Heartworm preventative (often called “heart wormer”) is a medication that kills immature heartworms in your dog’s body before they can grow into dangerous adults. These larvae enter through mosquito bites, and without regular prevention, they develop into foot-long worms that live in the heart and lungs. Preventatives work by wiping out the larvae accumulated over the previous 30 days, essentially resetting the clock each time you give a dose.
How Heartworm Preventatives Work
When a mosquito carrying heartworm larvae bites your dog, it deposits tiny worms (called L3 larvae) into the skin. Over the following weeks, those larvae grow and migrate through tissue, eventually reaching the bloodstream and settling in the heart and pulmonary arteries. Left alone, the whole process from bite to adult worm takes about six months.
Heartworm preventatives interrupt this cycle early. Each dose kills all the L3 and L4 stage larvae that have entered your dog since the last dose. At these stages the worms are still small and vulnerable, living in skin and muscle tissue. Once they mature into adults, standard preventatives can no longer touch them. That’s why consistent, on-time dosing matters so much: you’re clearing out baby worms before they become a permanent problem.
Beyond killing larvae, these medications can suppress reproduction in adult female worms and shorten adult worm lifespan. They also eliminate microfilariae, the microscopic offspring that circulating adult worms release into the bloodstream. Clearing microfilariae means an infected dog can’t pass the parasite to mosquitoes and on to other animals.
Types of Heartworm Prevention
Monthly Chewables
Oral chewable tablets are the most common form. Given once every 30 days, they come in flavored, treat-like forms that most dogs eat willingly. The active ingredients vary by brand but fall into a few categories: ivermectin (found in products like Heartgard Plus), milbemycin oxime (Interceptor, Sentinel), and moxidectin (found in newer combination products). All target the same larval stages through slightly different mechanisms.
Topical Treatments
Spot-on liquids applied to the skin between the shoulder blades also provide monthly protection. These typically contain either selamectin (Revolution) or moxidectin (Advantage Multi). They absorb through the skin and distribute through the body to reach larvae in tissue. Topicals can be a good option for dogs that refuse oral medications or have sensitive stomachs.
Long-Acting Injections
For owners who struggle with monthly dosing, injectable options provide either six months (ProHeart 6) or a full year (ProHeart 12) of protection from a single shot administered at the vet’s office. These use a slow-release form of moxidectin. Studies show compliance is significantly higher with injections compared to monthly products, simply because there’s nothing to remember at home between visits.
All-in-One Products
Many newer heartworm preventatives bundle protection against fleas, ticks, and intestinal parasites into one monthly tablet. Products like Simparica Trio, NexGard PLUS, and Credelio Quattro combine a heartworm-preventing ingredient with compounds that kill fleas and ticks on contact, plus dewormers for roundworms and hookworms. Credelio Quattro also covers tapeworms. These combination products simplify parasite control into a single monthly dose instead of stacking multiple medications.
How Effective Are They?
When given on schedule, heartworm preventatives are remarkably effective. In controlled studies, monthly products like NexGard PLUS and Simparica Trio achieved 99.5% to 100% efficacy, meaning virtually no treated dogs developed adult worm infections.
That said, heartworm resistance to preventatives has been confirmed in laboratory settings, with resistant worm strains identified in parts of the United States. The prevalence of heartworm disease is also increasing, and its geographic range is expanding into areas historically considered low-risk. Even against a known resistant worm strain, current combination products still achieved over 99% efficacy in trials, so prevention remains highly effective. But the emergence of resistance reinforces why year-round, consistent dosing is important rather than skipping months or using preventatives only during summer.
Why a Prescription Is Required
In the United States, all heartworm preventatives require a veterinary prescription. The primary reason is safety: giving a preventative to a dog that already has an adult heartworm infection can be harmful or fatal. If a dog is carrying microfilariae in its bloodstream, the preventative can cause those microfilariae to die suddenly, triggering a shock-like reaction. Heartworm preventatives do not kill adult worms, so a positive dog would continue getting sicker while appearing to be “protected.”
This is why vets require a heartworm test (a simple blood draw) before prescribing or refilling prevention. The test confirms your dog is negative, making it safe to start or continue the medication. Most vets recommend annual testing even for dogs on consistent prevention.
Breed-Specific Sensitivity
Some dog breeds, particularly Collies and related herding breeds, carry a genetic mutation that makes them extremely sensitive to certain drugs, including ivermectin at higher doses. The mutation affects a gene responsible for producing a protein that acts as a gatekeeper at the blood-brain barrier. Normally, this protein pumps drugs like ivermectin out of the brain. Dogs with the mutation produce a nonfunctional version of the protein, allowing the drug to accumulate in brain tissue and cause neurological toxicity.
Dogs that carry two copies of the mutation (homozygous) show the sensitivity, while dogs with one or zero copies do not. The doses of ivermectin used in heartworm preventatives are far lower than those that typically cause problems, so most affected dogs tolerate standard preventatives safely. Still, if you have a Collie, Australian Shepherd, Shetland Sheepdog, or similar breed, your vet may recommend a genetic test or choose a non-ivermectin product as a precaution.
Side Effects
For the vast majority of dogs, heartworm preventatives cause no noticeable side effects. Oral and topical products occasionally cause mild digestive upset or temporary lethargy. The injectable form (ProHeart) carries a slightly higher side-effect profile. Over a five-year monitoring period covering nearly 1.4 million doses, adverse events with the injectable occurred at a rate of roughly 14 per 10,000 doses. Younger dogs and certain breeds showed higher odds of a reaction. Most adverse events were mild and resolved without lasting issues.
Serious reactions are rare across all product types. The biggest risk isn’t from the preventative itself but from giving it to a dog with an undetected heartworm infection, which is why testing before starting prevention matters.