does my dog need heartworm meds  

If you give your dog heartworm preventives, or are considering them, you might want to read this article first.

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.

To save you some research time, I’ve pulled together some highlights from the Adverse Drug Experience (ADE) Reports filed with the US Food and Drug Administration (FDA) for the most popular heartworm drugs.

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 (oral)

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.

does my dog need heartworm meds


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.

Does my dog need heartworm meds

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

does my dog need heartworm meds


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.

does my dog need heartworm meds

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).

does my dog need heartworm meds

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.

does my dog need heartworm meds


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.

does my dog need heartworm meds


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.

does my dog need heartworm meds

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.

does my dog need heartworm meds


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, hypersalivation
  • General: depression, lethargy, anorexia, fever, weight loss, weakness
  • Dermatological: injection site pruritus/swelling, erythema multiforme
  • Neurological: seizures, ataxia, trembling, hind limb paresis
  • Hematological: leukocytosis, anemia, thrombocytopenia
  • Respiratory: dyspnea, tachypnea, coughing
  • Hepatic: elevated liver enzymes, hypoproteinemia, hyperbilirubinemia, hepatopathy
  • Urinary: 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.

does my dog need heartworm meds

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.

(Related: Heartworm Facts … Prevalence, Medication and Alternative Treatment)