Before you give your dog heartworm preventives, you need to know about the dangers of heartworm medication.
Even 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. And your vet will likely tell you they’re safe too.
So … are heartworm drugs safe or not? See what you think after you read the information below.
We’ve listed some of the main adverse effects reported to the FDA in Adverse Drug Experience (ADE) Reports for the most popular heartworm drugs. ADE reports are real side effects reported in dogs given these medications.
Keep in mind, ADEs are almost certainly under-reported. Many vets either don’t recognize the side effects were caused by the heartworm drugs they prescribed, or they don’t take the time to file a report.
Heartworm Medication Side Effects
Here are the drugs we researched.
- Heartgard Plus
- Iverheart Plus (oral) – generic for Heartgard Plus
- TriHeart Plus (oral) – generic for Heartgard Plus
- Sentinel (oral)
- Trifexis (oral)
- Interceptor (Oral)
- Revolution (topical)
- Advantage Multi (topical)
- Proheart6 and ProHeart12 (Injectable)
Do Heartworm Medications Prevent Heartworm?
It’s important to know the word “preventive” is inaccurate. 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!
How Heartworm Medications Kill Heartworms
Heartworm drugs are pesticides that work by paralyzing the worm’s nervous system. When you see the ADE reports, you’ll see that many of the side effects reported are conditions that involve the nervous system. Does that sound like a drug that’s safe for your dog?
These drugs also treat other parasites like intestinal worms … whether your dog has worms or not. If you do use heartworm drugs, it’s much safer to use the ones that only kill heartworms, not other type of parasites (that your dog probably doesn’t even have).
Is Heartworm Medication Harmful To Dogs?
You can decide this after you read the lists of side effects of heartworm medications. The reports filed with the FDA contain many pages for each drug … but we’ve just listed the top few adverse effects in each case.
Note that the first items on many of the lists are thousands of reports of ineffectiveness of the drugs in killing heartworm larvae or other parasites like hookworms … in other words, besides being risky, the drugs don’t always work.
Here are just a few of the top adverse effects reported.
Heartgard Plus, Iverhart Plus and TriHeart Plus
These medications 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 versions of Heartgard Plus.
Drug: Ivermectin, Pyrantel
- Ineffective – heartworm larvae, hookworms, ascarids.
- Vomiting
- Diarrhea
- Depression/lethargy
- Anorexia
- Convulsions
- Ataxia
- Death
Sentinel
The active ingredients in Sentinel are milbemycin oxime plus lufenuron, for heartworms, hookworms and roundworms.The side effects shown below are for the combination of milbemycin and lufenuron. Again, note that several of the problems reported are ineffectiveness of the drugs.
Drug: Milbemycin, Lufenuron
- Ineffective – heartworm larvae, hookworms, ascarids
- Vomiting
- Depression/lethargy
- Diarrhea
- Pruritis
- Anorexia
- Convulsions
- Ataxia
- Trembling
Interceptor
Interceptor contains milbemycin oxime and is sold for heartworms, roundworms and whipworms. Again, ineffectiveness of the drug is high on the list.
Drug: Milbemycin oxime
- Ineffective – heartworm larvae, ascarids, hookworms, whipworms
- Vomiting
- Diarrhea
- Depression/lethargy
- Anorexia
- Convulsions
- Ataxia
- Trembling
- Diarrhea, bloody
- Death
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.
Drug: Ivermectin, Praziquantel, Pyrantel
- Vomiting
- Ineffective – heartworm larvae, hookworms, tapeworms, ascarids
- Depression/lethargy
- Diarrhea
- Anorexia
- Trembling
- Convulsions
- Ataxia
- Polypnea
Trifexis
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, called 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 seen the side effects for milbemycin oxime above. Here are the problems listed for spinosad.
Drug: Spinosad
- Vomiting
- Depresson/Lethargy
- Ineffective – fleas
- Anorexia
- Puriritis
- Diarrhea
- Convulsions
- Trembling
- Ataxia
- Unpalatable, won’t eat
Revolution
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.
Drug: Selamectin
- Ineffective – heartworm larvae
- Vomiting
- Depression/lethargy
- Ineffective – fleas, ticks
- Diarrhea
- Pruritis
- Anorexia
- Convulsions
- Trembling
- Alopecia, application site
- Ataxia
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.
Drug: Imidacloprid, Moxidectin
- Depression/lethargy
- Vomiting
- Anorexia
- Pruritis
- Hyperactivity
- Diarrhea
- Ineffective – heartworm larvae
- Behavior disorder
- Trembliln
- Ataxia
- Anaphylaxis
- Hypersalivation
- Urticaria
- Convulsions
ProHeart 6 And ProHeart 12
ProHeart 6 has a controversial history. 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 … and there’s now a second, longer term formula, ProHeart 12, that lasts 12 months. If you’re considering using these drugs, read about some of the side effects first.
The manufacturer’s prescribing information for ProHeart 12 reports the following Post Approval Experience for ProHeart 6. It’s a long and troubling list that we’ve included in full.
“Post-Approval Experience (2018): The following adverse events are based on post-approval adverse drug experience reporting for ProHeart 6. ProHeart 12 and ProHeart 6 are the same formulation, but ProHeart 12 is three times the concentration of ProHeart 6. 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 reflected 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.
In some cases, death has been reported as an outcome of the adverse events listed above.“
ProHeart® 12(moxidectin)
For Extended-Release Injectable Suspension for Dogs
Keep in mind that with a long term injectable drug like this, if your dog has a bad reaction, you can’t just stop using it. It’s in your dog’s system for 6 or 12 months.
RELATED: Read more about the FDA approval of Proheart 12 …
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!
Is Heartworm Medicine Really Necessary?
If you want to protect your dog from heartworm without medication, the answer is, you can. There are several steps you can take to prevent heartworm disease in your dog. First, it’s important to understand the process of how dogs get heartworms, and then learn how to …
- Support your dog’s immune system to strengthen his defenses against disease
- Avoid mosquito bites
- Use herbal heartworm protection
- Do more frequent heartworm testing
RELATED: Read the full details of natural heartworm prevention without pills …
Many dog owners prevent heartworms successfully without drugs, even when they live in heartworm-endemic areas.