vet Veterinary Team
Disclaimer: The customer and expert messages presented herein may have been altered with artificial intelligence or modified to enhance readability and safeguard the privacy of individuals. This content is for informational purposes only and is not a substitute for professional veterinary advice, diagnosis, or treatment. If your pet is experiencing a medical emergency, contact your local veterinarian or emergency animal hospital immediately. Talk to a vet now.
