Interceptor prevents heartworm disease and treats several intestinal parasites in dogs. The original Interceptor covers heartworm, hookworms, roundworms, and whipworms. Interceptor Plus adds tapeworm protection to that list, making it one of the broader-spectrum monthly chewables available.
Parasites Covered by Interceptor
The original Interceptor formula uses a single active ingredient, milbemycin oxime, to target four types of parasites in dogs:
- Heartworm (prevention only, not treatment of existing infections)
- Hookworms
- Roundworms (two species)
- Whipworms
For cats, Interceptor covers heartworm, hookworms, and roundworms. Whipworm and tapeworm coverage does not apply to the feline version.
What Interceptor Plus Adds
Interceptor Plus contains a second active ingredient, praziquantel, which targets tapeworms. This is the key difference between the two products. Everything else, including heartworm, hookworm, roundworm, and whipworm coverage, remains the same.
The tapeworm coverage in Interceptor Plus is notably broad. It treats four species of tapeworm, including the type dogs pick up from swallowing fleas. It also covers two species that pose a public health concern because they can, in rare cases, infect humans. If your dog spends time around livestock, wildlife, or rodents, this broader tapeworm protection is particularly relevant.
Interceptor Plus is labeled for dogs only, not cats.
How It Prevents Heartworm
Interceptor does not repel mosquitoes or stop a dog from being bitten. Instead, it kills heartworm larvae that a mosquito may have deposited in the weeks since the last dose. Given once monthly, each dose clears any immature heartworms before they can grow into adults and settle in the heart and lungs. This is why consistent monthly dosing matters: a missed dose gives larvae time to mature past the point where the medication can eliminate them.
Your dog should be tested for heartworm before starting Interceptor. If a dog already has an adult heartworm infection, a preventive alone won’t resolve it, and the situation requires a different treatment approach.
How It Works Against Intestinal Worms
Milbemycin oxime, the ingredient shared by both products, works by forcing open specific channels in a parasite’s nerve cells. This floods the cells with chloride ions, which paralyzes the worm’s muscles, including its feeding apparatus. The worm can’t eat or move and is eventually expelled from the digestive tract. Even if some muscle movement returns briefly, the paralysis of the worm’s feeding structures tends to persist long enough to kill it.
Praziquantel, the second ingredient in the Plus version, works through a different mechanism that specifically targets tapeworms. Between the two ingredients, Interceptor Plus covers both roundworm-type parasites and flatworm-type parasites, which is why its coverage list is longer than most single-ingredient preventives.
Age and Weight Requirements
The minimum requirements differ slightly between the two products and between species:
- Interceptor for dogs: four weeks of age and at least two pounds
- Interceptor Plus for dogs: six weeks of age and at least two pounds
- Interceptor for cats: six weeks of age and at least 1.5 pounds
Both products come in weight-range-specific tablets, so your dog or cat receives a dose calibrated to their size. Using the wrong weight category can result in underdosing or overdosing.
Safety in Herding Breeds
Some herding breeds, particularly Collies, carry a genetic mutation (MDR1) that makes them sensitive to certain parasite medications. Milbemycin oxime has been specifically tested in dogs with this mutation. The FDA determined that Interceptor is safe for MDR1-affected dogs when used at the labeled dose. No adverse effects were observed in testing. This sets it apart from some other parasite preventives that carry stronger warnings for these breeds.
Side Effects
Interceptor Plus has a mild side effect profile. In FDA clinical trials, diarrhea was the only adverse reaction reported, and none of the affected dogs needed treatment for it. A separate clinical study found no treatment-related adverse reactions at all. Vomiting, lethargy, and decreased appetite are occasionally reported in post-market use but are uncommon.
What Interceptor Does Not Cover
Neither version of Interceptor kills fleas or ticks. If your dog needs flea and tick protection, you’ll need a separate product or a combination preventive that includes those ingredients. Interceptor also does not treat an existing heartworm infection. It only prevents new infections by killing larvae before they mature. And while Interceptor Plus covers several tapeworm species, it does not replace the need for flea control if your dog has a flea-transmitted tapeworm problem, since reinfection will continue as long as fleas are present.