Interceptor for Cats: Overview, Dosage & Side Effects

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Interceptor is the brand name for a monthly oral heartworm and intestinal parasite preventative manufactured by Elanco Animal Health. In this article, you’ll learn some general product information, how Interceptor targets parasites, side effects to monitor for, and some frequently asked questions.

Interceptor for Cats Overview

Medication Type:
Macrolide antiparasitic
Chewable tablet
Prescription Required?:
FDA Approved?:
Life Stage:
4 weeks of age or older
Brand Names:
Common Names:
Milbemycin oxime
Available Dosages:
5.75mg (brown, cats 1.5-6lb); 11.5mg (yellow, cats 6.1-12lb); 23mg (white, cats 12.1-25lb)
Expiration Range:
Products should be used before the expiration on the package. Interceptor tablets should be stored at room temperature.

About Interceptor for Cats


Interceptor is a monthly preventative medication that targets mosquito-borne heartworms (Dirofilaria immitis), and the intestinal parasites roundworms (Toxocara cati) and hookworms (Ancylostoma tubaeforme).

Interceptor’s active ingredient is milbemycin oxime. Milbemycins work primarily through specific glutamine-gated chloride ion channels in the nervous system of the parasite. This inhibits the electrical activity of nerve cells in worms, causing paralysis and death.

Milbemycins also enhance the release of gamma-aminobutyric acid (GABA), which is an inhibitory neurotransmitter, thereby blocking nerve conduction in worms.

Mammals don’t have the specific glutamate-gated chloride channels in their nervous systems, making milbemycins generally safe and nontoxic. Milbemycins also do not cross the blood-brain barrier, preventing them from reaching a mammal’s GABA receptors.

As a heartworm medication that is FDA approved for cats, Interceptor is a prescription item that can only be prescribed by a licensed veterinarian.

What Does Interceptor Do for Cats?

Interceptor is generally regarded as a safe and effective option for the prevention of heartworms, roundworms, and hookworms in cats.

Interceptor will actually kill adult roundworms and adult hookworms in cats, providing treatment for existing infections.

However, with heartworms, Interceptor has only been shown to be effective for up to 45 days after initial infection with heartworm larvae carried by a mosquito. Once the larvae mature further and develop into adult worms, Interceptor can no longer provide treatment.

There is no approved adult heartworm treatment for cats. While cats often have infections caused by very few worms and some cats may clear infections on their own, cats can also get severe chronic inflammatory changes to their lungs. Sudden death can also unfortunately be the only sign that a cat was infected with heartworms. Testing is unreliable in cats and this is why focus on prevention is very crucial in addressing heartworm disease in cats.

It is important to realize, as with all preventatives, what Interceptor does not do. It cannot treat or prevent any other intestinal parasites other than roundworms and hookworms, and it has no activity against fleas and ticks. Interceptor Plus, which also includes praziquantel for treating and preventing tapeworms in dogs, is not approved for use in cats.

Side Effects of Interceptor for Cats

Very few cats experience side effects from Interceptor.

According to the manufacturer, safety studies involving young cats and kittens given up to 10 times the recommended dose of Interceptor experienced no drug-related ill effects.

In studies involving nearly 150 cats of several different breeds, Interceptor was used safely with vaccines, dewormers, antibiotics, flea collars, steroids, shampoos, and dips.

It should be noted however, that any new oral medication has the potential to cause digestive upset. In dogs, the most common adverse reactions included vomiting, diarrhea, decrease in appetite, and excessive drooling/hypersalivation.

While neurologic effects have been seen in dogs, including abnormal gait, seizures, and convulsions, it is important to note that some dogs carry what is called the MDR1 mutation that increases their susceptibility of neurologic side effects to certain drugs. Cats do not carry this mutation and these side effects have not been reported in cats.

According to the manufacturer, safety in heartworm positive cats has not been established and they do recommend testing prior to starting Interceptor.

Heartworm testing in cats is not very reliable as cats may often have a single-sex worm infection and the test is based on detecting the female worm antigen. Therefore, if a cat has a single-sex infection by a male worm, the test will come up negative.

This is why it is important to consult with your vet prior to starting Interceptor, or any heartworm prevention medication for your cat. Cats may show other signs of illness that can be associated with heartworms, including asthma-like signs such as coughing or breathing difficulty.

Interceptor for Cats Dosage

Interceptor comes in different dosages intended for cats of varying weights.

Interceptor is approved for cats and kittens aged 6 weeks of age or older, and weighing at least 1.5 pounds.

There are three dosage sizes of Interceptor available for cats. Interceptor Green is for cats weighing 1.5 to 6 pounds (5.75 milligrams of milbemycin oxime). Interceptor Yellow is for cats weighing 6.1 to 12 pounds (11.5 milligrams of milbemycin oxime). Interceptor White is for cats weighing 12.1 to 25 pounds (23 milligrams of milbemycin oxime).

For a cat weighing more than 25 pounds, the manufacturer recommends combining tablets to achieve the necessary dose based on weight. For example, a 27 pounds cat would get one Interceptor White tablet and one Interceptor Green tablet.

Interceptor is intended to be given once monthly, preferably on the same date each month. The manufacturer recommends starting within 30 days of a cat’s exposure to mosquitoes (this is because the medication can still kill heartworm larvae for up to 45 days).

The chewable Interceptor tablets are formulated to be palatable and can be offered on their own as a treat. In the event your kitty will not take Interceptor on its own, the tablet can be administered in food, or broken up into smaller pieces.

If the dose of Interceptor is not fully consumed, the manufacturer does recommend giving another full dose as soon as possible.

While the manufacturer does indicate giving monthly during mosquito “season,” it is important to note that mosquitoes and heartworm disease can be a significant risk year-round in many regions, which is why veterinarians recommend year-round prevention, even for indoor cats. In studies, up to 25% of heartworm positive cats have been identified as indoor-only cats.


Interceptor is a common and reliable heartworm and intestinal parasite preventative that has been around for many years and is considered safe and effective. Its main downside is that it does not provide protection against fleas and ticks.

Frequently Asked Questions

Can Interceptor be given to cats?

Yes, Interceptor can be given to cats, but it is important to only use the correct dosage size, which include Interceptor Green, Yellow, and White. The packaging will indicate it is for cats and dogs, and a picture of both is on the front. 

Interceptor Plus, which contains praziquantel for tapeworms, is not approved for use in cats. 

What does Interceptor treat in cats?

Interceptor for cats can actively kill adult roundworms and hookworms, as well as provides prevention with continued administration. For mosquito-borne heartworms, Interceptor can kill newly introduced larvae for up to their first 45 days, as well as prevent new infections. 

Interceptor cannot kill later stage larvae or adult heartworms, and does not kill fleas and ticks. 

Is Interceptor still available?

Yes, Interceptor is still widely available. Interceptor Plus for dogs is now also available, but does not carry a label of approval for cats. 

How often should Interceptor be administered?

Interceptor is designed to be administered once every 30 days. The manufacturer recommends starting it within 30 days of mosquito exposure and continuing through the rest of mosquito “season.”

However, it is important to note that in many regions, mosquito-borne heartworm disease is a year round concern, leading most veterinarians to recommend using prevention every month all year. 

If a dose is missed within 30 days of its due date, the manufacturer recommends giving the next dose as soon as possible. If it has been beyond 30 days of the last due date, the manufacturer recommends re-evaluating a cat for possible heartworm disease first through blood testing and evaluation of any clinical signs by a licensed veterinarian. 

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About Dr. Chris Vanderhoof, DVM, MPH

Dr. Chris Vanderhoof is a 2013 graduate of the Virginia-Maryland College of Veterinary Medicine (VMCVM) at Virginia Tech, where he also earned a Masters in Public Health. He completed a rotating internship with Red Bank Veterinary Hospital in New Jersey and now works as a general practitioner in the Washington D.C. area. Dr. Vanderhoof is also a copywriter specializing in the animal health field and founder of Paramount Animal Health Writing Solutions, which can be found at Dr. Vanderhoof lives in the Northern Virginia area with his family, including 3 cats.

4 thoughts on “Interceptor for Cats: Overview, Dosage & Side Effects”

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  1. Chris Alston

    My cat won’t eat the Interceptor tab and is also suspicious of unknown tastes/smells in her tuna fish treats. Last night I added her monthly Interceptor tab to my cats refrigerated container of tuna, and I plan to divvy out the contents over the next several days. Do you foresee any problems with this method?

    1. Avatar photoDr. Chris Vanderhoof, DVM, MPH Post author

      Hi Chris, thanks for your question. Cats can be a little frustrating with how picky they can be with certain things so I can empathize there. The manufaturer indicates that the Interceptor tablets can be broken up into smaller pieces for cats to make it easier to give, so there should be no issue there. However, giving the medication over the course of several days I would be concerned could alter the absorption and the effective dose of the drug, which may cause it to be less effective. The manufacturer stresses the importance of giving it on the same day each month (as is appropriate for all medications acting as heartworm preventatives). So while I think you would be fine splitting it up to mix with a single meal or even feeding small pieces throughout a single day, I would try to avoid spreading the dose over several days.

  2. Rebecca Wolf

    “Since the feline MDR1 mutation was more recently discovered than the canine MDR1 mutation there has been less research on the topic. The WSU PrIMe research team is working to determine if other drugs commonly used for treating cats are substrates for feline P-glycoprotein (i.e., have the potential to cause toxicity in cats with the MDR1 mutation).”