Crisis! I Accidentally Gave My Dog 2 Doses Of Heartgard!

Accidents happen.

Even when it involves strong, prescription-only heartworm preventatives.

Maybe it occurred when both you and your partner unknowingly gave your dog a tablet on the same day… or when your dog decided it was going to have one straight out of the packet for a snack.

However it came to be- What’s going to happen now that you accidentally gave your dog 2 (or more) doses of Heartgard?

And what should you do if something like this has taken place?

It may come as a surprise to you, but even if your dog ate three (or even ten!) doses of Heartgard, it most likely won’t experience any serious negative symptoms at all. This is due to the extremely wide margin of safety of both its active ingredients, ivermectin and pyrantel pamoate.

The worst that’s realistically likely to happen if your dog ate two doses of Heartgard is a temporary bout of loose stools. Other than that, simply carry on as normal and make sure that you give the next monthly dose on time!

(Side note: I am a member of the Amazon Associates program. From time to time I like to recommend products in my posts that I feel may truly be helpful to readers and their pets. If you do end up buying something by clicking the links on my site, I may receive a tiny amount of commission from the big guys. 

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What’s In Heartgard, Anyway?

Heartgard Plus packet

Heartgard contains two active ingredients: Ivermectin and pyrantel pamoate.

Ivermectin is a chemical compound that is originally derived from fermentation processes of organisms that reside in the earth and soil.

First discovered on a Japanese golf course, it is now used in heartworm medications such as Heartgard and Iverhart to kill heartworms at the larval stage when they haven’t yet matured and migrated into the arteries of the lungs.

Not only is ivermectin used in tiny doses to kill heartworms in a dog’s body, it is also used in much higher concentrations (think 50-60x!) in other medications to combat tough parasites such as demodectic and sarcoptic mange.

Pyrantel pamoate is used as a dewormer in Heartgard, making it a complete protective solution by targeting stomach parasites such as hookworm and roundworm.

It works by paralyzing and killing worms in the stomach and intestines, and renders them unable to attach themselves to intestinal walls- resulting in expulsion in the feces.

It can take a few hours of digestion before the active ingredients ivermectin and pyrantel pamoate provide relief against heartworms.

Though rare, some dogs may experience mild side effects even with proper dosage of heartworm medication (1.1% of administered doses according to studies). These side effects can include vomiting, diarrhea, lethargy, and in serious cases seizures and allergic reactions.

Dogs may also experience a loss of appetite and a higher frequency of diarrhea and vomiting for the next 24 hours as parasites are eliminated from the body.

Some dogs are also naturally predisposed to experiencing reactions after ingesting heartworm medication- most commonly herding breeds like Collies and Heelers. Even still, adverse symptoms are rare and most frequently occur only when an overdose has taken place.

What Will Happen If My Dog Ate Too Much Heartgard?

Shiba Inu sitting on bed

As explained above, Heartgard contains the active ingredients ivermectin and pyrantel pamoate. You will be very glad to learn that it is actually very difficult for a dog to overdose on Heartgard due to its overall safety as a product.

Since in this case you only accidentally gave your dog a double dose of Heartgard, it likely won’t result in any adverse symptoms at all.

There will be no long term side effects to speak of, and if your pup does experience any negative side effects it will most likely just be in the form of mild, transient stomach upset.

It may still be helpful to learn what’s in a Heartgard tablet, just in case, you know, your dog decides to feast on 10 packets in the near future.


Ivermectin is considered to be very safe for dogs, as it does not have a long half-life in the body and is completely processed within 48 hours.

Not only that, it also has a very wide margin of safety, and dogs would have to consume over 0.45mg (450mcg) per pound of body weight of ivermectin to experience any symptoms of toxicity.

Even multiple accidental doses are unlikely to result in any long-term, or short-term, negative side effects for your dog.

Given that one Heartgard Plus tablet for dogs up to 25 pounds contains only 68 micrograms (mcg) of ivermectin, even a mere 10-pound dog would have to eat 66 tablets(!) for any potential poisoning to arise.

Chances are you don’t have 66 pills of Heartgard lying around at home!

Even so, herding breeds (such as Collies) are especially sensitive to the effects of ivermectin and may be affected by even a slight overdose.

This is due to a genetic mutation in the MDR-1 gene in their brain, which affects the blood-brain barrier. Therefore, it is advised to take the dog to the vet immediately if they have ingested a large amount of heartworm medication.

Younger dogs are also generally more susceptible than adult canines to the side effects of medications and overdoses.

When overdosed, ivermectin can negatively affect the dog’s central nervous system and impede nerve transference.

Some of the symptoms of ivermectin toxicity can include:

  • Disorientation
  • Uncoordinated, ‘drunk’ movements
  • Dilated, unfocused pupils (Mydriasis)
  • Excessive salivation
  • Tremors and shivering
  • Stepping with legs lifted too high (“Goose-stepping”)
  • Hypersensitivity to sudden touch or noises
  • Depression
  • Loss of consciousness and coma.

Pyrantel Pamoate

Similarly to ivermectin, pyrantel pamoate is very safe and can be given safely at higher-than-indicated doses due to its low GI absorption. Dogs can safely be administered up to 14.5mg of pyrantel pamoate per kilogram of body weight.

Symptoms of toxicity may begin to show if a dog ingests an amount of pyrantel pamoate greater than 690mg per kilogram of body weight.

Given that one Heartgard Plus tablet for dogs up to 25 pounds contains only 57 milligrams (mg) of pyrantel pamoate, even a mere 10-pound (4.5kg) dog would have to eat 54 tablets(!) for any potential poisoning to arise.

(Just a note that mathematics is not my strong suit: Though I am quite certain the numbers are correct, the overall point I’m trying to get across is that it takes an EXAGGERATED amount of Heartgard tablets for an overdose to occur.)

When it is somehow overdosed, signs of pyrantel pamoate poisoning can include:

  • Excessive salivation and sweating
  • Accelerated heart rate
  • Vomiting
  • Diarrhea
  • Uncoordinated movements and shaking
  • Convulsions
  • Excitability.

Can I Give My Dog A Higher Dose Of Heartgard?

Hand feeding dog heartworm pills
Hand feeding pet dog with chewable to protect and treat from heartworm disease, on white background

Heartgard is a very safe product, and even if you give your dog a higher dose it most likely won’t cause any health issues.

With that said, there is really no need to give your dog a higher dose of Heartgard.

It won’t make the medication any more effective than if you give it in proper amounts, and it is always best to follow medication dosage guidelines in case your dog does have extra sensitivity to the drug.

If it’s a situation where you have truly forgotten whether you have given your dog Heartgard this month (see below section), then the truth is that it may be safer to give it a potential extra dose of Heartgard than to miss a dose.

This will ensure that your dog remains protected from heartworm infection, while the risk of medicinal side effects is kept small.

However, consult your vet first before giving the extra dose, and if your dog does display symptoms of diarrhea, vomiting or lethargy, take it to the animal hospital immediately for an examination.

Can I Give My Dog A Lower Dose Of Heartgard?

Dachshund hiding in blankets

You may be worried about giving your dog a monthly dose of heartworm preventatives due to the chemical content contained within.

However, the fact remains that the risks of heartworm prevention are much, much lower than those that can occur if you leave your dog vulnerable to heartworm infestation. Heartworm disease can lead to fatal consequences– something you definitely do not want to encounter.

While heartworms can be treated when they have established a presence inside a dog, it becomes a much more complicated process and involves much stronger (and more dangerous) arsenical medication.

An infected dog requires hospitalization and constant monitoring during treatment, as there is a constant risk of worm debris and blood clots blocking blood vessels. After that, the dog has to be kept inactive for up to 6 weeks to make sure that it recovers properly.

The same dilemma applies for expired heartworm medication. Medication past its use-by date will have most likely lost some level of effectiveness, leaving your dog vulnerable to heartworm infestation in the interim.

Therefore, if you find that you gave your dog expired Heartgard, it would be best to consult with your vet on the timing of the next dosage. It may also be prudent to have heartworm tests done to check for the possible presence of parasites.

What Should I Do If I Can’t Remember If I Gave My Dog Heartworm Medicine?

Dog looking quizzical

Underdoses and overdoses are not particularly desirable situations when it comes to heartworm medicine. Both can lead to consequences that affect your pet in a very negative way, so it’s important to know what to do if you can’t remember whether you gave your dog its pills or not!

The first thing that you should do is to contact your vet for advice in this situation and to double check the proper dosage.

By checking with the vet you will be able to confirm your dog’s weight at the time of the checkup, from which you will then be able to determine a margin of safety for the medication.

This will allow you to make an informed decision on whether you can give your dog a heartworm pill soon, or whether you should wait for a while. Once you have been able to determine that, remember to put it in your calendar!

The best way to do so is not only to mark a physical calendar, but also to put it in your phone. Most phones have settings you can toggle that will allow you to set an alarm or reminder for the same date of every month. This way, you won’t forget whether or not you gave medicine.

Designate only one person in your household to hold the responsibility of giving your dog its heartworm medication. This will prevent instances where many people each give the dog a pill on the same day.

Using a pill case can also make life much, much easier as it simply becomes a case of loading the compartments once and giving the pills once each month until it is completely empty. To avoid confusion, don’t reload medications until the current dosages have all been completed!

Is It Dangerous If I Gave My Dog A Heartworm Pill Early?

Rest assured: It won’t be dangerous if you give your dog a Heartgard pill early, whether intentionally or by accident.

At the same time, it won’t make prevention more or less effective either.

According to 1800PetMeds, most heartworm medicines have a 15-day safety margin even though they are meant to be given monthly. This means that as long as you give your dog a heartworm pill within a 45-day span, it will still offer the same protection window.

If you happen to give your dog a heartworm pill early, or if you miss it altogether for a month, simply restart the monthly dosage schedule again immediately and carry on as normal!

Missing or giving an early dose once will not have any long-term health implications. However, the monthly schedule should be adhered to once started. This is because irregularity can lead to increased resistance of the parasites, making them more difficult to deal with.

What Should I Do If My Dog Ate Too Much Heartgard?

Dog in sweater lying on bed

The first thing you should do in a situation where your dog has eaten heartworm medication (or any other type of medication, like Melatonin or Excedrin) is to establish exactly how much was eaten!

It is also extremely helpful to salvage (as much of) the packaging as possible, so that you can take it to any potential vet visit. This will aid the vet in being able to accurately determine the dosage that was ingested and whether there will be any toxicity.

As mentioned in the above section, heartworm medications do generally have a wide margin of safety and if the toxic threshold was not exceeded then no treatment will be necessary.

If the vet suspects that the dog has eaten a toxic dose, he may induce vomiting if the medication was recently swallowed. Making a dog throw up medication will only work if less than 2 hours have passed, as after that the ivermectin will already have made its way into the bloodstream.

There is no antidote for ivermectin poisoning, but the vet may still be able to provide supportive treatment by administering medications or substances (such as activated charcoal) to limit further absorption.

Additionally, the vet may also perform tests to examine the condition of your dog, such as:

  • Pancreas, liver, and kidney chemistry tests to determine function and sugar levels
  • Electrolyte tests to make sure there is no dehydration or imbalances
  • Blood cell count to rule out blood-related disorders

In some serious cases, recovery can be very slow and take weeks- or even months. Dogs may be sedated for much of the time, or may be otherwise recumbent, in a coma, or be unable to eat or drink.

In Summary

Even if you accidentally gave your dog 2 doses of Heartgard, the good news is that the margin of safety for heartworm medications is very wide and your dog will most likely be just fine.

Heartworm infections can be truly devastating, so even though an overdose is never recommended under any circumstances, it may in this situation be preferable to underdosing as this can subsequently put your dog at risk for infestation.

Ideally, you will of course want to give your dog just the right amount of Heartgard! Heartgard contains two active ingredients: Ivermectin and pyrantel pamoate.

Ivermectin should be given monthly at a proper dosage of 2.72mcg per pound of body weight, while pyrantel pamoate should be administered at a dosage of 2.27mcg per pound of body weight.

Overdoses, though extremely improbable, can still occur with Heartgard- especially among certain dog breeds such as Collies. Therefore, like any other medication it needs to be treated with respect.

Otherwise, negative side effects mainly affecting the central nervous system can occur, such as disorientation, excessive drooling, trembling, ‘goose-stepping’, depression, hypersensitivity to stimuli, and even loss of consciousness and coma.

In order to keep track of the monthly doses, use of tools such as a calendar and pill case is highly recommended. Whenever an overdose or underdose is suspected, the best course of action is to contact your vet for confirmation and further instruction.

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