The best heartworm and flea prevention for most dogs is an all-in-one monthly chewable that covers heartworms, fleas, ticks, and intestinal worms in a single dose. Three products currently lead this category: Simparica Trio, NexGard PLUS, and Credelio Quattro. All three are FDA-approved, require a prescription, and have been tested against both standard and drug-resistant heartworm strains with efficacy rates above 99%.
Which one is “best” depends on your dog’s age, breed, weight, and lifestyle. Here’s what you need to know to make that choice.
How All-in-One Chewables Compare
Simparica Trio, NexGard PLUS, and Credelio Quattro all work the same basic way. Each contains an ingredient from the isoxazoline class that kills fleas and ticks, plus two additional compounds that prevent heartworm disease and eliminate common intestinal parasites like roundworms and hookworms. The main differences come down to one thing: Credelio Quattro also covers tapeworms, which the other two do not.
In a head-to-head clinical trial testing both NexGard PLUS and Simparica Trio against a drug-resistant heartworm strain from the Mississippi Delta region, NexGard PLUS achieved 99.5% efficacy and Simparica Trio reached 99.8%. Against a standard heartworm strain, NexGard PLUS hit 100% prevention with a single dose. These are essentially equivalent results, so the choice between them rarely comes down to effectiveness alone.
All three are flavored chewable tablets given once a month. Most dogs take them willingly, and the convenience of covering four or five parasites in one chew is what makes these products so popular. If your dog needs flea, tick, heartworm, and intestinal worm protection (and nearly all dogs do), a combination product eliminates the need to juggle multiple medications and schedules.
Why Year-Round Prevention Matters
The American Heartworm Society recommends giving prevention every month, all year long. This isn’t just a conservative recommendation. Heartworm-carrying mosquitoes can appear earlier and later in the season than most people expect, and a single missed month creates a window where larvae can establish themselves in your dog’s body. The preventive works retroactively: each monthly dose kills any heartworm larvae your dog picked up in the preceding weeks before they can mature into adults.
Compliance is a major problem with monthly products. Research tracking real-world purchasing behavior found that only about 35% of dog owners who buy monthly heartworm prevention actually purchase all 12 doses in a year. Fewer than half buy enough doses to cover even six months. That gap between what’s recommended and what actually happens is a significant reason dogs still get heartworm disease even when their owners believe they’re protected.
Injectable Prevention: A Set-It-and-Forget-It Option
If remembering a monthly chew sounds unreliable for your household, there’s an injectable alternative. ProHeart 12 is a single injection given by your veterinarian that prevents heartworm disease for a full 12 months. ProHeart 6 covers six months. Because the vet administers it directly, compliance is essentially guaranteed for every dog that receives it.
The tradeoff is that ProHeart only prevents heartworms. It does not kill fleas, ticks, or intestinal worms, so you’d still need a separate product for those parasites. It also can’t be purchased online or given at home, and the full annual cost is paid upfront rather than spread across monthly purchases. For owners who are consistent with a monthly routine, an all-in-one chewable covers more parasites in fewer steps. For owners who know they’ll miss doses, the injectable can be a smarter foundation to build around.
When to Start and What’s Required First
Puppies should begin heartworm prevention as early as possible, and no later than 8 weeks of age. Most combination chewables have minimum weight requirements (typically around 2.8 to 3.3 pounds depending on the brand), so very small puppies may need to wait a few weeks to reach the threshold.
For adult dogs or any dog that has gone more than a few months without prevention, a heartworm test is needed before starting or restarting medication. This isn’t optional. If a dog already has adult heartworms living in the heart and lungs, giving a preventive alone won’t clear the infection, and complications can occur. The test is a simple blood draw with results typically available the same day. Annual testing is recommended even for dogs on year-round prevention, because no product is 100% effective in every real-world scenario.
Side Effects and Safety Concerns
The most common side effects of combination chewables are mild: vomiting, diarrhea, and lethargy that typically resolve within a day or two. These affect a small percentage of dogs.
The more serious concern involves the isoxazoline class of ingredients found in all three major combination products. The FDA has issued an alert noting that isoxazolines have been associated with neurologic reactions in some dogs, including muscle tremors, loss of coordination, and seizures. These events are uncommon, and most dogs tolerate isoxazolines without any issues. However, seizures have occurred even in dogs with no prior history of neurologic problems. If your dog has a seizure disorder or a history of neurologic issues, this is worth discussing with your vet before choosing a product in this class.
Breed-Specific Drug Sensitivities
Certain herding breeds carry a genetic variant called MDR1 that makes them more sensitive to some medications. Breeds commonly affected include Collies, Australian Shepherds, Shetland Sheepdogs, Old English Sheepdogs, and German Shepherds, though it can appear in mixed breeds with herding ancestry as well.
The ingredient most associated with MDR1 toxicity concerns is ivermectin, found in some older heartworm preventives. At the low doses used in FDA-approved heartworm products, ivermectin is still considered safe for MDR1-sensitive dogs. Problems arise when dogs are exposed to higher, off-label doses. If you have a herding breed, a simple genetic test can determine MDR1 status, and your vet can choose products accordingly. The newer combination chewables use different active ingredients for heartworm prevention (moxidectin rather than ivermectin), which is another option for sensitive breeds.
Drug-Resistant Heartworms
Heartworm strains resistant to standard preventive medications have been documented in the Mississippi Delta region of the United States, and there’s concern that resistance could spread to other areas. This is one reason year-round, consistent dosing matters so much. Skipping months or giving inconsistent doses creates conditions that favor the survival of partially resistant larvae.
The good news is that current combination products still show very high efficacy even against known resistant strains in controlled studies. Consistent monthly dosing with an FDA-approved product remains the most effective strategy available, and combining it with practical mosquito reduction (eliminating standing water, limiting outdoor time at dawn and dusk) adds another layer of protection in high-risk regions.
Choosing the Right Product for Your Dog
For most healthy adult dogs, any of the three major combination chewables will provide excellent, broad-spectrum protection. Simparica Trio and NexGard PLUS perform nearly identically in clinical trials. Credelio Quattro is worth considering if your dog has exposure to tapeworms, since it’s the only combination product that covers them.
If your dog has a history of seizures, talk to your vet about whether a non-isoxazoline flea product paired with a standalone heartworm preventive might be a better fit. If compliance is your biggest challenge, ask about ProHeart 12 for heartworm coverage and add a separate flea and tick product on top. The best prevention plan is ultimately the one you’ll actually follow through on every single month.