Heartworm medicine contains a type of antiparasitic compound called a macrocyclic lactone, which kills heartworm larvae before they can mature into adults. The specific compound varies by product, but the four used in current preventatives are ivermectin, milbemycin oxime, moxidectin, and selamectin. Most products also include a second or third active ingredient to cover additional parasites like intestinal worms, fleas, or ticks.
The Core Ingredient: Macrocyclic Lactones
Every heartworm preventative on the market relies on a macrocyclic lactone as its heartworm-killing ingredient. These compounds work by targeting specific chloride channels in the nervous system of parasitic worms. When the drug binds to these channels, it forces them open, flooding the nerve cells with chloride ions. This paralyzes the larvae and kills them.
The key detail is that heartworm preventatives don’t kill adult heartworms. They eliminate the immature larvae (called L3 and L4 stages) that enter your dog’s body through a mosquito bite during the previous month. This is why monthly dosing matters: each dose clears out any larvae that have been developing since the last dose. The amount of drug needed to kill these larvae is remarkably small. Ivermectin, for example, is dosed at just 6 to 12 micrograms per kilogram of body weight for heartworm prevention, a fraction of the dose used for other parasitic conditions.
Active Ingredients in Monthly Chewables
Oral chewable tablets are the most common form of heartworm prevention for dogs, and nearly all of them are combination products. The heartworm-preventing ingredient is paired with one or more additional compounds that target different parasites. Here’s what you’ll typically find:
- Ivermectin + pyrantel pamoate prevents heartworm while pyrantel treats roundworms and hookworms in the gut. Some formulations add praziquantel to also cover tapeworms.
- Milbemycin oxime handles both heartworm prevention and intestinal worms on its own. It’s often combined with praziquantel for tapeworm coverage, lufenuron for flea control, or spinosad to kill adult fleas.
- Moxidectin + sarolaner + pyrantel pamoate is a newer triple-combination approach. Moxidectin prevents heartworm, pyrantel covers intestinal worms, and sarolaner (an isoxazoline compound) kills fleas and ticks.
The inactive ingredients in chewable tablets are mostly flavoring and binding agents. Popular products use real beef or pork liver to make the chew palatable enough that most dogs eat it as a treat.
Topical and Injectable Formulations
Topical (spot-on) heartworm preventatives use either selamectin or moxidectin as their active ingredient. Selamectin is applied alone, while moxidectin is typically paired with imidacloprid, which kills fleas on contact. These are applied to the skin at the back of the neck once a month and absorb into the bloodstream.
For dogs whose owners prefer not to remember a monthly dose, injectable moxidectin is available in 6-month and 12-month formulations. The 12-month version uses moxidectin embedded in glyceryl tristearate microspheres, tiny fat-based particles that slowly dissolve in the body and release the drug over the course of a year. A veterinarian administers the injection, and the dog is protected for the full duration without any at-home dosing.
Cat Products Have Fewer Options
Cats have a narrower selection of heartworm preventatives. The only oral option for cats is milbemycin oxime. Most cat products are topical: selamectin (alone or combined with sarolaner for flea and tick coverage), moxidectin with imidacloprid, moxidectin with fluralaner, or a newer combination of eprinomectin with praziquantel and a flea-killing compound. Eprinomectin is another macrocyclic lactone but is used exclusively in cat products.
What the Extra Ingredients Target
Because dogs and cats are exposed to multiple parasites at once, manufacturers bundle heartworm prevention with coverage for other common threats. The additional active ingredients fall into a few categories:
- Pyrantel pamoate paralyzes adult roundworms and hookworms in the intestines so they’re expelled naturally. It’s been in use since 1977.
- Praziquantel targets tapeworms by disrupting their ability to maintain their outer covering, which causes them to break apart and be digested.
- Isoxazolines (sarolaner, afoxolaner, lotilaner, fluralaner) kill fleas and ticks by overexciting their nervous systems. These are the newest class of ingredients in combination heartworm products.
- Lufenuron prevents flea eggs from hatching but doesn’t kill adult fleas.
- Imidacloprid kills adult fleas on contact and is used in topical formulations.
- Spinosad kills adult fleas rapidly and is found in some oral combinations.
Preventatives vs. Treatment Drugs
Heartworm preventatives and heartworm treatment drugs are fundamentally different medications. Preventatives contain macrocyclic lactones at very low doses to kill larvae. Treatment for an established adult heartworm infection requires melarsomine dihydrochloride, an arsenic-based compound given by deep intramuscular injection. The treatment protocol typically involves three injections of melarsomine along with an antibiotic (doxycycline) and a macrocyclic lactone given over several months. This is a far more intensive, expensive, and risky process than monthly prevention.
An alternative non-arsenical treatment protocol uses higher doses of moxidectin combined with doxycycline to slowly kill adult worms over a longer period, though this approach takes much longer than the standard arsenical treatment.
Safety for Sensitive Breeds
Some dog breeds, particularly Collies, Australian Shepherds, and related herding breeds, carry a genetic mutation (called MDR1) that makes them more sensitive to macrocyclic lactones. This mutation affects a protein that normally keeps drugs from crossing into the brain.
The good news is that heartworm preventative doses are so low they’re safe even for dogs with this mutation. Ivermectin-sensitive Collies tolerate up to 60 micrograms per kilogram with no adverse effects, and the heartworm prevention dose is only 6 to 12 micrograms per kilogram. Problems arise only at much higher doses: mild neurological signs like unsteadiness and dilated pupils appear around 100 to 120 micrograms per kilogram, and severe toxicity begins above 200 micrograms per kilogram. These higher doses are sometimes used for treating mange, not for heartworm prevention.
Why Year-Round Prevention Is Recommended
The American Heartworm Society recommends giving heartworm preventatives every month, all year long, regardless of climate. Year-round dosing eliminates the risk of gaps in coverage, which is particularly important given that some heartworm populations have shown signs of reduced susceptibility to preventatives. Annual heartworm testing with both an antigen test and a microfilaria test is recommended alongside prevention, since no preventative is 100% effective. Puppies should start preventatives by 8 weeks of age, with their first heartworm test at 7 months old, since the earliest an infection can be detected is about 5 months after a mosquito bite.