Heartgard® for Dogs: Risks, Side Effects, Dosage, and More

Protecting your dog from silent killers is at the top of your priority list. But what is an example of a disease that lurks, trying to evade early detection to continue progressing? Heartworms. This condition is insidious, wreaking havoc along the path to eventual death. But there’s something simple you can do about it to prevent its development. Learn more about Heartgard® for dogs, including when it’s indicated, what the side effects are, and what the proper dosage is for your pup!

Heartgard® for Dogs: Risks, Side Effects, Dosage, and More

Why Do Dogs Need Heartgard®?

Heartworm Prevention

Your pup has to be at least six weeks old before you can begin to administer Heartgard® for heartworm prevention. Heartworms develop after an infected mosquito bites a pup, spreading infected larvae right into the bite wound. The heartworms don’t develop right away. It can take up to seven months before the larvae fully develop into adult heartworms. Symptoms following the development of the disease include weight loss, coughing after physical activity, lethargy, weakness, and irregular heartbeat, among others.

Additional Worm Prevention

Although heartworm prevention is vital for healthy pups, there are also other worms that Heartgard® helps to prevent. The ingredients in this medication include ivermectin and pyrantel pamoate, which offer protection against roundworms and hookworms as well. However, keep in mind that Heartgard® does not protect against tapeworms. For tapeworm prevention, you need a separate medication like NexGard, for instance.

Side Effects of Heartgard® for Dogs

Vomiting and Diarrhea

Like with any supplement or medication, your dog may experience some side effects after administration. Generally, Heartgard® is tolerated well by dogs but if your dog is sensitive to new medication, you may notice some changes. For example, gastrointestinal upset is a minor possibility (occurs in 1.1% of administered doses), and you may notice vomiting, diarrhea, or both. If your pup vomits within an hour after you’ve given them Heartgard®, you may need to speak with your veterinarian so they can let you know how to move forward. Diarrhea should clear up on its own but if it does persist, that requires a call to your vet as well.

Appetite Loss/Anorexia

Your usually hungry pup may not be interested in food after you have administered Heartgard. However, this side effect is only temporary, and it should clear up on its own. You will have your regular, enthusiastic eater again shortly after.

Lethargy/Depression

You know something is up when your typically energetic dog is not acting like itself. After administering Heartgard®, you may notice that they’re acting a little bit more lethargic or looking a bit depressed. Again, this is a relatively normal side effect that resolves on its own but keep an eye on your dog to see if any symptoms persist.

Drooling/Hypersalivation

Another side effect you may notice is excessive drooling, also called hypersalivation, after administration. Not all dogs have this response to the medication but if they do, it typically clears up rather quickly.

Mydriasis

Mydriasis is a fancy way to describe dilated pupils. This side effect is not as common, but you may notice that your dog’s pupils dilate. Sometimes, it’s just one eye and sometimes it’s both.

Ataxia

If you notice that your dog looks uncoordinated when walking, they could be experiencing ataxia. This nervous system incoordination presents with an abnormal gait, but you may notice that your dog’s head tilts or that they experience abnormal spinal reflexes. If you notice some of these startling side effects, contact your veterinarian to discuss the proper next steps.

Convulsions

One of the main ingredients in Heartgard® is ivermectin and one of its potential side effects is convulsions or seizures. Generally, this medication is safe for dogs but there have been some reported adverse events that include convulsions. This is especially the case if your pup is sensitive to ivermectin (which certain breeds are).

Signs of Heartgard® Overdose or Allergies in Dogs

Weakness

While most dogs can take Heartgard® without even batting an eye, those with sensitivities to the ingredients may experience some rare side effects. If you notice that your dog is displaying weakness, they may have either overdosed on the medication or may be allergic to one of the ingredients in the formula.

Swelling

If your dog has an allergic reaction to Heartgard® or takes more than the appropriate dosage, you may also notice swelling along with other symptoms.

Hives/Itching

Your pup may develop hives which will cause itching. After administering Heartgard®, make sure you keep a close eye on your dog to pick up on any changes, even if they’re subtle. Some of these effects are significantly more noticeable (like if your dog is constantly scratching) but some of them may be harder to spot.

Blindness

More severe effects following overdose include blindness. Be sure to monitor your pup’s eyes to note any changes.

Coma

An overdose of Heartgard® may result in a coma. This may occur after their heart rate lowers, their body temperature changes and their behavior becomes abnormal.

Death

Overdose or allergic reactions to Heartgard® may result in death. To prevent an adverse reaction, avoid the medication if your pup is sensitive to ivermectin, and keep any extra chewable tablets locked away in a place your dog can’t access.

Heartgard® Risks to Consider

Allergic Reactions

Your dog requires your due diligence before administering medication. Be sure to double-check with your veterinarian to determine if your pup is sensitive to ivermectin. This can prevent discomfort and adverse events.

Overdose

To prevent overdose, follow the manufacturer’s dosage instructions (there’s an example of dosages in the table below). Keep this and any other medications out of reach of your pup, especially if you’ve got a clever floof that knows how to unlock doors and open drawers!

Drug Interactions

Heartgard® may interact with other drugs, so be sure to check with your vet. Some of those drugs include blood pressure medications and anti-fungal medications, among others.

Sensitivity to Ivermectin

Some dog breeds are sensitive to ivermectin, which means they’re not good candidates for this heartworm preventative. Some of those breeds include German shepherds, border collies, Australian shepherds, Shetland sheepdogs, and old English sheepdogs.

When Is Heartgard® Not an Option for Your Dog?

Genetic Mutation

Though ivermectin is safe for most dogs, certain breeds have a mutation in the MDR1 gene. For these dogs, Heartgard® is not an option, as it can cause neurological damage and worse.

Heartgard Dosage Chart

Heartgard® is a chewable tablet that you give to your dog once per month each year. The package comes with a total of 12 tablets and a space on the back of the box where you can note the date you administer each chewable.

Alternatives to Heartgard®

When Heartgard® is not appropriate for your pup, you have other options. There are other products that are milbemycin-based instead of ivermectin-based. These include Trifexis and Interceptor, both of which are orally administered. Aside from options that don’t include ivermectin as an ingredient, you have several heartworm preventatives to select from, including Tri-Heart Plus, NexGard, and Simparica, to name a few.