Heartgard for Dogs with Seizures: Is It Safe?

Name: Heartgard for Dogs with Seizures: Is It Safe?
Creator: Dr. Zubair Khalid

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-10-03

Heartgard for dogs with seizures is usually safe when it is given at the correct monthly dose. The amount of ivermectin in standard heartworm prevention is low, and most dogs with epilepsy tolerate it without any change in seizure control. The main exception is dogs with the MDR1 gene mutation, who can react badly even to normal doses [1].

This article is educational and is not a substitute for veterinary diagnosis or treatment.

Quick Answer

Heartgard's active ingredient is ivermectin, which kills heartworm larvae before they grow into adult worms [1].
Standard preventive doses are not likely to cause a problem in dogs that are not sensitive to ivermectin, even if they get one or two extra tablets [1].
Collies, Border collies, and Old English sheepdogs are among the breeds that can be extra sensitive to ivermectin because of a multidrug resistance gene mutation [1].
Never skip or stop heartworm prevention because your dog has seizures. Heartworm disease itself is far more dangerous than the preventive.
Ask your veterinarian before switching products, and tell them every seizure medication your dog takes.

What Heartgard Is and Why Veterinarians Prescribe It

Heartgard is a heartworm preventive that veterinarians prescribe for dogs and cats. Its active ingredient is ivermectin, a compound that kills heartworms at the larval stage [1]. That timing matters. Mosquitoes deposit heartworm larvae in your dog's skin, and those larvae travel through the body for months before maturing into adult worms in the heart and lungs. Ivermectin clears them out during that window.

Most dogs receive Heartgard on a monthly basis [1]. The chewable form is the one owners know best, and many dogs take it willingly because it is flavored. The goal is simple: one dose every month, all year, so no larvae ever reach adulthood.

Heartworm prevention is not optional for most dogs in the United States. Heartworm disease is transmitted by mosquitoes in every state, and treatment for an established infection is expensive, hard on the dog, and carries real risk. For a dog with epilepsy, the safer choice is almost always to stay on a preventive instead of to stop it.

How Ivermectin Works in the Body

Ivermectin belongs to a drug class called macrocyclic lactones. These drugs open channels in the parasite's nerve and muscle cells, which paralyzes and kills the larvae. The drug has a much stronger effect on the parasite than on the dog because of how it distributes in each body.

The dog's own nervous system is largely protected by a pump called P-glycoprotein. This pump sits in the blood-brain barrier and pushes drug molecules back out of the brain. In dogs with a normal P-glycoprotein system, very little ivermectin reaches the brain at preventive doses.

This is where the MDR1 gene comes in. MDR1 codes for that pump. Dogs with a mutation in this gene make a faulty pump, so ivermectin and similar drugs can build up in the brain and cause neurologic signs. Collies, Border collies, and Old English sheepdogs are among the breeds known to carry this mutation, which makes them extra sensitive to ivermectin [1]. A regular dose may adversely affect these dogs [1].

If you own one of these breeds, your veterinarian can test for the mutation with a simple cheek swab or blood sample. Knowing the result changes which preventives are safe for your dog.

Dosing Basics for Dogs on Seizure Medication

Your veterinarian decides the dose. It is calculated from your dog's body weight and health status, and the label dose is set well below the level that causes neurologic problems in normal dogs. Do not split, double, or guess at a dose on your own.

Give the chewable with or right after a meal. Most dogs take it as a treat. If your dog is on phenobarbital or another anti-seizure drug, tell your veterinarian which one and at what dose. They will factor that into the plan.

If you miss a month, give the dose as soon as you remember if it has been less than a few weeks. If more than a month has passed, call your veterinarian. They may want to test for heartworm exposure before restarting, because a gap in coverage leaves your dog unprotected.

If your dog vomits shortly after taking the dose, call your veterinarian. They will tell you whether to redose. Do not give a second dose on your own.

Side Effects: What to Watch For

Most dogs have no reaction at all. When signs do appear, they are usually mild and short-lived.

Side effect	How common or serious	What to do
Vomiting, diarrhea, drooling	Uncommon, mild	Watch for 24 hours. Call your vet if it continues.
Lethargy or poor appetite	Uncommon, mild	Call your vet if it lasts more than a day.
Tremors, wobbling, unsteadiness	Rare in normal dogs, more likely in MDR1 dogs	Call your vet the same day.
Dilated pupils, blindness, disorientation	Rare, serious	Treat as an emergency. Call your vet or poison control now.
Worsening or new seizures	Not established as a direct effect of preventive doses	Call your vet the same day. Do not stop the drug on your own.

The neurologic signs on that list are the ones tied to ivermectin sensitivity. They show up when too much drug reaches the brain, which is what happens in dogs with the MDR1 mutation [1].

One reassuring point: a recent study looked at retinal function and pupil responses in healthy dogs receiving oral macrocyclic lactone heartworm preventives and found no association between the medication and abnormal results [2]. That does not rule out every possible effect, but it supports what veterinarians see in practice.

If your dog has a seizure after a dose, write down the time, how long it lasted, and what it looked like. That record helps your veterinarian sort out whether the two events are related or coincidental. For step by step guidance during a seizure, see Dog Seizures: Emergency Steps and When to Go to the Vet.

Who Should Not Take Heartgard

Dogs with a known MDR1 mutation should not receive ivermectin preventives. Your veterinarian will choose a different heartworm preventive from a different drug class.

Collies, Border collies, and Old English sheepdogs without a confirmed test result deserve extra caution, since these breeds are more sensitive to ivermectin than other breeds [1]. Testing removes the guesswork.

Puppies under the labeled age and weight should not take the product until they meet the minimums on the package. Pregnant or nursing dogs need a veterinarian's approval first.

Drug interactions matter too. If your dog takes multiple anti-seizure medications, your veterinarian will review the whole list before prescribing. Dogs with refractory epilepsy are often on two to six anti-seizure drugs at once, which is a common situation in canine neurology practice [3]. That makes a full medication review essential at every visit.

Some owners ask about cannabidiol (CBD) for seizure control. If your dog is on CBD, mention it. Studies of CBD in dogs with drug-resistant epilepsy have shown reductions in seizure frequency, but they also reported ataxia as a side effect and increased serum alkaline phosphatase levels [4][3]. Your veterinarian needs the complete picture to judge interactions.

Heartgard Overdose and When to Call for Help

The amount of ivermectin in standard heartworm prevention is not likely to cause an emergency in dogs that are not sensitive to the drug, even if they eat one or two tablets above their normal dose [1]. That is the good news for most households.

The risk rises with large numbers of tablets or with medication intended for a larger animal [1]. A dog that eats a whole box, or a horse dewormer paste, needs immediate attention. Signs of overdose include vomiting, drooling, wobbling, tremors, dilated pupils, and seizures.

Call your veterinarian right away if you suspect an overdose. If your clinic is closed, call a poison control hotline. The ASPCA Animal Poison Control Center is available at (888) 426-4435, and the Pet Poison Helpline is available at (855) 764-7661. Have the product packaging and your dog's weight ready.

Do not wait for symptoms to appear before calling. Treatment is easier and safer when it starts early.

Limitations and When to Contact a Veterinarian

No article can tell you whether your specific dog should take a specific drug. That decision belongs to you and your veterinarian, who know your dog's weight, breed, seizure history, and full medication list.

Call your veterinarian the same day if your dog shows tremors, wobbling, dilated pupils, unusual lethargy, or vomiting after a dose. Call the same day if seizures become more frequent or more severe after starting any new medication.

Go to an emergency clinic now if a seizure lasts more than five minutes, if seizures cluster with no recovery between them, or if your dog is unresponsive, struggling to breathe, or not waking up. These are emergencies regardless of what preventive your dog takes. For more on what can be done during and after a seizure, see What Can Be Done For Dog Seizures.

Schedule a routine visit if you want to discuss switching preventives, testing for the MDR1 mutation, or reviewing your dog's seizure medications. Before any new seizure drug is started, your veterinarian will typically run baseline lab work, and you can read about that process in What Tests Does Your Vet Run Before Prescribing Seizure Medication for Dogs?.

Frequently Asked Questions

Can Heartgard make my dog's seizures worse?

At correct preventive doses, there is no well established link between Heartgard and worsening seizures in dogs with a normal MDR1 gene. If your dog has the mutation, ivermectin can reach the brain and cause neurologic signs, which may include seizures [1]. Tell your veterinarian about any change in seizure pattern after a dose.

Should I stop heartworm prevention if my dog has epilepsy?

No. Stopping prevention leaves your dog open to heartworm disease, which is a serious and sometimes fatal condition. If you are worried about ivermectin, ask your veterinarian about a preventive from a different drug class instead of skipping prevention altogether.

How do I know if my dog has the MDR1 mutation?

Your veterinarian can order a genetic test using a cheek swab or blood sample. Breeds such as Collies, Border collies, and Old English sheepdogs are more likely to carry the mutation and are more sensitive to ivermectin [1]. Testing is the only way to know for certain.

What should I do if my dog has a seizure right after taking Heartgard?

Call your veterinarian the same day and describe the seizure, including how long it lasted and what your dog did before and after. Do not give another dose until you have spoken with them. If the seizure lasts more than five minutes or repeats without recovery, go to an emergency clinic.

Are other heartworm preventives safer for dogs with seizures?

Some preventives use drugs from different classes, and those may be a better fit for a dog with the MDR1 mutation. The right choice depends on your dog's test result, age, weight, and other medications. Your veterinarian can match the product to your dog's specific situation.

References

Can Dogs Overdose on Hartgard. Pet Poison Helpline.
Effects of Oral Macrocyclic Lactone Heartworm Preventatives on Retinal Function and Chromatic Pupillary Light Reflex in Healthy Companion Dogs. Veterinary ophthalmology, 2026.
An Exploratory Study of Cannabidiol as an Adjunctive Treatment for Refractory Epilepsy in Dogs. Animals : an open access journal from MDPI, 2025.
Cannabidiol treatment for refractory idiopathic epilepsy in dogs: A systematic review with risk of bias assessment. Preventive veterinary medicine, 2025.