Interceptor is a monthly chewable medication for dogs and cats that prevents heartworm disease and treats several types of intestinal worms. Made by Elanco, it’s one of the most widely prescribed parasite preventatives in veterinary medicine. The product comes in two versions: the original Interceptor (for dogs and cats) and Interceptor Plus (for dogs only), which covers a broader range of parasites.
What Interceptor Protects Against
The original Interceptor prevents heartworm disease and treats and controls three types of intestinal parasites: roundworms, hookworms, and whipworms. Interceptor Plus adds tapeworm coverage to that list, bringing the total to five major parasite types in a single monthly chew.
Heartworm prevention is the primary reason most vets recommend Interceptor. Heartworm disease is caused by parasitic worms that live in the heart and lungs, transmitted through mosquito bites. Left untreated, it can be fatal. Interceptor works by killing heartworm larvae before they mature into adults, breaking the cycle within the first month after a mosquito bite.
The intestinal parasites Interceptor targets are common in dogs. Roundworms and hookworms are particularly prevalent in puppies, while whipworms tend to affect dogs that spend time in contaminated soil. Tapeworms, covered only by the Plus version, are often picked up from fleas or contact with wildlife.
How It Works
Interceptor’s active ingredient, milbemycin oxime, belongs to a family of compounds called macrocyclic lactones. It works by binding to specific channels on parasite nerve cells, forcing chloride ions to flood in. This essentially short-circuits the parasite’s nervous system, causing paralysis and death. These channels are unique to invertebrates, which is why the drug kills worms without affecting your dog or cat’s nervous system.
Interceptor Plus pairs milbemycin oxime with a second ingredient, praziquantel, which specifically targets tapeworms. Praziquantel has demonstrated 100% effectiveness against four species of adult tapeworms in controlled studies. The original Interceptor contains only milbemycin oxime, so it doesn’t cover tapeworms.
Interceptor vs. Interceptor Plus
The distinction matters when choosing between the two products:
- Interceptor (milbemycin oxime only): Prevents heartworm disease and treats roundworms, hookworms, and whipworms. Available for both dogs and cats.
- Interceptor Plus (milbemycin oxime + praziquantel): Covers everything the original does, plus four species of tapeworms. Available for dogs only.
If your dog has any risk of tapeworm exposure (which includes any dog that could encounter fleas), the Plus version offers more complete protection. For cats, the original Interceptor remains the available option in this product line.
How to Give It
Both versions are given by mouth once a month, with or without food. The chewable tablet can be broken into pieces if your pet has trouble with it, but the key is making sure your dog actually chews the tablet rather than swallowing it whole. If your pet spits out part of the chew or doesn’t finish it, give a fresh one.
Interceptor Plus is approved for dogs and puppies six weeks of age or older that weigh at least two pounds. Timing matters: doses should be given as close to every 30 days as possible. If you miss a dose, give it as soon as you remember and continue the monthly schedule from there. Don’t double up to make up for a missed dose.
In areas where mosquitoes are active year-round, most vets recommend giving Interceptor every month without seasonal breaks. In colder climates, some vets allow a seasonal schedule, but year-round prevention is generally considered safer since it also keeps intestinal parasites in check.
How Effective It Is
In FDA-reviewed studies, Interceptor achieved 100% effectiveness against heartworm infection when given once monthly for six consecutive months following exposure. That’s the gold standard result for heartworm preventatives, though the FDA notes that one or two isolated doses may not provide complete protection. Consistency is what makes Interceptor reliable: missing months creates gaps where larvae can mature into adult heartworms.
Side Effects
Interceptor has a strong safety profile. In clinical trials submitted to the FDA, no adverse effects were definitively attributed to the drug. Some pet owners reported vomiting, diarrhea, or weight loss, but investigators traced those symptoms to other causes like diet changes or underlying health issues rather than the medication itself. Interceptor was also tested alongside other common pet medications and showed no problematic interactions.
Most dogs and cats tolerate Interceptor well, including breeds that carry the MDR1 gene mutation. This mutation, common in Collies and related herding breeds, makes certain dogs highly sensitive to some parasite drugs (notably ivermectin at higher doses). However, milbemycin oxime at its labeled dose has been studied specifically in dogs with this mutation. In one study, Collies that were confirmed sensitive to related drugs showed no signs of toxicity when given milbemycin oxime at doses up to 10 times the maximum label dose. This makes Interceptor one of the safer choices for MDR1-affected breeds, though your vet may still want to confirm your dog’s genetic status before prescribing any parasite preventative.
What Interceptor Doesn’t Cover
Interceptor does not kill fleas, ticks, or mites. If your pet needs protection against those parasites, you’ll need a separate product or a combination preventative from a different brand. Some pet owners pair Interceptor with a flea and tick product to get full-spectrum coverage, which is a common approach vets recommend. Interceptor also does not treat an existing adult heartworm infection. It only prevents new infections by killing larvae. Dogs should be tested for heartworm before starting Interceptor to make sure they aren’t already infected.