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How Does Flea Medicine Work for Cats? Vet Approved Facts & FAQ

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Last Updated on November 16, 2023 by Jordyn Alger

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	Dr. Lorna Whittemore Photo

Reviewed & Fact-Checked By

Dr. Lorna Whittemore


The information is current and up-to-date in accordance with the latest veterinarian research.

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Fleas are the most common parasites that cats suffer from. Even if you have an indoor cat, he can get fleas from other pets or rodents that may sneak their way into the house. Regardless of how he gets them, they need to be treated before they multiply out of control.

Plenty of feline flea medications are on the market, and it can be hard to tell exactly how they work. The instructions on the product may tell you how to use it safely but may still leave you in the dark about what the chemicals are doing. Part of the challenge to understanding flea medications is that there are several types that all work differently from each other. Not all are safe for cats, so it is vital to know the differences and which ones are safe.

Keep reading below if you want to learn more about how flea treatment products work.

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The Different Types of Flea Medication

Topical Treatments

Various topical flea medications tend to block  the conduction of channels in a flea’s nervous system to kill them. These topical treatments usually kill off adult fleas within 24 hours of the first application, preventing them from laying eggs and allowing a new generation of fleas to rise. Commonly seen active ingredients in topical treatments are Fipronil and Imidacloprid. Some topical treatments also help to reduce the environmental flea burden as they are shed into the cat’s environment.

Oral Treatments

There are many oral flea medicines with varying active ingredients. A more commonly used class of medications called the Isoxazolines block nerve channels (GABA and glutamate gated chloride channels) in the fleas leading to hyper-excitation and then death of the flea. They get to work quickly and can have a duration of action for between 1 and 3 months.

Collar Treatments

The safety and efficacy of flea collars varies widely, as with the other flea products. It is recommended you discuss flea treatment products with your veterinarian. Commonly found active ingredients in flea collars are Imidacloprid and Amitraz. Again these work by interfering with the flea’s nervous system.

cat wearing flea and tick collar
Image Credit: Anna Krivitskaya, Shutterstock

Environmental Flea Treatments

The trouble with fleas (and other pests) is that they are not limited to running around in your cat’s fur. If your cat has developed a flea infestation, there is a high chance that the fleas have spread to other areas in the house. In fact, that may be how your cat got fleas in the first place.

In that case, you will need to treat your cat and your entire home. Thankfully, there are some products available that can help you to do just that. You can find insecticides meant to kill fleas, or you can employ professional pest control companies to sweep through your home and eradicate any pests.

The environmental sprays will need to be used anywhere the fleas, eggs, or larvae may be dwelling. Think about your cat’s favorite spots—these are most likely to be hosting a family of fleas by now. This can include couches, beds, pillows, carpets, and more. You will want to take care and ensure that the underside of rugs and furniture is also being sprayed, as fleas can be sneaky.

Even with the insecticide spray, some fleas may persist. You will want to wash anything you can with hot water, such as your cat’s bedding. In some instances, you may even want to replace some items in your home. If you use a vacuum to clean up fleas, you should throw away the vacuum bag so that fleas do not thrive inside of it.

Where Is It Used?

Three of the most common flea medicines are used topically, orally, or as a collar.

Topical Medication

Topical flea medicines are applied to the skin and coat. These treatments are liquid, often oily, and are meant to be put on the skin. Typically, topical products are meant to be used on one single spot between your cat’s shoulders.


cleaning cat by shampoo on water bath
Image Credit: angnokever, Shutterstock

are another form of topical flea medication. These medicated shampoos are applied to your cat’s skin and coat during a bath, helping to kill off the pesky fleas that have made their home in your cat’s fur.


Oral Medication

Oral flea medicines are usually chewable pills that you can hide in your cat’s food to convince him to ingest it.

Collar Medication

Flea collars can be effective for cats, but only if the proper ones are used. It is extremely important to make sure that you are putting a feline flea collar on your cat rather than a canine flea collar since canine flea collars are very poisonous to your cat.

cat with green flea collar
Image Credit: Inga Gedrovicha, Shutterstock

Advantages of Feline Flea Medicines

Other than the benefit of getting rid of fleas, there are other advantages to feline flea medicines. For instance, flea medications come in several varieties and forms. As we’ve discussed, topical, oral, and flea collars can rid your cat of fleas, but additional treatments can be used to supplement those products. Some of these include medical shampoos and flea combs that remove fleas from your cat’s fur. Other products include flea wipes, sprays, and creams. So, if oral medications don’t work for you and your cat, you still have plenty of options.

Another great benefit to flea medication is that the signs of flea infestation are usually eradicated very quickly. Many topical flea treatments can kill almost all adult fleas within 24 hours of the first application. Some oral medications can start to kill fleas within 30 minutes of ingestion. That means that your cat’s scratching and irritation will dwindle rapidly after applying most flea medicines.

cat and flea comb
Image Credit: Simone Hogan, Shutterstock

Disadvantages of Feline Flea Medicines

Aside from catching and applying the flea medications, the main disadvantage regarding flea medicines is the risk of toxicity if misused. It is always important to read carefully and understand the instructions regarding any product for your cat, but it is especially the case with flea treatments. If misused for your cat’s weight, age, and other factors, your cat may suffer side effects from the medicine. You must never use dog flea products on cats.

Signs of flea medicine toxicity include tremors, drooling, diarrhea, and vomiting. Your cat may also experience lethargy and difficulty breathing. If your cat’s pupils are constricted, that could be another warning sign. In more severe instances, your cat may collapse.

However, this is not so much a disadvantage of the product as it is a sign of poor use. That is why it is vital to double-check any products you use on your cat to ensure that they are suitable for him; if you are ever in doubt, consult your veterinarian.

applying flea treatment to cat
Image Credit: Csaba Deli, Shutterstock

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Frequently Asked Questions (FAQs)

1. How Do I Know if the Flea Medication is Working?

The effects of flea medication may be seen as early as 24 hours, but they may occur even earlier. However, if the infestation is extensive, it may take much longer to eliminate the pests entirely. Since the flea’s life cycle is around 3 months, some cases of infestation may take months to improve.

As the treatment works, you will notice fewer fleas in your cat’s skin and fur. This number may go up and down for a while if a new generation of eggs hatched in the middle of your treatment. Dead fleas will start falling off your cat as the treatment works. The medication may not suit your situation if you do not notice a decrease in fleas. Talk to your vet to devise a new treatment plan, if necessary.

2. What Effect Can Fleas Have on My Cat?

Fleas are so tiny that it can be easy to think that they aren’t that big of a deal. However, left unchecked, they absolutely can be. If fleas repeatedly bite your cat, he may develop an allergy. This will cause your cat to over-groom or scratch himself excessively, which can lead to skin infections.

If you have a kitten or an older cat, fleas may even cause anemia. This is because adult fleas feast on your cat’s blood throughout their lifespan. Fleas are also hosts for tapeworms. If your cat grooms himself and ingests a flea by accident, a tapeworm may develop in his digestive system. Fleas are also the vector for other serious illnesses in cats such as feline infectious anemia caused by Haemoplama bacteria.

3. I’ve Tried Flea Medication—Why Does My Cat Still Have Fleas?

Flea medicine failure is most often due to improper use. If you have been applying flea medication but have not noticed any relief for your cat, double-check the instructions to verify that you are using the product correctly. If you are, consult your vet about your next plan of action. Do not increase the dosage without first talking to your vet.

Other reasons your cat may not be free from the fleas include an insufficient cleaning of the home or another exposure to fleas. If fleas have spread into your home, your cat may be re-exposed to fleas consistently enough to keep them stuck to him.

kitten wearing green flea collar
Image Credit: Fayzulin Serg, Shutterstock

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Fleas can be troublesome pests, but thankfully, there are plenty of effective flea medications that can make those troubles disappear. No matter what your cat needs, there will be a product for him, whether that’s a topical medicine, an oral treatment, or a flea collar. Be sure to double-check all flea product instructions before using them to ensure your cat’s health and safety.

Featured Image credit:, Shutterstock

About the Author

Jordyn Alger
Jordyn Alger
Cats are Jordyn’s passion. Whenever she has the opportunity she loves to learn about animals and share her knowledge with everyone. She is devoted to her sassy tabby cat named Jack and works hard to buy him his favorite treats and toys. Discovering new ways to better care for her cat is always on her to-do list, and she is eager to help pet parents everywhere. In Jordyn's spare time, she loves to write, read, and take long walks. She has a creative spirit, which often helps her to come up with funny cat names.

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