There isn’t a single “best” heartworm medicine for every dog. The right choice depends on your dog’s size, breed, lifestyle, and what other parasites you want to cover at the same time. All FDA-approved heartworm preventatives are highly effective when given on schedule, so the real question is which format and coverage combination fits your situation. Here’s how to sort through the options.
How Heartworm Preventatives Work
Every heartworm preventative on the market uses one of four active ingredients, all from a drug class called macrocyclic lactones. These compounds kill heartworm larvae that mosquitoes have deposited under your dog’s skin before the larvae can mature and reach the heart. The four ingredients are ivermectin (used in Heartgard Plus and its generics), moxidectin (used in ProHeart, Simparica Trio, and Advantage Multi), milbemycin oxime (used in Interceptor, Sentinel, Trifexis, and others), and selamectin (used in Revolution).
All four are effective against heartworm larvae when used as directed. The differences come down to delivery method, what else the product kills, and how often you need to give it.
Three Delivery Methods Compared
Monthly Chewables
Oral chewables are the most popular option. Products like Heartgard Plus, Interceptor Plus, Simparica Trio, and Sentinel Spectrum are flavored tablets or soft chews given once a month. Most dogs eat them willingly, which makes compliance straightforward. The downside is that you have to remember every single month, and a missed dose creates a gap in protection.
Topical Solutions
Topical preventatives like Revolution and Advantage Multi are liquids applied to the skin between your dog’s shoulder blades, also monthly. These work well for dogs that refuse chewables or have food sensitivities. The trade-off: you need to avoid bathing your dog for a day or two after application, and the product can leave an oily spot on the fur.
Injectable Prevention
ProHeart 6 and ProHeart 12 are injections given by your veterinarian that protect against heartworm for six or twelve months, respectively. These use moxidectin in a slow-release formula. If you struggle to remember monthly doses, the injectable option essentially eliminates compliance gaps. The catch is that it only covers heartworm (and hookworms), so you’d still need a separate product for fleas, ticks, or other intestinal parasites.
What Each Product Covers Beyond Heartworm
Most heartworm preventatives pull double or triple duty against other parasites. This is often the deciding factor when choosing between products.
- Heartgard Plus: Heartworm, roundworms, hookworms.
- Interceptor Plus: Heartworm, roundworms, hookworms, whipworms, tapeworms.
- Simparica Trio: Heartworm, roundworms, hookworms, fleas, ticks. One of the few single products that covers both heartworm and ticks.
- Sentinel Spectrum: Heartworm, roundworms, hookworms, whipworms, tapeworms, plus flea egg development prevention (does not kill adult fleas).
- Trifexis: Heartworm, roundworms, hookworms, whipworms, adult fleas.
- Advantage Multi (topical): Heartworm, roundworms, hookworms, whipworms, fleas.
- Revolution (topical): Heartworm, roundworms, hookworms, fleas, ear mites, sarcoptic mange.
- ProHeart 12 (injectable): Heartworm and hookworms only.
If your dog spends time in wooded or grassy areas where ticks are common, a product like Simparica Trio that includes tick protection can replace what would otherwise be two separate medications. If tapeworms are a recurring problem, Interceptor Plus or Sentinel Spectrum cover those. The broader the coverage, the fewer products you need to manage.
When to Start and How Dosing Works
The American Heartworm Society recommends starting puppies on prevention as early as the product label allows, and no later than 8 weeks of age. Dosing is based on body weight, not age, so you’ll need to update the dose as your puppy grows. Most products come in weight-range packages (for example, 1 to 25 pounds, 26 to 50 pounds), and your vet will match the right one to your dog’s current size.
Before starting any heartworm preventative, dogs older than about seven months need a heartworm test. Giving prevention to a dog that’s already infected can cause serious, even life-threatening reactions. Annual testing is recommended even for dogs on year-round prevention, because no product is 100% foolproof in real-world conditions.
Breed Sensitivity and the MDR1 Gene
Certain breeds carry a genetic mutation called MDR1 that makes them more sensitive to some drugs, including the ingredients in heartworm preventatives. Collies are the most commonly affected breed, with roughly 70% carrying the mutation. Australian Shepherds (about 50%), Long-haired Whippets (50%), McNabs (30%), and Shetland Sheepdogs (15%) are also at elevated risk. German Shepherds carry it at around 10%, and it shows up in about 5% of mixed-breed dogs.
The good news: the FDA has determined that all four major heartworm prevention ingredients (ivermectin, milbemycin, moxidectin, and selamectin) are safe for MDR1-positive dogs when used at standard label doses. The danger comes from off-label use at higher doses, particularly with ivermectin. If you own one of these breeds, a simple cheek-swab test can identify the mutation, and sticking to labeled heartworm products at the recommended dose keeps your dog safe.
Resistance and Why Compliance Matters Most
There are confirmed heartworm strains in parts of the United States, particularly the Mississippi River valley, that show some genetic resistance to macrocyclic lactones. Researchers have identified specific genetic markers in these resistant parasites. However, this resistance has not been confirmed in Europe or Australia, and in most of the U.S. it remains uncommon.
The far bigger risk factor is inconsistency. Missing doses, giving them late, or stopping prevention during winter months accounts for the vast majority of heartworm infections in dogs that were supposedly “on prevention.” Factors like the drug’s potency, natural variation in heartworm susceptibility, and owner compliance all play into real-world effectiveness. Of these, compliance is the one you control directly.
What to Do If You Miss a Dose
If you’re late by a month or less, give the missed dose immediately and continue on your regular schedule. Your vet will likely want to run a heartworm test at the next visit if it falls more than seven months from the missed dose, since that’s roughly how long it takes for an infection to become detectable.
If the lapse is two months or longer, restart the preventative right away. Your vet may also prescribe a short course of an antibiotic that weakens heartworm larvae, improving the odds that the preventative will clear any early infection. If you’ve gone more than seven months without protection, a heartworm antigen test should be done promptly, because at that point an established infection could be present.
Choosing the Right Product for Your Dog
For most dogs, the best heartworm medicine is the one you’ll actually give consistently. A monthly chewable like Heartgard Plus or Interceptor Plus works well for owners who are organized with reminders. If you know you’ll forget, the ProHeart 12 injection removes that variable entirely. If you want to minimize the number of products you’re buying, a broad-spectrum option like Simparica Trio or Trifexis can collapse heartworm, flea, tick, and intestinal parasite protection into a single monthly dose.
Cost varies widely depending on your dog’s weight and whether a generic is available. Ivermectin-based products like generic Heartgard tend to be the least expensive. Combination products that include flea and tick protection cost more per dose but can save money overall compared to buying separate products. Many vet clinics offer rebates or bulk pricing on 12-month supplies, and purchasing a full year at once also eliminates the risk of running out mid-season.