What is in heartworm medicine?

You might think that heartworm is a disease exclusively for the royalty of worms, but it poses a real threat to our canine companions. Luckily, there are countless treatments on the market that help keep dogs from falling prey to these sneaky parasites. Today we’ll peek under the hood of various heartworm medicine products and browse through their active and inactive ingredients.

First things first: what’s heartworm?

Heartworm (Dirofilaria immitis) has nothing to do with either hearts or worms (you heard it here first). It’s actually a long spaghetti-like parasite species that live in dogs’ blood vessels and in rare cases even humans’. That’s why vets always push you towards using some kind of preventative care like chewable tablets or topical solutions.

Why use preventatives

It can take months for your dog show symptoms after being infected with larval stage heartworms so neglecting prevention could lead them into serious health issues down the line (actually immediately not just “down the line”, so don’t procrastinate). You’d want them by your side no matter what size they are unless you’re not interested in having an extremely loyal companion who thinks you’re his personal brunch buffet.

But if convincing you based solely on paying enormous out-of-pocket expenses fails – let me tell ya – parasitic infections really aren’t a walk in the park for anyone involved – except maybe earthworms because they are used as natural alternatives recently shown to be efficient against coccidia larvae…Wait where were we? Right – doggos! So yeah…one day your furry friend might start coughing profusely one moment, develop muscle wasting, lethargy or general unfitness; next thing you know they’re at death’s door all because their owner couldn’t find 5 minutes per month to apply treatment ointment or make sure they swallowed pills with some delicious bacon (or a vegan equivalent). However, all hope is not lost, and there are tons of options out there to ensure that your best bud stays healthy.

What’s in the heartworm medicine?

Before we start drooling over the ingredient list let’s clarify one thing – most of us aren’t talented chemists or veterinary experts but scientists who designed these products are. They pick specific molecules and blend them together into something very effective in keeping our four-legged pals safe from parasites’ ploys – simple enough right? So, without further ado here’s just a taste of what goes inside popular OTC dog heartworm meds:

Milbemycin oxime

The reason you might never see your furry friend scratch their nether regions raw for months in despair- well it could go unnoticed as dogs are more prone to chewing than scratching themselves silly-. Milbemycin inhibits nerve transmission causing paralysis by preventing neurotransmitter release when binds certain glutamate-gated chloride channels found specifically only on nematode larvae (aka filarial/non-biting heartworms), fleas or mange mites. This compound should be administered monthly orally or topically and is often combined with other drugs like praziquantel (highlight) for extended disease prevention coverage.


A sister chemical to milbemycin oxime although their activity mechanisms differ wildly I mean completely distinctively(highlight)– ivermectin blocks communication between GABA molecules(don’t know this guy) thereby disrupting neuronal activity leading yet againto death through paralysis-causing muscular relaxation. In layman terms: ‘it makes those nasty little worms suffocate till done dead’ (cue dramatic music)! Same as above – you can pick whether oral treatment or subdermal implantation’s convenient.

Pyrantel pamoate

This particular compound is barely used for heartworm prevention as it won’t offer efficacy against adult worms but it’s often added in high weight dog treatments to diminish intestinal roundworm and hookworm-related infections. It works on the same GABA receptors just like ivermectin in blocking an inhibitory neurotransmitter gradient between neurons which leads to depolarization, muscle contraction and finally spastic paralysis correlated with nematode death…. Sounds pretty cool, right?

Advantage Multi

Some products out there provide a more comprehensive approach by working not only against heartworms but other pesky creatures like fleas or ticks ABOVE ALL (so don’t even think of using this drug solely)! One of them is topical solution Advantage Multi that has moxidectin (this guy acts similar to milbemycin oxime) paired with imidacloprid (yawn) that has somehow magically different mechanismof action (here’s your proof: tellschistosoma mansoni helminths(??!) whose life cycle stages involve exposure to minimum two hosts before reproduction).

What are the non-active ingredients?

Let’s start analysing common inactive constituents listed on labels:

  • Magnesium stearate
  • Lactose monohydrate
  • Hydroxypropyl methylcellulose

What do they have in common? Probably nothing just classic ‘fillers’ from our daily vitamin/mineral pills.

The Final Verdict

The list can go on involving fancier sounding names plus structures resembling squiggly children drawings…Imagine reading ingredient labels while constantly humming the “chemistry set” song – all helps create quite humorous scenery doesn’t it? Consider trying out OTC or prescribed meds containing any form of mentioned active compounds if you want some peace dealing with worm risks festering inside your buddy. Your pup will thank you for it – as will the parasitic worm royals whose demands you’ll no longer have to satisfy regularly.

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