The original NexGard chew does not prevent heartworm disease. It only kills fleas and ticks. However, a newer product called NexGard Plus does include heartworm prevention, combining flea and tick protection with coverage against heartworms and intestinal worms in a single monthly chew.
This is a common point of confusion because the products share a name. If your dog is currently on regular NexGard, it is not protected against heartworm and needs a separate preventive unless you switch to NexGard Plus or add another heartworm medication.
NexGard vs. NexGard Plus: Key Differences
Original NexGard contains one active ingredient that targets fleas and ticks. That’s all it does. It works well for external parasites but offers zero protection against heartworms or intestinal worms. If your vet prescribed NexGard alongside a separate heartworm preventive, that combination was intentional, and dropping the heartworm medication would leave your dog unprotected.
NexGard Plus contains two additional active ingredients on top of the original flea-and-tick compound. One of those extra ingredients belongs to a class of antiparasitics that kills immature heartworm larvae before they can develop into adult worms. The other targets intestinal parasites. The result is a single chew that covers:
- Fleas
- Ticks (six species, including black-legged, brown dog, American dog, lone star, Gulf Coast, and longhorned ticks)
- Heartworm disease
- Roundworms
- Hookworms
Both products are given once every 30 days as a flavored chew. NexGard Plus is approved for dogs and puppies eight weeks of age and older that weigh at least four pounds.
How Heartworm Prevention Works in NexGard Plus
Heartworm prevention doesn’t create a shield that blocks mosquitoes or stops bites from happening. Instead, it works retroactively. When a mosquito bites your dog, it can deposit microscopic heartworm larvae under the skin. Those larvae take several months to mature into adult worms that lodge in the heart and lungs. The heartworm-prevention ingredient in NexGard Plus kills the larvae during that early stage, before they ever reach the heart.
This is why monthly dosing matters so much. Each dose eliminates any larvae your dog picked up in the previous 30 days. Miss a dose or give it late, and larvae from that gap period can survive long enough to mature beyond the point where the medication can kill them. If you realize you’ve missed a dose, give it as soon as you remember and stay on schedule going forward. Dogs that have had a gap in prevention for two months or more typically need a heartworm test before restarting, since giving preventives to a dog with an active adult heartworm infection can cause serious complications.
What About Cats?
NexGard Plus is only for dogs. For cats, the NexGard line offers a separate product called NexGard Combo, which is a topical liquid applied to the skin once a month. It covers fleas, ticks, heartworms, roundworms, hookworms, and tapeworms in cats. It is not interchangeable with the dog products.
Side Effects to Know About
NexGard Plus belongs to a drug class called isoxazolines, which the FDA has flagged for a specific safety concern. These products have been associated with neurologic reactions in some dogs, including muscle tremors, loss of coordination, and seizures. Most dogs tolerate them without any neurologic issues, but seizures have occurred even in dogs with no prior history of them. Dogs that already have a seizure disorder may be at higher risk.
More common, less serious side effects can include vomiting, diarrhea, and decreased appetite after taking the chew. These tend to be mild and short-lived.
Do You Need to Switch?
Whether NexGard Plus makes sense for your dog depends on what your current parasite prevention setup looks like. If you’re already giving original NexGard for fleas and ticks plus a separate heartworm preventive, switching to NexGard Plus could simplify things into one monthly chew. If you’re giving original NexGard and nothing else, your dog has no heartworm protection, and that’s a gap worth addressing, especially in areas where mosquitoes are active for much of the year.
Dogs need a negative heartworm test before starting any heartworm preventive for the first time. Starting prevention in a dog that already has adult heartworms can trigger a dangerous inflammatory reaction as the medication interacts with the existing infection. A simple blood test at the vet confirms whether your dog is clear to start.