Interceptor Plus is considered safe for most dogs. It’s FDA-approved for puppies and dogs six weeks of age or older weighing at least two pounds, and clinical trials showed minimal side effects. The most commonly reported reaction was mild diarrhea that resolved without treatment. For the vast majority of dogs, this is a well-tolerated monthly chewable that protects against heartworm and several intestinal parasites.
What Interceptor Plus Protects Against
Interceptor Plus contains two active ingredients that together cover a broad range of parasites. The first, milbemycin oxime, prevents heartworm disease and treats roundworms, hookworms, and whipworms. The second, praziquantel, targets tapeworms. That combination makes Interceptor Plus one of the wider-spectrum monthly preventives available, covering five types of dangerous worms in a single chewable tablet.
The tapeworm coverage is worth noting because many competing heartworm preventives don’t include it. If your dog is prone to picking up tapeworms (common in dogs that swallow fleas or hunt small animals), Interceptor Plus handles that without needing a separate dewormer.
Side Effects in Clinical Trials
In FDA-reviewed clinical studies, side effects were rare and mild. One pivotal study found diarrhea in five dogs receiving Interceptor Plus compared to two dogs in the placebo group, and none of those dogs needed treatment. A second clinical study reported no adverse reactions related to the product at all.
The most commonly reported side effects across broader use include diarrhea, vomiting, decreased appetite, and lethargy. These tend to be short-lived. Serious reactions are uncommon at the labeled dose.
Safety for Collies and MDR1-Mutation Breeds
Certain breeds, particularly Collies, Australian Shepherds, and related herding dogs, carry a genetic mutation (called MDR1 or ABCB1) that makes them more sensitive to some antiparasitic drugs. This is a common concern for owners of these breeds, and for good reason: the mutation affects how efficiently their bodies clear certain compounds from the brain.
The good news is that the manufacturer specifically tested Interceptor Plus in dogs with the MDR1 mutation. No adverse effects were observed at label doses, and the FDA determined the product is safe for these dogs when used as directed. Milbemycin oxime is structurally similar to ivermectin but is less toxic to mammals, which contributes to its wider safety margin. If your dog is a herding breed or a herding-breed mix, Interceptor Plus at the correct dose for their weight is generally a safe choice.
What Happens if a Dog Gets Too Much
Accidental overdose is a realistic concern, especially if a dog chews into the box or gets a dose meant for a larger dog. At the standard preventive dose (0.5 to 1.0 mg/kg of milbemycin oxime), toxicity is extremely unlikely. Problems start at much higher doses.
In normal, healthy dogs, mild signs like unsteadiness, drooling, dilated pupils, and lethargy have been documented at doses 10 to 20 times the standard amount. Dogs with the MDR1 mutation may show those signs at lower overdose levels, roughly 5 to 10 times the label dose. At very high exposures, more serious neurological signs can develop, including tremors, disorientation, blindness, and in severe cases, coma.
Because milbemycin oxime has a long half-life, overdose symptoms can last days to weeks depending on how much was consumed. The most dangerous overdose scenarios involve dogs accidentally getting into concentrated large-animal deworming products, not from eating an extra chewable tablet or two. Still, if your dog consumes more than their prescribed dose, contact your vet or an animal poison control hotline promptly.
Puppies, Pregnant Dogs, and Breeding Dogs
Interceptor Plus can be given to puppies as young as six weeks old, provided they weigh at least two pounds. That’s a relatively early start compared to some other preventives, which matters in areas where heartworm-carrying mosquitoes are active year-round.
Milbemycin oxime has been tested in pregnant females, breeding males and females, and puppies over two weeks of age across more than 75 breeds. At the standard once-monthly dose, it was well tolerated. Safety studies did find that when pregnant dogs received three times the normal dose every single day from mating through weaning (a deliberately exaggerated regimen), the drug appeared in their milk and affected nursing puppies. At the normal monthly dosing schedule, this isn’t a practical concern.
Heartworm Testing Before Starting
Before giving your dog Interceptor Plus for the first time, or if you’ve missed doses, your vet will want to run a heartworm test. This isn’t just a formality. Monthly preventives like Interceptor Plus kill heartworm larvae picked up in the previous 30 days. They don’t treat an existing adult heartworm infection. Starting a preventive without testing could mask an active infection and delay treatment, allowing the disease to progress and cause serious heart and lung damage.
A Note on Resistant Heartworm Strains
One area where Interceptor Plus has limitations involves certain drug-resistant heartworm strains that have emerged, primarily in the Mississippi Delta region. In controlled laboratory studies using two resistant strains (known as ZoeLA and JYD-34), Interceptor Plus showed significantly reduced efficacy: roughly 21% against one strain and 55% against the other, compared to over 95% for susceptible heartworm populations. These strains are not yet widespread, and for most dogs in most regions, Interceptor Plus remains highly effective at preventing heartworm. But it’s a reason to keep up with annual heartworm testing even if you never miss a dose.
Weight-Based Dosing
Interceptor Plus comes in four tablet sizes based on your dog’s weight. Using the correct size matters for both safety and effectiveness. Underdosing leaves gaps in protection, while consistent overdosing increases risk over time. If your dog is near the boundary between two weight ranges, weigh them at your vet’s office rather than guessing. Growing puppies may need to move up a tablet size as they gain weight through their first year.