You’ve Dewormed Your Cat: A Vet Explains What Happens Next

Deworming has become an extremely common practice in veterinary medicine and is pivotal to the health and wellbeing of our pets. Gastrointestinal parasites can be prevalent amongst certain populations of cats including outdoor kitties, kittens from breeding colonies, those from shelters, and any cat not treated routinely with preventatives. Indoor cats may be at risk too! Not only are parasites gross for us to encounter, but they can also have significant impacts on the health of kittens, adult cats, and humans alike. These parasites can cause poor nutrition, diarrhea, vomiting, weight loss, anemia, and even death if left untreated. This is why it is extremely important to perform routine deworming, especially in kittens and high-risk adult cats.

Deworming is often prophylactic. This means that it is used to prevent disease before it is apparent. It can also be performed as a treatment after fecal testing is positive for parasites or worms are seen/suspected. The type of deworming agent used in your cat is guided either by fecal results, worms that are seen in the stool, or your veterinarian’s preference.

While deworming is a common and important practice, like all medications, it can come with certain side effects. We will discuss these below.

Most common side effects of deworming treatments in cats

While significant side effects are uncommon, these are the ones cats may experience within 24 hours of treatment:

  1. Vomiting
  2. Diarrhea
  3. Loss of appetite
  4. Drooling
  5. Irritation at the site of administration – if topical medication is used
  6. Worms present in vomit or stools
  7. Lethargy
  8. Irritability

Rare side effects include:

  1. Tremors
  2. Seizures
  3. Gastrointestinal obstructions from dead worms

How long do these side effects usually last?

After deworming, side effects are typically self-limiting and usually resolve within 24 hours. Diarrhea, drooling, and poor appetite are the most common side effects reported by owners. These side effects may persist for a day or two but usually do not require medical treatment.

If your pet vomits more than twice, has ongoing liquid diarrhea, blood in the stools, or has stopped eating, please contact a veterinarian right away. Any severe side effects like tremors or seizures need to be addressed immediately.

Does the type of dewormer (external or internal) affect side effects?

Different side effects may be seen depending on the route of administration of the medication. Oral products are more likely to cause gastrointestinal upset such as vomiting, diarrhea, or poor appetite. These side effects may be avoided if you give the deworming medication after a meal. If tummy upset occurs, make sure your cat stays well hydrated and continues to eat. You can feed small frequent meals of a wet diet, or even a bland diet like boiled chicken and rice.

Topical products may lead to hypersalivation, irritability, and skin irritation. Some cats can have reactions to topical skin products and lose hair or develop an allergic reaction. If a severe reaction is seen secondary to topical medication, consider bathing your cat in Dawn dish soap right away and have them seen by a veterinarian immediately.

View Cat-World’s guide to worming products here.

Is deworming 100% effective? Will my cat still have worms after the treatment?

No treatment for parasites is 100% effective. Proper administration is a must to increase the likelihood of success. It is also extremely important to consider repeated fecal testing as directed by your veterinarian to ensure that there is no ongoing presence of parasites. Some parasites can be resistant to treatment. Other treatments may only target adult parasites, so treatments must be repeated one to two times to break the worm lifecycle.

What does it mean if I notice worms in my cat’s feces after deworming? Is it normal for worms to come out after a treatment?

Seeing dead or dying worms in the feces after treatment is extremely normal! It may be alarming to owners to see a large number of worms in the feces, but this means that the treatment is working. Cats may shed dead worms for a few days to a week following treatment. The lack of worms in the feces does not mean that treatment isn’t working. The parasites may be too tiny to see with the naked eye.

How long after deworming until all the worms are gone? Will my cat need a second treatment?

Typically worms will begin to die within hours of administration of deworming medications. Each dewormer works differently to eliminate worms, so some work quicker than others. Some dewormers only target adult parasites and not larvae, so repeated deworming is necessary. Kittens require repeated deworming every 2-3 weeks until they are 2 months old.

To determine if worms are still a problem, consider repeated fecal testing to assure your pet is no longer positive for parasites. You can also go by clinical signs and consider repeated deworming if your pet is losing weight, has a pot belly, or parasites are seen in the feces.

If my cat still has live worms, how long should I wait in-between treatments?

Repeated treatment is determined by the type of parasite and dewormer administered. In kittens, dewormer is often repeated every 2-3 weeks. The same is true for treatments that target adult parasites and not the larva. Certain topical and oral treatments are repeated monthly. Your veterinarian will help you formulate a treatment schedule that is best for your kitty.


How often should I deworm my cat?

  • Kittens: Deworming every 2 weeks from 2 weeks until 2 months of age is recommended. After 2 months of age, broad-spectrum deworming agents should be used regularly.
  • Outdoor cats, shelter animals, or breeding cats: Deworming is recommended every 3 months or as guided by fecal testing results.
  • Indoor cats: Deworming may be recommended on an as-needed basis as determined by clinical signs or positive fecal testing. Fecal tests are recommended every 6 months to a year. Monthly prevention is still strongly encouraged.

When do you deworm a kitten?

  • Deworming should begin in kittens at 2 weeks of age and continue until at least 6 months of age.

Can you deworm a pregnant cat?

  • Yes! It is very important to prevent and treat parasites in pregnant and nursing animals. Some parasites are easily passed from mom to kitten, so prevention is key. Discuss products that are safe to use in pregnant or lactating cats as some are not safe for the kittens.

Should I feed my cat after deworming?

  • Ideally, you should feed your cat before deworming to try to eliminate the side effects of an upset stomach. If this is not an option, consider feeding your cat small frequent meals after deworming. If vomiting occurs, let your cat’s stomach settle for 4-6 hours before offering a meal. Consider feeding a bland diet like boiled chicken and rice for a few days to help support good digestion.

Can I bathe my cat after deworming?

  • When using oral medications, bathing is very safe and sometimes recommended to prevent reinfection from parasites passing in the stool. With topical medications, bathing is not recommended for at least 24-72 hours. Bathing prior to this time frame may wash away the active ingredients of the deworming medication and decrease efficacy. Chat with your veterinarian to determine how long you should wait.

Should I clean my house after deworming my cat?

  • Sanitizing the home is very important after your pet has been diagnosed with worms. Some worms pass eggs and larvae in the feces that can cause reinfection of your cat. These parasites can be transmitted to other pets in the home. Some parasites are zoonotic, meaning they can be transmitted from animals to people. Therefore, proper cleaning and hand hygiene are so important. If your pet has been diagnosed with parasites, always wash your hands after touching them. Be extra mindful about cleaning the litter box.
  • Areas your cat frequents in the home must be disinfected. This will kill the worms and eggs that live in the home. Make a cleaning solution with diluted bleach and use this to clean solid surfaces that your pet comes in contact with. Use this solution to clean litter boxes or any area your pet has had an accident. Steam clean your furniture and wash all bedding your cat has been in contact with.
  • It is important to continue this regimen during the course of deworming treatment in your pet. You do not want to risk them becoming reinfected.

Learn more about cleaning your house after deworming your cat here

Can indoor cats even get worms?

  • Even though outdoor kitties are at high risk, indoor pets can get worms too! This is most often caused by the transmission of tapeworms by fleas. Cats can also pick up parasites brought in by other animals or even through our natural movement in and out of the home. Therefore, monthly preventatives are so important.

Learn more about indoor cats getting worms here.


  • Dr Paula Simons, Veterinarian

    Dr Paula Simons graduated from the Ontario Veterinary College with a Doctorate in Veterinary Medicine (DVM) in 2019. She is currently working at 'Cornell University Veterinary Specialists' (CUVS) in Connecticut as an Emergency and Critical Care veterinarian resident (see her work profile). CUVS is a 24/7 Emergency and Critical Care Facility certified by the Veterinary Emergency & Critical Care Society, indicating the highest level of patient care.

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