Heartgard Side Effects in Dogs: What to Expect

Name: Heartgard Side Effects in Dogs: What to Expect
Creator: Dr. Zubair Khalid

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

Most dogs take Heartgard every month with no problems at all. When heartgard for dogs side effects do appear, they are usually mild and short lived, such as a little vomiting or diarrhea. Serious reactions are rare and are seen mostly in dogs with a genetic sensitivity to the drug's active ingredient.

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

Quick Answer

Heartgard is a monthly heartworm preventive whose active ingredient is ivermectin, a drug that kills heartworm larvae [1].
The large majority of dogs tolerate it well, and most owners never see a reaction.
The most serious risk is neurologic signs (wobbliness, dilated pupils, tremors) in dogs with a multidrug resistance gene mutation, which is more common in Collies, Border collies, and Old English sheepdogs [1].
If you see vomiting, diarrhea, or lethargy after a dose, call your veterinarian. If you see wobbliness, tremors, or trouble standing, treat it as an emergency.
Never give extra doses or a second tablet to "catch up." Follow the schedule your veterinarian sets.

What Heartgard Is and What Veterinarians Use It For

Heartgard is a prescription heartworm preventive that veterinarians prescribe for dogs and cats [1]. Its active ingredient is ivermectin, a compound that kills heartworms at the larval stage [1]. That timing matters. Heartworm prevention works by killing the young worms your dog picks up from a mosquito bite before they grow into adult worms in the heart and lungs.

Most of the time, Heartgard is given on a monthly basis [1]. Your veterinarian decides whether it is the right preventive for your dog based on weight, age, health status, and where you live. Heartworm disease is a serious and sometimes fatal condition, so staying on a preventive is one of the simplest things you can do for your dog's long term health.

Heartgard is not a treatment for adult heartworm infection. If your dog already has adult heartworms, the plan looks different, and your veterinarian will guide you through it.

How It Works

Ivermectin belongs to a group of drugs called macrocyclic lactones. These drugs interfere with the nervous system of the parasite, which paralyzes and kills the larval worms before they can mature.

The same drug class is used in many heartworm preventives, including products built around moxidectin [2][3]. Because these drugs act on the parasite's nervous system, the main concern in dogs is what happens if a dog's own nervous system is exposed to more of the drug than it can clear. That is where genetics come in.

Dosing Basics

Who decides the dose

Your veterinarian calculates the dose from your dog's weight and health, and the product comes in weight ranges for that reason. Heartgard has specific dosage requirements that need to be followed [1]. Do not split, double, or guess at a dose on your own.

How it is given

Heartgard is a chewable given by mouth, usually once a month. Most dogs eat it willingly. If your dog is picky, ask your veterinarian about the best way to give it without breaking the dose apart.

What to do about a missed dose

Call your veterinarian for guidance on a missed month. Do not give two doses at once to make up for it. Giving more than the labeled amount is how overdoses happen, and the risk goes up if the extra tablets were meant for a larger animal [1].

Heartgard Side Effects: Common vs. Serious

Most reactions owners report are mild and pass quickly. The table below separates the everyday issues from the ones that need a phone call or a trip to the clinic.

Side effect	How common or serious	What to do
Vomiting	Uncommon, usually mild	Withhold food briefly, offer water, call your vet if it repeats
Diarrhea	Uncommon, usually mild	Watch for blood, worsening, or lasting more than a day
Lethargy or poor appetite	Uncommon, usually mild	Call your vet if it lasts past a day
Itching, hives, facial swelling	Rare, can be serious	Call your vet right away, this can be an allergic reaction
Wobbliness, staggering, dilated pupils	Rare, serious	Emergency visit now, especially in herding breeds [1]
Tremors, seizures, trouble standing	Rare, serious	Emergency visit now [1]

The neurologic signs in the bottom rows are the ones tied to ivermectin sensitivity. Some dog breeds, including Collies, Border collies, and Old English sheepdogs, are extra sensitive to ivermectin in Heartgard, so a regular dose may adversely affect them [1]. These breeds carry a multidrug resistance gene mutation that makes them sensitive to this class of drugs [1].

That sensitivity is not limited to those breeds, and it is not limited to dogs. A case report described a young Maine Coon cat that developed tongue protrusion, dilated pupils, weakness, and ataxia after a routine application of a product containing a macrocyclic lactone at label dose [4]. Testing found the cat was homozygous for a variant that leaves it without a working P-glycoprotein drug transporter, which raises the risk of neurotoxicity from this drug class [4]. The takeaway for dog owners is the same: an individual animal's genetics can change how it handles these drugs.

If your dog has a cough you are worried about, it helps to know that a cough is not a typical Heartgard reaction. A cough in a dog on heartworm prevention is more often linked to other causes, and it is worth reading about How to Tell If Your Dog's Cough Is a Side Effect of Heartworm Medication so you can describe what you are seeing to your veterinarian.

Do these drugs affect vision?

A recent study looked at whether oral macrocyclic lactone heartworm preventives change retinal function or the pupillary light reflex in healthy dogs [5]. In the retrospective portion, there was no association between heartworm preventive use and the electroretinogram measurements studied [5]. This is reassuring for owners of healthy dogs, and it does not replace an exam if your dog seems to be bumping into things or squinting.

Who Should Not Take It and Drug Interactions

Tell your veterinarian about every medication, supplement, and topical product your dog receives. The concern with this drug class is not a long list of interactions so much as stacking. If your dog is already receiving another macrocyclic lactone, such as a moxidectin based product, your veterinarian needs to know so the total exposure stays appropriate [2][3].

Dogs with a known multidrug resistance gene mutation, or breeds known to carry it, need a conversation with their veterinarian before any ivermectin containing product is used [1]. The same is true for dogs with liver or neurologic disease, dogs on multiple parasite preventives, and puppies at the low end of the weight range for a product size.

If your dog takes other long term medications, side effect questions come up often. Owners managing heart or seizure drugs sometimes ask how those compare, and a look at What Are the Side Effects of Vetmedin for Dogs? can help you keep the two conversations separate when you call your clinic.

Overdose: What Happens and When to Call

An overdose happens when Heartgard instructions are not followed [1]. The amount of ivermectin in standard dog heartworm prevention is not likely to cause an emergency in non-sensitive dog breeds if they consumed one or two tablets above their normal dosage [1]. Ingesting a large number of tablets, or medication intended for a larger animal, may be a concern [1].

That distinction matters. A dog that ate one extra chew is usually a watch and call situation. A dog that emptied a bottle, or ate a large animal dewormer, needs immediate help.

Call your veterinarian first. If your clinic is closed, call a pet poison hotline. Have the product name, the strength, the number of tablets, your dog's weight, and the time of ingestion ready. Do not wait for symptoms to start before you call.

Limitations and When to Contact a Veterinarian

Individual cases need a veterinarian. No article can tell you whether your dog's specific signs are from the medication, from something else, or from a combination of both.

Call your veterinarian the same day if your dog vomits more than once after a dose, has diarrhea that lasts more than a day, refuses food, or seems unusually tired.

Go to an emergency clinic now if your dog is wobbly, staggering, drooling heavily, has dilated pupils, has tremors, has a seizure, cannot stand, or has facial swelling, hives, or trouble breathing [1].

Call your veterinarian for a routine visit if you have questions about the product, want to switch preventives, or your dog has a breed background that includes Collies, Border collies, or Old English sheepdogs [1]. Genetic testing for the multidrug resistance mutation is available, and your veterinarian can tell you whether it makes sense for your dog.

If your dog is on other medications and you are sorting out which drug is causing which sign, it helps to review each one. Owners on steroid therapy often keep a separate list of what to watch for, and What Are the Side Effects of Prednisone in Dogs? covers that ground.

Frequently Asked Questions

How long do Heartgard side effects last in dogs?

Mild stomach upset usually settles within a day once the dose is digested. Neurologic signs are different and should be treated as an emergency, not something to wait out. If any sign is still present after 24 hours, call your veterinarian.

Can my dog have a reaction to Heartgard after years of taking it?

Yes, a reaction can show up at any point, though it is uncommon. Sensitivity is often genetic, so a dog may handle the drug fine for years and then show signs when something else changes, such as a new medication or a different product added to the routine. Report any new sign to your veterinarian.

Is ivermectin the same as Heartgard?

Ivermectin is the active ingredient in Heartgard, and Heartgard is one specific product built around it [1]. Other heartworm preventives use different macrocyclic lactones, such as moxidectin [2][3]. They are related drugs, so tell your veterinarian about every preventive your dog receives.

What should I do if my dog ate extra Heartgard chews?

Call your veterinarian or a pet poison hotline right away with the number of chews and your dog's weight. One or two extra tablets are not likely to cause an emergency in a non-sensitive breed, but a large number of tablets or a product meant for a bigger animal may be a concern [1]. Do not wait for symptoms.

Are some dog breeds more sensitive to Heartgard?

Yes. Collies, Border collies, and Old English sheepdogs are more sensitive to ivermectin than other breeds because of a multidrug resistance gene mutation [1]. If your dog is one of these breeds, or a mix, talk with your veterinarian before starting or continuing any ivermectin containing preventive.

Can Heartgard cause seizures in dogs?

Seizures are a rare but serious sign of ivermectin toxicity, and they are most likely in dogs with the multidrug resistance mutation [1]. A seizure after a dose is an emergency. Go to a veterinary clinic immediately and bring the product packaging with you.

Do I need to stop Heartgard if my dog has a reaction?

Do not stop or restart a heartworm preventive on your own. Stopping leaves your dog unprotected from heartworm disease, which is far more dangerous than most side effects. Your veterinarian can switch your dog to a different preventive class if needed.

References

Can Dogs Overdose on Hartgard. Pet Poison Helpline.
Efficacy of a novel chewable tablet (Credelio Quattro™) containing lotilaner, moxidectin, praziquantel, and pyrantel for the prevention of heartworm disease (Dirofilaria immitis) in dogs. Parasites & vectors, 2026.
Efficacy of a single oral administration of a formulation of fluralaner, moxidectin and pyrantel (BRAVECTO® TriUNO) in dogs for the treatment and prevention of angiostrongylosis. Parasites & vectors, 2026.
Inherent P-glycoprotein deficiency and macrocyclic lactone toxicity in an Australian Maine Coon cat homozygous for ABCB11930_1931del TC. JFMS open reports, 2026.
Effects of Oral Macrocyclic Lactone Heartworm Preventatives on Retinal Function and Chromatic Pupillary Light Reflex in Healthy Companion Dogs. Veterinary ophthalmology, 2026.