# Interceptor for Heartworm Prevention in Cats Heartworm disease in cats is a serious, mosquito-borne infection that can damage the lungs and sometimes the heart, and Interceptor (milbemycin oxime) is a monthly chewable preventive that stops the parasite before it matures. Even indoor cats can get heartworm, so year-round prevention is often recommended. This article explains how Interceptor heartworm for cats works, how to give it, and what to watch for. This article is educational and is not a substitute for veterinary diagnosis or treatment. ## Quick Answer - Heartworm is spread only by mosquito bites, and both indoor and outdoor cats can be infected [1]. - Interceptor contains milbemycin oxime, a macrocyclic lactone that kills heartworm larvae before they grow into adult worms [2]. - It is given once a month, ideally on the same date each month, and many veterinarians recommend giving it year-round in the United States [2]. - A missed dose leaves a gap in protection, so call your veterinarian for guidance if you forget. - Your veterinarian sets the exact dose based on your cat's weight and health, and testing may be recommended before starting. ## What Interceptor Is and What Veterinarians Use It For Interceptor is a monthly oral preventive that contains milbemycin oxime. Milbemycin belongs to a group of drugs called macrocyclic lactones, which are the mainstay of heartworm prevention in both dogs and cats [2]. In cats, the same medication is also used to treat certain intestinal worms, so your veterinarian may prescribe it for more than one purpose. Heartworm disease is caused by a parasitic worm called Dirofilaria immitis. Mosquitoes pick up the microscopic larvae (called microfilariae) from an infected animal and pass them to a new host when they bite [1]. Once inside a cat, the larvae travel through the body and eventually settle in the blood vessels leading from the heart to the lungs. In advanced infections, worms may also enter the heart itself [1]. Cats are not the natural host for this parasite, so the disease behaves differently than it does in dogs. The worms often do not survive as long, and a cat may carry only one or two. Even so, the presence of the parasite stresses the heart, blood vessels, and lungs, and severe complications can occur when worms die [1]. Veterinary scientists use the term heartworm associated respiratory disease, or HARD, to describe the lung disease heartworms cause in cats [3]. Because treatment options for adult heartworms in cats are limited and risky, prevention is the safest approach. That is why veterinarians focus on stopping the infection before it takes hold. You can read more about the broader picture in [Heartworm Prevention for Cats: Why It's Important and How It Works](/knowledge/veterinary-medicine/preventive-and-routine-care/heartworm-prevention-for-cats-why-its-important-and-how-it-works). ## How Interceptor Works Against Heartworm Milbemycin oxime works by interfering with the parasite's nervous system. When a cat takes the medication, the drug is absorbed into the bloodstream. If a mosquito has recently deposited heartworm larvae into the cat, those larvae are exposed to the drug and are killed before they can develop into adult worms. This is why timing matters. The medication does not stay in the body at full strength for a full month, and it does not kill adult worms that are already established. It works on the early larval stages. Macrocyclic lactones like milbemycin are given at 30-day intervals during the transmission season to keep that window of protection open [2]. In the United States, many veterinarians recommend giving heartworm preventives throughout the year instead of only during warm months [2]. Mosquitoes can survive indoors and in warm pockets even during cooler weather, and year-round dosing removes the guesswork. It helps to understand that prevention is not the same as treatment. If a cat already has adult heartworms, milbemycin will not clear them. Managing an established infection is a different process, which is covered in [Heartworm Disease in Cats: Feline Diagnosis and Supportive Care](/knowledge/veterinary-medicine/parasite-control/heartworm-disease-in-cats-feline-diagnosis-and-supportive-care). ## Dosing Basics for Interceptor Monthly Cat Use ### Who Decides the Dose Your veterinarian decides the dose. Interceptor comes in different strengths, and the right one depends on your cat's weight and overall health. Never split, guess, or borrow a dose from another pet. Cats and dogs have different products and different dosing, and a dog product is not a substitute. Before starting, your veterinarian may recommend a heartworm test. Testing helps confirm your cat is not already infected, which matters because preventive medications are not designed to treat adult worms. Testing in cats can be tricky. Antigen tests may give false-negative results in cats with male-only infections, so the true prevalence is likely underestimated [4]. Your veterinarian will interpret results in the context of your cat's history and signs. ### How It Is Given Interceptor is typically given as a flavored chewable. Most cats take it willingly, but some are picky. If your cat refuses the chewable, ask your veterinarian about the best way to give it. Do not crush or mix it into a full meal unless your veterinarian says it is fine, because a cat that eats only part of the food may not get the full dose. Give it at the same time each month. Mark your calendar, set a phone reminder, or pair it with another monthly task so it becomes a habit. ### What to Do About a Missed Dose If you miss a dose, call your veterinarian. In general, the goal is to get back on schedule as soon as possible, but the exact advice depends on how late the dose is and your cat's risk level. Do not double up unless your veterinarian tells you to. A gap in protection means your cat may have been exposed without coverage. If you are traveling or boarding your cat, plan ahead so the monthly dose is not missed. If someone else cares for your cat, leave clear written instructions. ## Side Effects of Interceptor in Cats Most cats tolerate milbemycin oxime well. When side effects occur, they are usually mild and short-lived. The table below summarizes what owners commonly ask about. | Side effect | How common or serious | What to do | |---|---|---| | Vomiting | Uncommon, usually mild | Watch for one episode. If it repeats or your cat seems unwell, call your veterinarian. | | Diarrhea | Uncommon, usually mild | Offer water and monitor. Call if it lasts more than a day or your cat stops eating. | | Drooling or lip smacking | Uncommon, often related to taste | Offer fresh water. Call if it continues or your cat seems distressed. | | Lethargy | Uncommon, usually brief | Let your cat rest. Call if it lasts more than a day or worsens. | | Loss of appetite | Uncommon | Call your veterinarian if your cat skips more than one meal. | | Tremors, wobbling, or stumbling | Rare but serious | Call your veterinarian right away or seek emergency care. | | Seizures | Rare but serious | Treat as an emergency. Call your veterinarian or an emergency clinic immediately. | | Severe allergic reaction (facial swelling, hives, trouble breathing) | Rare but serious | Seek emergency veterinary care immediately. | If your cat vomits shortly after taking the chewable, the dose may not have been absorbed. Do not give another dose without asking your veterinarian first. Some cats have underlying health conditions that make any medication riskier. Tell your veterinarian about all medications, supplements, and any history of seizures or neurologic problems before you start. ## Who Should Not Take It and Drug Interactions Milbemycin oxime is generally well tolerated, but it is not right for every cat. Your veterinarian may avoid it or adjust the plan if your cat: - Has a known allergy to milbemycin or related drugs - Has a history of seizures or neurologic disease - Is very young, underweight, or debilitated - Is pregnant or nursing (your veterinarian will weigh the risks and benefits) - Is already receiving other parasite preventives that contain macrocyclic lactones Drug interactions matter. Macrocyclic lactones include ivermectin, selamectin, abamectin, eprinomectin, milbemycin oxime, and moxidectin [2]. If your cat is already on a product containing one of these, adding another can increase the total exposure. Always tell your veterinarian everything your cat receives, including over-the-counter products and flea treatments. You can read about flea products in [Flea Prevention Dogs Cats](/knowledge/veterinary-medicine/parasitic-diseases/flea-prevention-dogs-cats). Some cats are also on other dewormers for intestinal parasites. If your cat has roundworms, your veterinarian may choose a product that covers both. More on that is available in [Roundworm Treatment for Cats: Safe Dewormers and Prevention](/knowledge/veterinary-medicine/parasitic-diseases/roundworm-medicine-for-cats). If your cat has kidney or liver disease, your veterinarian will decide whether Interceptor is appropriate and whether the dose needs adjustment. ## Overdose and When to Call a Veterinarian or Poison Control An overdose means your cat received more than the prescribed amount. This can happen if a chewable is dropped and eaten, if two doses are given by mistake, or if a cat gets into a package. Signs of overdose can include vomiting, diarrhea, drooling, lethargy, tremors, wobbling, or seizures. These signs can also occur with other problems, so do not wait to see how things go. If you suspect an overdose, call your veterinarian right away. If your regular clinic is closed, call an emergency veterinary hospital or a pet poison control hotline. Have the product packaging, your cat's weight, and the estimated amount swallowed ready. Do not induce vomiting unless a veterinarian tells you to. Some products can cause harm if vomited and inhaled. If your cat has a seizure, trouble breathing, or collapses, treat it as an emergency and go to the nearest emergency clinic. ## Limitations and When to Contact a Veterinarian Interceptor prevents heartworm infection, but it does not treat adult heartworms, and it does not protect against every parasite. It also does not replace mosquito control or other preventive measures. No preventive is 100 percent effective in every situation, and missed doses are a common reason for breakthrough infections [2]. Contact your veterinarian the same day if: - Your cat vomits right after taking the dose - You miss a dose or are unsure whether it was given - Your cat has diarrhea, drooling, or seems off for more than a day - You are unsure whether your cat is due for a heartworm test Seek emergency care now if: - Your cat has a seizure - Your cat has tremors, wobbling, or trouble standing - Your cat has facial swelling, hives, or trouble breathing - Your cat collapses or seems unresponsive Schedule a routine visit if: - Your cat is due for a refill or a heartworm test - You are switching from another preventive - Your cat has new coughing, breathing changes, or exercise intolerance Individual cases need a veterinarian. Your cat's age, weight, health history, and risk of exposure all affect the plan. ## Frequently Asked Questions ### Is Interceptor safe for cats? Milbemycin oxime is generally well tolerated in cats when given at the dose your veterinarian prescribes. Some cats have mild stomach upset or drooling, and serious reactions are rare. Tell your veterinarian about any health conditions or other medications before starting. ### Can indoor cats get heartworm? Yes. Both indoor and outdoor cats can be infected because mosquitoes come inside and bite cats indoors [1]. The critical factor is being bitten by a mosquito carrying the infectious larvae, not whether your cat goes outside. ### Does Interceptor kill adult heartworms? No. Interceptor works on the early larval stages of heartworm, not adult worms. If your cat already has adult heartworms, your veterinarian will discuss a different management plan. ### What if I miss a month of Interceptor? Call your veterinarian for guidance. The goal is usually to get back on schedule as soon as possible, but the advice depends on how late the dose is and your cat's risk. Do not double up unless your veterinarian tells you to. ### Do I need to test my cat before starting Interceptor? Your veterinarian may recommend a heartworm test before starting prevention. Testing helps confirm your cat is not already infected, which matters because preventives are not designed to treat adult worms. Testing in cats can be challenging, and results are interpreted along with your cat's history and signs [4]. ## Related Articles - [Heartworm Prevention for Dogs and Cats: What Owners Need To Know](/knowledge/veterinary-medicine/parasitic-diseases/heartworm-prevention-dogs-cats) - [Heartworm Prevention for Cats: Why It's Important and How It Works](/knowledge/veterinary-medicine/preventive-and-routine-care/heartworm-prevention-for-cats-why-its-important-and-how-it-works) - [Flea Prevention Dogs Cats](/knowledge/veterinary-medicine/parasitic-diseases/flea-prevention-dogs-cats) - [Heartworm Disease in Cats: Feline Diagnosis and Supportive Care](/knowledge/veterinary-medicine/parasite-control/heartworm-disease-in-cats-feline-diagnosis-and-supportive-care) - [Roundworm Treatment for Cats: Safe Dewormers and Prevention](/knowledge/veterinary-medicine/parasitic-diseases/roundworm-medicine-for-cats) - [What to Expect When Your Dog Needs a Heartworm Test Before Getting Prevention](/knowledge/veterinary-medicine/pet-medication-guide/dog-heartworm-test-before-prevention) - [Interceptor Dewormer for Cats: What It Treats and Safety](/knowledge/veterinary-medicine/pet-medication-guide/interceptor-dewormer-for-cats) - [Interceptor for Cats: Side Effects, Sizes, and Dosing](/knowledge/veterinary-medicine/pet-medication-guide/interceptor-cats-side-effects-dosing) - [ProHeart for Cats: Is the Heartworm Injection Safe?](/knowledge/veterinary-medicine/parasitic-diseases/proheart-cats-heartworm-injection) ## References 1. [Heartworm Disease in Cats - Cat Owners](https://www.merckvetmanual.com/cat-owners/heart-and-blood-vessel-disorders-of-cats/heartworm-disease-in-cats). Merck Veterinary Manual. 2. [Macrocyclic lactone resistance in Dirofilaria immitis: risks for prevention of heartworm disease](https://pubmed.ncbi.nlm.nih.gov/34717929/). International journal for parasitology, 2021. 3. [Heartworm in Cats](https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/heartworm-cats). Cornell University College of Veterinary Medicine. 4. [Feline heartworm (Dirofilaria immitis) infection in stray cats in Ulsan, Korea](https://pubmed.ncbi.nlm.nih.gov/41668261/). Parasites, hosts and diseases, 2026. ## Further Reading - [Heartworm Disease in Dogs, Cats, and Ferrets - Circulatory System (Merck Veterinary Manual)](https://www.merckvetmanual.com/circulatory-system/heartworm-disease/heartworm-disease-in-dogs-cats-and-ferrets) - [American Veterinary Medical Association: First Aid Tips for Pet Owners](https://www.avma.org/resources-tools/pet-owners/emergencycare/first-aid-tips-pet-owners) - [Merck Veterinary Manual: Cat Owners](https://www.merckvetmanual.com/cat-owners)