Health Parasites

How Flea Preventatives Work in Cats

Learn how spot-on, collars, and oral flea preventatives work in cats.

Flea preventatives are keeping those pesky little blood suckers away, but do you know how they really work? Are they safe? Let’s take a practical and easy-to-understand look at flea preventatives.

Are Flea Preventatives Safe for Your Cat?

When used properly, yes, flea preventatives are usually safe for your cat (remember everything carries a risk). Each flea preventative contains ingredients dosed at the proper amount for the animal type (dog, cat, etc.), size, and application (topical, oral, etc.) This keeps the animal’s exposure to a minimum, but is enough to kill the fleas. We’ll get into how they work shortly.

Grey and white cat outside
Many cats like Beaker love to be outside. Make sure to treat cats that go outdoors with flea preventative.

A very simplistic way to view this is think about how we use certain products in small doses, but if inhaled or ingested in a large quantity, we would get sick. You put lotion on your body, but you wouldn’t want to eat it, but if you got a little in your mouth, you’d probably be okay.

Also, insect bodies work differently from mammalian bodies, so chemicals that affect them don’t always have the same effect on us.

Keep Your Cat Safe: Follow Directions

Three cats snuggling on a bed
Fleas can carry parasites, making flea preventative a crucial part of managing your cat’s health.

According to an EPA study, the biggest cause for a negative reaction to flea preventative in cats is when the owner doesn’t follow the directions. Make sure you always:

  • Talk to your vet about a safe and effective flea preventative.
  • Select a flea preventative labeled for use in cats. Some preventatives are only for use in dogs.
  • Choose a flea preventative that is the correct dose for the age and weight of your cat.
  • Follow the instructions when applying or administering flea preventative. If topical, administer between shoulder blades on back of neck where your cat can’t lick it.
  • Follow the directions for reapplication (topical is usually 30 days; flea collars are a few months, etc.).
  • Keep in mind any living thing can have a reaction to a medication, vaccination, supplement (I got sick from a Vitamin C supplement once), or even food. If your cat gets sick, reach out to your vet and discuss if your cat needs immediate medical attention or if the symptoms will pass on their own. Common side effects for flea preventatives include GI upset and diarrhea.

Avoid a Flea Preventative That’s Only Labeled for Use in Dogs

Chrysanthemum flowers contain the chemical pyrethrin which is a natural insecticide and repellent. Pyrethrin is an active ingredient in some dog flea preventatives. Scientists have created a more stable synthetic version of pyrethrin, called pyrethroid. Most pyrethroids are toxic to cats (and fish), but are found in some d0g flea preventatives.

How can it be safe for dogs but toxic for cats and fish? The same way avocados are good for humans but toxic for African grey parrots. We are all different.

Topical or Spot-on Flea Preventative

Grey cat
Topical or spot-on flea preventatives work by mixing with the natural oils on your cat’s body.

Topic or spot-on flea preventatives are probably the most popular. Simply apply the liquid to the back of the cat’s neck. The chemicals absorb into the cat’s skin and enter sebaceous glands. In laywoman’s terms, sebaceous glands produce sebum, a group of oils that lubricate the skin and coat. The active ingredient in the flea preventative is released in the oils and ends up on your cat’s skin and coat. Most of the active ingredients work by attacking the insect’s nervous system or blocking nerve receptors, causing paralysis and death. Let’s take a look at the active ingredients in popular spot-on flea preventatives.

Imidacloprid, active ingredient in Advantage II

Imidacloprid is designed to imitate nicotine. It’s much more toxic to insects than mammals because it binds to the receptors of insect cells. If ingested by a mammal, it’s broken down by the liver and excreted in feces.

Fipronil & S-Methoprene, active ingredients in Frontline Plus

Fipronil collects in the cat’s hair follicles. Although not made to work in the bloodstream, a very small amount ends up there, and is passed in the cat’s feces. Fipronil affects the brain and spinal cord of insects, causing them to be hyperactive until they die. S-methoprene is an insect growth regulator, and prevents baby fleas from growing into adult fleas. It also prevents the eggs from hatching.

Selamectin, active ingredient in Revolution Plus

Used to kill many parasites (including fleas and all types of worms, even heartworms), selamectin cannot cross the blood-brain barrier in mammals, which is why it’s safe to use on cats. It does enter the cat’s bloodstream, which is how it’s able to get rid of worms, but also is present on the skin, controlling fleas and ticks. In parasites, the selamectin mimics the amino acid glutamate. The chloride ions flow unchecked into the parasite’s nerve cells, causing paralysis and death.

Flea Collar

Tabby cat with green eyes
Fleas are very hard to spot on tabby cats.

Flea collars are unique in that they typically last more than six months. Some of them are just repellants, killing fleas before they bite. Other collars are repellants and kill fleas when they bite too. Similar to topical preventatives, collars have active ingredients that are absorbed into the glands and then released.

Some flea collars include imidacloprid, as mentioned above, as well as flumethrin. Although it’s in the pyrethroid family, cats tolerate flumethrin fairly well in small doses.

Also, there are concerns and controversy around certain flea collars and their ingredients, which is why I’m not listing brands here. Be very careful when handling any flea collar containing tetrachlorvinphos (TCVP), because of rising concerns around its safety to humans. If you want to use a flea collar, ask your vet for safe recommendations.

Beware of collars with “natural ingredients,” as they are not always effective, and can also cause some skin irritation.

Oral Flea Meds

There are a handful of oral flea preventatives for cats, but they work differently. Some of them just kill life fleas, while others provide prevention.

Spinosad, active ingredient in Comfortis

Spinosad was discovered in a bacteria. It moves from the cat’s stomach, to the bloodstream. Once a flea bites the cat, it’s muscles will flex uncontrollably until it dies.

Nitenpyram, active ingredient in Capstar

Nitenpyram is another chemical that mimics nicotine. It goes from the cat’s stomach to their bloodstream, starts working in less than 30 minutes and remains effective for about 24 hours. Feeding adult fleas ingest the nitenpyram, which attacks their nervous system. Baby fleas and eggs don’t bite your cat, so it will not kill them. Nitenpyram is a great and fast-acting first step to tackling fleas, but you’ll likely need to take other measures. Capstar followed by a spot-on can be a great way to kill all the buggers.

Follow Directions

This was just a very basic overview of flea preventatives. Remember to always follow directions and talk with your vet. This will lower the risk of any complications.


Coates, Jennifer. PetMD. How to Choose the Safest Flea Treatment for Your Cat.

Entirely Pets. Capstar Flea Treatment Side Effects.

EPA Evaluation of Pet Spot-on Products: Analysis and Plans for Reducing Harmful Effects.

Kvamme, Jennifer. PetMD. How Do Common Flea Medications Work?

Leonardi, Lauren. PetCareRx. How Do Flea Collars Work?

Meggitt, Jane. Does Topical Flea Treatment Go Into the Bloodstream?

NPIC, National Pesticide Information Center – This site was not secure so I’m not linking to it, but I referenced fact sheets on imidacloprid, permethrin, and spinosad.

Pet Shed. What Is S-Methoprene?

Pucheu-Haston, Cherie. Today’s Veterinary Practice. The Flea-Infested Pet: Overview of Current Products.

Ruben, Dawn. PetPlace. Fipronil (Frontline) for Dogs and Cats.

By LizsKittyBootCamp

Hi, I'm Liz, and I'm a cat behaviorist who provides advice and insights on cat behavior.

3 replies on “How Flea Preventatives Work in Cats”

Thank you for this useful information. Would you please consider doing a follow-up story with more information. For example, can you give a Capstar and apply Frontline at the same time? Also, is it best to rotate between the different types of treatments (ie, will the fleas become resistant to Frontline if it’s used several consecutive months)? And, is it safe to apply a topical worm treatment and a topical flea treatment at the same time or should you wait a period of time between those treatments?

Thank you Cathy. Glad you enjoyed it. I’m not a vet but I am happy to research and provide more info

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