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Companion Animal Business


Active Ingredient

The active ingredient in Interceptor is milbemycin oxime. This single compound when administered monthly to your dog, prevents heartworm and treats and controls several intestinal parasites. The family of milbemycins was discovered by Sankyo Company, Limited of Tokyo, Japan in 1970 and was first reported on in 1974. The first isolate was made from fermentation broth of Streptomyces hygroscopicus subspecies aureolacrimosus.

Milbemycin was first introduced in Canada in 1993. Interceptor may be administered monthly to dogs to prevent heartworm disease and treat and control intestinal parasites. Interceptor is safe to give to puppies and kittens as young as 2 weeks of age. Milbemycin is a component of SENTINEL and Interceptor and is GUARANTEED by Novartis Animal Health.

Milbemycin oxime is manufactured by a combination of fermentation and chemical synthesis. The raw drug substance is a pale yellow crystalline powder. Below is the chemical structure of milbemycin oxime.

Chemical structure of milbemycin oxime

Mode of Action

Interceptor works by interfering with the transmission of nerve impulses in parasitic worms such as heartworm, and intestinal parasites such as whipworm in dogs only and roundworms & hookworms. Interceptor interferes with nerve transmission within these invertebrates by binding to receptor sites on the cell membrane's glutamate-gated chloride channels. This binding enhances the flow of chloride (C1-) ions into the cell, paralyzing, then killing the parasite. The result is that with each monthly dose, heartworm disease is prevented and the pet is cleared of any of these intestinal worms it may have picked up in the environment. It is recommended that all pets that have not visited a vet be tested for the presence of adult worms before beginning treatment with a preventive such as Interceptor. Your veterinarian will advise you of the heartworm testing policy for your clinic.

Activity Spectrum

Heartworm (Dirofilaria immitis larvae)
Interceptor is labeled effective against the third and fourth larval stage (L3-L4) of D. immitis, which is the stage of the disease when the larvae, after the pet is bitten by an infective mosquito, are beginning to migrate through the tissues. They continue to migrate for 50-70 days and it is during this period that they are susceptible to removal with Interceptor . It is the time frame of these L3 and L4 larvae that give Interceptor one of the greatest strengths.Regular monthly dosing is recommended.

Heartworm Microfilarie (Dirofilaria immitis microfilarie)
There are no labeled microfilaricides currently available to veterinarians today. Interceptor is not indicated as a microfilaricide and any such use is considered an off label application and must be authorized and supervised by your veterinarian. One of the recommendations of The American Heartworm Society is that regular monthly dosing with Interceptor is effective in removing microfilarie from dogs that are being treated for the disease. This application is only indicated after the adults have been removed with a heartworm adulticide and the dog has recovered from any adulticide related effects. When trial dogs were treated with Interceptor to remove microfilarie, some dogs exhibited mild transient reactions due to the microfilaricidal activity of the drug. Alive, the microfilarie pose no immediate threat, but once killed the presence of them in the bloodstream may cause brief reactions in some dogs. All dogs that experienced these mild shock-like symptoms recovered rapidly. Following the second and third treatment, no negative reactions were recorded. Your veterinarian will be able to discuss a treatment regime with you should your dog test positive for adult heartworm and need to undergo treatment with a microfilaricide.

Hookworm (Ancylostoma caninum)
Interceptor is labeled for the treatment and control of A. caninum. Results of a series of studies on both naturally occurring and experimentally induced hookworm infections showed Interceptor to have an efficacy against A. caninum as high as 99.6% at the recommended dose of 0.5 mg/kg. Hookworm is a particularly dangerous parasite in dogs due to its effect on younger animals.

Roundworms (Toxocara canis and Toxascaris leonina)
Interceptor is labeled for the treatment and control of T. canis and T. leonina. Studies show success rates of 100% with T. canis and 97.5% with T. leonina in pre-clinical trials. This excellent level of control is especially important because roundworm is easily passed from mother to puppy through the mother's milk and can cause stillbirth and neonatal death in severe cases. In addition, roundworm can be dangerous to humans, especially children.

Whipworm (Trichuris vulpis)
Interceptor is labeled for the treatment and control of T. vulpis. Studies with Interceptor indicate success rates as high as 99.4% in laboratory and field trials. Whipworm can cause chronic diarrhea, anemia, and in some dogs even death. They may also be present with few or no clinical signs.


Milbemycin is safe in over 75 dog breeds, including collies and toy breeds. If you are at all concerned about a reaction with a Novartis product please see your veterinarian, or contact us directly. Common difficulties that some dogs experience are vomiting or diarrhea, and these are usually transient and passing. Ensuring the dose is given after the full meal is one way to relieve this. Sometimes direct dosing of a dog with the tablet can stimulate the gag reflex and cause vomiting which is unrelated to the drug itself. If this occurs repeatedly you will need to alter your dosing method, such as mixing the dose with a meal.

Interceptor Flavor Tabs contain an artificial beef protein. This protein allows dogs to enjoy the taste of beef, while still remaining safe for dogs with beef allergies.

Preventive Regime

Because dogs can become infected with and/or spread some intestinal worms year-round, your dog should be treated with Interceptor (or Sentinel) every month, regardless of the season.

Visit the Farm Animal Business


Learn more about the internal parasites that can infect your pet.