Updated: Dec 14, 2019

Heartworm is one of the fiercest nightmare of any dog owner. It is that silent and unseen predator that can strike your healthy and lovely dog, even when you do all things right and with one tiny bite from one tiny mosquito or flea, bring down your beloved friend in a short time.

The infected insects pass on eggs into the blood stream of the worm blooded animals and the eggs develop into adult worms which crawl into arteries and heart in a few short weeks.

The worms then fill the heart and arteries and cause blockages that cannot be cleared with anything short of surgeries. Even with treatment, the blockage caused by dead worms can cause death quickly.

Heartworm can be a devastating disease. Dogs with moderate or severe infestations display a chronic cough and can’t engage in much activity, as worms choke their heart and major blood vessels, reducing their blood (and thus oxygen) supply. The disease often leaves its victims incapacitated, incapable of doing much more than a slow walk without gasping for air, and kills many dogs. Even the treatment for heartworm disease can be deadly, regardless of which method is used, so it is important to understand the risks that you take if you choose not to give your dog heartworm preventative.

Hence, and at this point of the conversation the number one nightmare of any dog owner begins and they cling to any and all methods of prevention and never let go.

I frequently see people allege that as long as you have a healthy dog, feed a raw diet, and do not over-vaccinate, your dog will not get heartworms. If only this were true! These measures may help to some degree, but they are not foolproof. The only way to know for sure that your dog is protected is to give heartworm preventatives.

What are those treatments?

Mostly veterinarians prescribe a monthly chewable tablet that contains drugs such as Ivermectin, Panacur, or anything similar. And that is how it has been since time began.

Well, not exactly since then, but right about the time that pharmaceuticals became interested in the market that pets created for them and unwanted drugs and those banned for humans, their interest in pet market increased. With such low oversight from FDA in pet pharmaceutical, these companies were free to market these formerly banned medications available to dogs and horses and still continue on production, but now with different branding: "For Veterinarian Use Only"

Heartworm preventatives can cause serious side effects in some dogs, including depression, lethargy, vomiting, anorexia, diarrhea, dilation of the pupil, loss of balance, staggering, convulsions, and hy-persalivation. Some dogs are especially prone to side effects from ivermectin, the main ingredient in one of the most widely used heartworm preventatives. Also, some of the preventatives are combined with drugs aimed at killing other pests such as fleas, mites, roundworms, and hookworms.


With the use of Natural Remedies on the rise, more and more every day people turn to them. The natural prevention methods already in the market rely heavily on some herbal dewormers, such as wormwood and black walnut, are potentially toxic when used at dosage levels needed to control intestinal parasites.

(Keep this part in mind. We will get back to Natural Remedies later in the article)


Here is a much deeper look into veterinarian prevention medication for both heartworm and other internal parasites:

Unless you’ve done some pretty thorough research – or in the unlikely event the veterinarian who prescribed these drugs warned you about the side effects – you may not be aware of the risks of giving heartworm drugs to your dog.

If you read the package inserts, the manufacturers usually list a few adverse reactions your dog might experience … but they also tell you that their drugs are safe.

So … are they safe or not?  See what you think after you read the information I’ve compiled for you.

Side effects have been reported in all these medications … and if you don’t know about them, you might want to read about them before giving these drugs to your dog.

ADE reports are real side effects experienced by dogs given these medications.

As you read about these side effects, keep in mind, ADEs are almost certainly under-reported. Many vets either don’t associate the side effects with the heartworm drugs they prescribed, or they don’t bother filing a report, because it’s tedious and time-consuming to do so.

Heartworm Preventives

Here are the drugs I researched.

Heartgard Plus

Iverheart Plus (oral) – generic for Heartgard Plus

TriHeart Plus (oral) – generic for Heartgard Plus

Sentinel (oral)

Sentinel Spectrum (oral)

Iverhart Max (oral)

Trifexis (oral)

Interceptor (Oral)

Revolution (topical)

Advantage Multi (topical)

Proheart6 (Injectable)

First, A Couple Of Points

The word “preventive” is a misnomer.  None of these drugs actually “prevents” heartworm. Instead, they work by killing heartworm larvae that may already be in your dog’s body.  So they are really treatment drugs, not preventive drugs.  And they treat a condition your dog may not even have!

Heartworm drugs are pesticides that work by paralyzing the worm’s nervous system.  What might it be doing to your dog’s nervous system?

When you see the ADE reports from the FDA’s website, you’ll see that many of the side effects reported are conditions that involve the nervous system.

Does this sound like something that’s safe for your dog to take?

And all these drugs also treat other parasites like various types of intestinal worms … whether your dog has worms or not.

Again, why would you treat your dog for something he doesn’t have?

The Side Effects

If you look at the side effects below and think “those numbers aren’t very big, compared to the number of dogs that take the drugs” … remember again, the adverse effects are very likely under-reported.

There are many pages of ADEs for each drug, listed by number of times reported, so I’ve just including the top 10 or so in each case.

Heartgard Plus, Iverhart Plus and TriHeart Plus

I’m showing these three brands together as they all have the same active ingredients – ivermectin and pyrantel pamoate.

Ivermectin is for heartworms, and pyrantel is to treat roundworms and hookworms. Iverhart Plus and Tri-Heart Plus are generic drugs for Heartgard Plus.

The ADEs on the combination of ivermectin and pyrantel are 16 pages long.

Note that the first three on the list are thousands of reports of ineffectiveness of the drugs in killing heartworm larvae, hookworms and roundworms (ascarids) …

… so besides being risky, this drug doesn’t always work.

Death ranks as the 11th most frequent adverse effect, with 264 deaths reported.


The active ingredients in Sentinel are milbemycin oxime plus lufenuron, for heartworms, hookworms and roundworms.

The list below is for the combination of milbemycin and lufenuron and contains 14 pages.

There are 87 deaths reported.

Again, note that several of the problems reported are ineffectiveness of the drugs.

There are also separate reports for milbemycin and lufenuron individually, so scroll down to see those too. They have plenty of problems of their own, even when not combined together.

The milbemycin report is under the Interceptor header.

Lufenuron seems comparatively problem-free compared to some of the other drugs, but still has 8 pages of side effects, including 15 deaths.


Interceptor contains milbemycin oxime and is sold for heartworms, roundworms and whipworms.

Milbemycin has 16 pages of adverse effects, with 279 deaths ranking 13th on the list.  Again, ineffectiveness of the drug is also high on the list with over 9,300 reports.

Sentinel Spectrum

This drug adds a third ingredient, praziquantel, to the ingredients (milbemycin and lufenuron) that are in the basic Sentinel.  The combination claims to tackle heartworms, roundworms, hookworms, whipworms and tapeworms.

There’s no report for the combination of all three ingredients, but you’ll see below that while praziquantel has a relatively short list of adverse effects, it still accounts for 13 deaths … along with 87 for the milbemycin/lufenuron combination).

Iverhart Max

This drug is a different combination of some already familiar ingredients … ivermectin for heartworms, pyrantel for hookworms and roundworms, and praziquantel for tapeworms.

There are 25 deaths reported, at number 15 on the list of adverse effects.


This drug’s achieved quite a bit of notoriety so you may have read or seen TV reports about it.  There’s even a Facebook page about it: Does Trifexis Kill Dogs?

The active ingredients are milbemycin oxime plus spinosad, and it’s supposed to stop heartworms, hookworms, roundworms, whipworms and fleas.

You’ve already seen the milbemycin report under Sentinel, with its 279 deaths.

Spinosad alone is responsible for 222 reports of death and the adverse effects list is 17 pages long.


Revolution is a topical drug containing the active ingredient selamectin.

It claims to stop heartworms, roundworms and hookworms as well as fleas, American dog ticks, ear mites and sarcoptic mange mites.

And in case you think a topical medication is safer than an oral drug …

… it may surprise you to see that there are 17 pages of adverse effects for selamectin and there are 236 deaths reported!

And it doesn’t seem to be very effective either, with nearly 6,000 reports of ineffectiveness against heartworm larvae.

Advantage Multi

Another topical option, Advantage contains imidacloprid, which is said to paralyze fleas, plus moxidectin against heartworms and intestinal worms.

The list of adverse effects shows a lot of skin issues associated with this drug, and there are 23 deaths too.


ProHeart 6 is another controversial drug.  The active ingredient is moxidectin, delivered via injection; it’s said to protect against heartworms and hookworms for six months.

First introduced in 2001, it was recalled in 2004 after over 5,500 adverse event reports, including about 500 deaths.

It’s back on the market now …

… but if you’re considering using it, read about some of the side effects first.

The prescribing information reports the following Post Approval Experience, revised in 2010.  It’s a long and troubling list.

The following adverse events are based on post-approval adverse drug experience reporting. Not all adverse reactions are reported to FDA/CVM. It is not always possible to reliably estimate the adverse event frequency or establish a causal relationship to product exposure using these data.

The following adverse events are listed in decreasing order of frequency by body system.

Immune: anaphylaxis and/or anaphylactoid reactions, urticaria, head/facial edema, pruritus, pale mucous membranes, collapse, cardiovascular shock, erythema, immune-mediated hemolytic anemia, immune-mediated thrombocytopenia (signs re ected in other system categories could be related to allergic reactions, i.e., gastrointestinal, dermatologic, and hematologic)Gastrointestinal: vomiting (with or without blood), diarrhea with or without blood, hypersalivationGeneral: depression, lethargy, anorexia, fever, weight loss, weaknessDermatological: injection site pruritus/swelling, erythema multiformeNeurological: seizures, ataxia, trembling, hind limb paresisHematological: leukocytosis, anemia, thrombocytopeniaRespiratory: dyspnea, tachypnea, coughingHepatic: elevated liver enzymes, hypoproteinemia, hyperbilirubinemia, hepatopathyUrinary: elevated BUN, elevated creatinine, hematuria, polydipsia, polyuria Cardiopulmonary signs such as coughing and dyspnea may occur in heartworm positive dogs treated with ProHeart 6.

In some cases, death has been reported as an outcome of the adverse events listed above.

In 2013, the company presented a Risk Minimization Action Plan for the drug to the FDA.

The ADEs on the FDA website are 19 pages long and there are 496 deaths reported.

Can You Protect Your Dog Without These Drugs?

So, now that you’ve read this long and scary list of side effects, you’re probably wondering if there’s a way to protect your dog without risking his health – or worse, his life!

Yes, there is!

Test your dog for heartworms on a regular basis. Depending on where you live, this may be every 4 months.


Test your dog. Check with your vet, but do not underestimate the power of Holistic healing and remedies.

It will shock you to learn that Holistic and Natural Prevention ranks in potency among the absolute highest levels of any and all of these medications. No modern medicine can be as powerful as the power of nature, when it comes to dealing with parasite control.

And toward that purpose, we have managed to tap into that power and combine the ancient recipes with modern science and formulate some of the best and safest remedies that nature could offer.

Defense is a combination of herbs and mushrooms and flowers and plants and is capable of doing what any of those medication can do, except with no side effect and no resistance and no damage to cell structure of the most sensitive animals, with 100% accuracy.

Unlike the modern chemical heartworm medications which are mostly poisonous and affect the nervous system, Defense simply dissolves the eggs and the worms at cellular level, without any slight change to the animals cellular structure.

It does not contain traditional herbs such as wormwood and black walnut and similar dewormers and anti-parasite natural remedies which cause severe toxicity issues at the levels required to destroy the worms.

Defense is a new take on old ancient recipes, verified by modern science and tested and approved by 1000s of dogs now living parasite free and or healed of Heartworm after being diagnosed.

Why treat your dogs for heartworm with toxic medications, when they are not even sick?

Why treat with toxic natural herbs or chemicals, when there are completely safe formulas available that cause no side effects and work every single time?!

Defense can prevent heartworm infection, destroy the eggs, and also full grown parasites and worms in the most infected animals. Even though it is best to use Defense as prevention and not risk the health of your Super Dogs, but should you find that the unspeakable has happened and the organs are already infected, administer Defense as early as possible.