What Chemical Ingredients Are in Flea Treatments?

Greetings fellow felines! Skye Blake here, reporting in with the scoop on active chemical ingredients in flea treatments.

The more you know, the more comfortable you’ll be when choosing flea treatments for your favorite feline.

The information here is for general knowledge… always see your vet with questions about your cat’s individual needs.

The best way to keep you and your cat safe is KNOW WHAT YOU’RE USING and KNOW YOUR CAT!

Click These Trails…

Who Is Skye Blake?
Chemical Flea Treatment Ingredients
Chemical Ingredients List (in alphabetical order)
A-C
D
E
F
G-I
J-M
N
O-Pi
Pr-Py
R
- Resmethrin (Pyrethroid Family)
S
T-Z
Sources
List of Sources

Who Is Skye Blake?

Skye Blake, Cat Info Detective, is a curious cat researcher (not a veterinarian or scientist) who sniffs out expert, reliable sources about cats, studies their information, then passes it on to you!

The information here is for general knowledge… always see your vet with questions about your cat’s individual needs.

All sources are at the bottom of each page so you can do more snooping.

Chemical Flea Treatment Ingredients

drawing of lab test equipment; medicine, chemical ingredients

The chemical ingredients listed below are various forms of insecticides.

I have not yet been able to dig up any studies about the effects of these flea products over a cat’s lifetime.

Your goal is to find what kills fleas best while still being safe for your cat.

It’s always best to ask your vet, especially if your cat has a weakened immune system (kittens, sick and senior cats).

Chemical Ingredients List (in alphabetical order)

Scientist holding test tube - vet medicine; chemical ingredients

When you read the chemical information below, you’ll see cautions about toxicity with various ingredients.

Most are skin and other irritations from either too much (overdose) or improper application of the product (getting into mouth, eyes, nose, etc.).

There are a few notable ingredients that are very dangerous to us fabulous felines!

A-C

Bifenthrin (Pyrethroid Family)

Bifenthrin is a broad-spectrum insecticide that interferes with an insect’s nervous system, used for many different types of insects.

Bifenthrin can cause reactions in animals and people if touched, eaten or inhaled.

These possible reactions could include vomiting, diarrhea, lethargy, twitching, drooling, hyperactivity followed by incoordination.

Cats, in particular, cannot handle being around Bifenthrin. Don’t use it in anything in a house with cats! ¹ ²

C

Cyhalothrin (Pyrethroid Family)

Science chemicals in beakers - vet medicine

There’s also Lambda-cyhalothrin and Gamma-cyhalothrin. Gamma is a refined form of Lambda.

It’s a more efficient chemical ingredient than either Cyhalothrin or Lambda-cyhalothrin.

It disrupts the nervous system of insects.

Lambda-cyhalothrin is a restricted chemical pesticide, mixed with other pesticides, and used only by certified professionals.

It’s toxic if swallowed and can cause convulsions and coma. Cats are especially susceptible. ³⁴⁵⁶ ⁷⁸⁹

D

Di-n-propyl Isocinchomeronate (MGK Repellent 326)

This is a chemical ingredient that’s always mixed with other insect repellents to make them more effective at repelling fleas.

Use it on cats topically but not on cuts or open wounds.¹⁰¹¹¹²¹³

Dioctyl Sodium Sulfosuccinate

Goes by many different names. Since DSS is a form of the chemical “sodium” (salt), it pulls water out of critters, which helps kill them.

“It is used to kill and control not only fleas but also mites, ticks, lice and other pests.

It works by affecting the waxy out covering of a flea’s physical makeup, slowly softening it.

Eventually, the internal organs of the pest are also affected leading to the death of the insect.”¹⁴

DSS is also for constipation in humans (“Colace”) and animals.¹⁵ It has many other uses as a chemical ingredient in deodorants, air fresheners, etc.¹⁶¹⁷

E

Esfenvalerate (Pyrethroid Family)

It quickly kills insects and is one of the chemical ingredients in broad spectrum insecticide concentrates.

Esfenvalerate is popular because of its low toxicity to people and is safe around pets when used as directed.¹⁸ ¹⁹

Etofenprox (Pyrethroid Family)

A pyrethroid ether pesticide (rather than pyrethroid ester). Disrupts the membrane of nerve cells.

Low concentrations make insects hyperactive. High concentrations paralyze and kill them.

“Etofenprox seems to be less toxic for cats than other synthetic pyrethroids. BUT: cats are more suseptible to synthetic pyrethroids than dogs and do not support doses that are harmless for dogs.” ²⁰

Cats lack the liver enzyme glucuronidase.

This keeps them from breaking down synthetic pyrethroids, which remain in their body much longer than dogs and other mammals.²¹²²²³²⁴²⁵²⁶²⁷

F

Fipronil (Phenylpyrazole Family)

Scientist pouring liquid into a test tube - vet

Used for a wide variety of pests (ants, beetles, cockroaches, fleas, ticks, etc.)

It kills insects when they eat or touch it by affecting their central nervous system.

Fipronil is more toxic to insects than mammals, since it’s more likely to bind to their nerve endings.

Fipronil can cause some skin irritation in people or pets.

If eaten, it can cause sweating, nausea, vomiting, headache, stomach pain, dizziness, weakness and seizures.²⁸²⁹

Flumethrin (& Imidacloprid)

This chemical is in cat collars (Seresto® brand) and is effective for about 8 months.

Some cats won’t tolerate collars and might scratch the area, opening a wound where the chemicals can enter the bloodstream.

This might cause a reaction but there’s no evidence of side effects with normal use. ³⁰³¹³²

March 3, 2021: An article in USA Today claimed there’s an alarming number of deaths and injuries to dogs and cats caused by Seresto® flea collars.

Is it legitimate or alarmist? See “Cat Flea Collars” for more about this claim.

Fluralaner (Bravecto)

Apply Fluralaner (sold as Bravecto®) topically between the shoulder blades and neck so your cat can’t lick it off.

It goes throughout the body and enters the flea or tick when they bite the cat, which then dies within hours.

Fluralaner kills only adult fleas after they bite the host animal and is effective for 12 weeks in cats over 6 months of age.³³

Cats: “The most commonly reported adverse reactions include vomiting, itching, diarrhea, hair loss, decreased appetite, lethargy, and scabs/ulcerated lesions.

BRAVECTO is not effective against American dog ticks beyond 8 weeks of dosing.

For topical use only. Avoid oral ingestion.

The safety of BRAVECTO has not been established in breeding, pregnant and lactating cats.

Neurologic adverse reactions have been reported in cats receiving isoxazoline class drugs, even in cats without a history of neurologic disorders.

Use with caution in cats with a history of neurologic disorders.”³⁴³⁵

G-I

Imidacloprid (Neonicotinoid Class)

Scientist holding test tube - vet medicine

Neonicotinoids are synthetic insecticides similar to nicotine but safer and more effective against insects.³⁶

Imidacloprid disrupts the ability of nerves to send a normal signal so the whole nervous system stops working properly.

It is much more toxic to insects than mammals and birds because it binds better to the receptors of insects.

In humans and pets, Imidacloprid doesn’t absorb well through the skin but can cause some irritation.

If breathed or swallowed, it can cause dizziness, breathlessness, confusion, vomiting in people.

If swallowed by pets, it can cause vomiting, drooling, trouble walking, tremors, tiredness.³⁷³⁸https://webinars.veteducation.com.au/wp-content/images/Insecticide-Toxicity-in-Cats-Lecture-Notes.pdf[/mfn]³⁹

March 3, 2021: An article was published in USA Today claiming there’s an alarming number of deaths and injuries to dogs and cats caused by Seresto® flea collars.

Is it legitimate or alarmist? See “Cat Flea Collars” for more about this claim.

Indoxacarb

kitten standing on an open book-popular topics

An oxadiazine pesticide that kills flea larvae (among others) by blocking neuronal sodium channels.

Use it carefully since some insects become resistant when exposed.⁴⁰⁴¹⁴²⁴³⁴⁴⁴⁵

J-M

Lotilaner

Lab beakers - vet medicine; chemical ingredients

A newer isoxazoline used in chewable tablet form that can be fed to cats with or after their food.

It’s claimed these tablets have a vanilla-yeast flavor that cats like, so it’s a no-stress way to give an oral medication.

Data shows that lotilaner™ (sold as Credilio™ by Elanco™) is highly effective against both fleas and ticks.⁴⁶⁴⁷

Methoprene (Insect Growth Regulator)

Siamese, ginger, white tabby cats fighting

It comes in 2 different forms – S-Methoprene and R-Methoprene

S-Methoprene works by acting like an important hormone in insects and interferes with insect growth and development.

It’s used frequently since it affects insects that survive exposure to other pesticides.

Methoprene can cause mild irritation in skin, eyes and lungs.

abyssinian cats sitting together; chemical ingredients

“In one study with very high doses (10 g/kg), dogs… showed signs like vomiting, dilated pupils, changes in behavior, breathing, and body movements.

When researches cut the dose in half (5 g/kg), the dogs had no observable signs or symptoms.”⁴⁸⁴⁹⁵⁰

Keep in mind that cats cannot tolerate many things that dogs can, even in smaller doses.

Moxidectin

This is an anthelmintic drug used to kill or expel parasitic worms from the host body. It’s used in dogs, cats, horses, cattle and sheep.

Moxidectin kills many parasites by binding to certain nerve and muscle cells.

This disrupts neurotransmission which paralyzes and kills the parasite.

Anthelmintic drugs get rid of the worms without hurting the animal.

Moxidectin is sometimes used as an ingredient in combination with other flea and tick pesticides.

Must be used very carefully in dogs and cats since it can be deadly if used incorrectly.

Most of the information I found was for dogs. Moxidectin is also used in products for humans. ⁵¹⁵²

“Do not use in sick, debilitated, or underweight cats.”⁵³

N

Nitenpyram (Neonicotinoid Class)

Scientist holding flasks of liquid - vet medicine; chemical ingredients

Nitenpyram (sold under the name Capstar®) is an oral flea treatment that absorbs into the blood of the host and gets into the fleas when they bite.

It works quickly to kill adult cat fleas and is effective for about 72 hours.

Neonicotinoids are neurotoxins that act similarly to nicotine.

They block neural signals in the central nervous systems of insects.

Nitenpyram is considered safe for humans but should still be handled carefully and not ingested.⁵⁴ ⁵⁵

N-octyl bicycloheptene dicarboximide (MGK-264) (Synergist)

A man-made chemical that works with pesticides to increase their ability to kill insects.

Synergists are not pesticides themselves. MGK-264 is often mixed with pyrethrins or pyrethroid insecticides.

MGK-264 has the ability to stop some of the insect’s enzymes from working, thus giving the pesticide more time to work without being resisted.

It’s considered low to very low toxicity with no skin irritation. Some eye irritation occurred in rabbits exposed to it.

As with other products, adverse reactions depend on the dosage… the higher the dose, the more likely a reaction will occur.⁵⁶⁵⁷⁵⁸⁵⁹⁶⁰

O-Pi

Permethrin (Pyrethroid Family)

cat looking at dropper of CBD oil; chemical ingredients

A pyrethroid insecticide used for many things, including flea products for dogs.

It’s the most widely used mosquito killer in the U.S., because of “its low cost, high effectiveness, low incidence of pest resistance, and broad labeling”.⁶¹

“Health effects from permethrin will depend on how someone is exposed to it.

Dogs and cats that have permethrin on their skin may act strangely, and flick their paws, twitch their skin or ears, or roll on the ground.

Animals that have licked treated skin may drool a lot or smack their lips.

Cats that have been exposed by accident to products with high (45-65%) levels of permethrin may seem anxious and can’t walk normally.

They may also have muscle tremors and seizures and they may die from the exposure.” ⁶²⁶³⁶⁴⁶⁵⁶⁶

Phenothrin (Pyrethroid Family)

kitten looking out window; chemical ingredients

(Sumithrin, D-phenothrin) A pyrethroid that kills adult fleas and ticks by disrupting their nervous system. It’s also used to kill head lice on humans.

Phenothrin is often combined with Methoprene. Some studies in Europe have shown widespread resistance to Phenothrin.

Phenothrin is no longer used in cat products as it has been linked to hair loss, salivation, tremors, seizures, and death in cats and kittens.⁶⁷⁶⁸⁶⁹⁷⁰

Piperonyl Butoxide (PBO) (Synergist – Insect Growth Regulator)

White cat with one blue, one gold eye, licking paws

PBO is an organic compound that helps pesticides be more effective at killing bugs.

It’s often combined with pyrethrins or pyrethroids.

PBO stops some enzymes in the flea and gives the pesticides time to work.

It’s considered “low to very low toxicity if eaten, inhaled, or touched. PBO is slightly irritating if it gets in the eyes or on skin. It does not produce skin allergies.”⁷¹⁷²⁷³⁷⁴⁷⁵⁷⁶

Pr-Py

Prallethrin

Lab tech looking through microscope - vet medicine; chemical ingredients

This pyrethroid insecticide controls mosquitoes, ants, fleas, etc.

It’s an effective ingredient against fleas and isn’t very toxic to humans, however, it can cause skin and eye irritation and other problems at higher dosages or greater exposure.⁷⁷⁷⁸⁷⁹

Pyrethrins

A mixture of six chemicals found in some chrysanthemum flowers that are naturally toxic to insects.

They help control mosquitoes, fleas, flies, moths, ants, etc. Chrysanthemum cinerariifolium is currently the flower used to extract pyrethrins.

Pyrethrum powder is whole crushed mum flowers.

vet listening to cat's heart with stethoscope; medicine; chemical flea treatments

Pyrethrins are usually separated from the flowers.

Synthetic pyrethrins (pyrethroids) make chemicals longer lasting and more effective, better ingredients.

Pyrethrins are usually mixed with synergists (e.g, MGK-264) to increase their effectiveness.

Scientist holding leafy plant stem - vet medicine

Pyrethrins damage the nervous system of insects that touch or eat it, quickly paralyzing and killing them.

They’re low in toxicity to people and other mammals but can be irritating on the skin.

Pyrethrins are a popular insecticide for organic gardeners.⁸⁰⁸¹ ⁸²⁸³⁸⁴⁸⁵

You can make your own pyrethrin pesticide from daisies (Chrysanthemum cinerariifolium)… just be careful when attempting any homemade products.

Pyrethroid

Test tubes; veterinary medicine; chemical ingredients in flea treatments

These are synthetic insecticides that are similar to pyrethrins.

It’s important to note that resistance to pyrethroids by insects is becoming widespread.

“Not all pesticides in the same class are equally toxic, or equally effective the same pests. Nevertheless, most pyrethroid insecticides share the following characteristics:

ginger, orange tabby kitten sleeping with pink fuzzy toy

Low in toxicity to mammals and birds

High in toxicity to fish if applied directly to water

Require very low doses to kill insects (high arthropod toxicity)

Fast-acting

Especially effective against chewing insects, though many pyrethroid insecticides can be absorbed by the insect pest when it merely walks over the dry residue

Bind tightly to soil and organic matter (therefore not as effective in penetrating soil to kill underground pests)

Dissolve very poorly in water

Pyrethroids became popular as consumer insecticides in the 1990s as replacements for older pesticides, like diazinon and Dursban® that were phased out for environmental and human health reasons.

Some pyrethroid insecticides last a long time in the environment (days or weeks), especially when protected from sunlight.

Others, such as allethrin and resmethrin, break down within a few minutes to a few hours after application.” ⁸⁶ ⁸⁷ ⁸⁸

Pyri

Pyridine

Scientist holding a beaker-enzymes - vet; chemical ingredients

An organic compound with the chemical formula C₅ H₅ N.

Pyridine is a solvent and building block ingredient in many products, including insecticides.

Some products made from it have antiviral, anticancer, antimicrobial, antidiabetic, and antitubercular properties.⁸⁹

Too much pyridine is toxic and can cause dizziness, nausea, headaches, anorexia.⁹⁰⁹¹

Pyriproxyfen (Nylar)

(Synergist-Insect Growth Regulator) An ingredient that inhibits “the hatching of flea eggs and the development of flea larvae.”

It doesn’t kill the adult flea or any stages of ticks.

Pyriproxyfen has a reputation for being mildly toxicity to humans and mammals.

It doesn’t bother skin but can cause mild irritation if it gets in the eyes.

Moderate to large amounts can cause problems.⁹²⁹³⁹⁴

R

Resmethrin (Pyrethroid Family)

Lab beakers-representing enzyme cleaners - vet; chemical ingredients

Resmethrin has been off the market since December 31, 2015.

It’s an insecticide ingredient that controls mosquitos, midges and blackflies.

Resmethrin is a chemical highly toxic to fish.⁹⁵⁹⁶⁹⁷⁹⁸

S

Sarolaner (Isoxazoline Class)

Scientific equipment in lab - veterinary medicine; chemical ingredients

Sarolaner is a chemical in topical (not oral) flea treatments for cats.

It kills fleas and other insects by affecting the nervous system.⁹⁹ ¹⁰⁰¹⁰¹¹⁰²¹⁰³

Selamectin

Selamection is a topical parasiticide and anthelmintic (dewormer) for dogs and cats.

It’s sold under the names Revolution® and Stronghold®.

In cats it “prevents heartworms, fleas, ear mites, hookworms, and roundworms”. ¹⁰⁴

Selamectin kills fleas when they bite and suck the animal’s blood, and is safe for cats and dogs, with less than 1% having side effects.

2 White/orange, ginger cats rubbing heads; chemical ingredients in flea & tick products

These usually consist of temporary irritation or loss of hair at the application site.

Other symptoms would be from high volume of dying worms or other parasites in the animal’s body, not the selamectin itself.¹⁰⁵

Don’t use in sick, debilitated or underweight animals, or kittens under six weeks. ¹⁰⁶¹⁰⁷¹⁰⁸¹⁰⁹¹¹⁰¹¹¹

Spinetoram (Spinosyn Class)

tabby cat looking at camera; chemical ingredients in flea treatments

Spinosyns act completely differently than any other known chemical insecticide ingredients.

“Spinetoram was more effective against fleas than fipronil/(S)-methoprene on cats. Spinetoram treatment significantly reduced signs of flea allergy dermatitis in cats…[and] resulted in a low adverse incident rate in cats.”¹¹²¹¹³¹¹⁴¹¹⁵

Spinosad

2 tabby cats lounging on floor, one yawning

This is a selective, broad-spectrum insecticide.

“Spinosad is a natural substance made by a soil bacterium that can be toxic to insects… a mixture of two chemicals called spinosyn A and spinosyn D.”

It kills insects that eat or touch it by damaging their stomach and nervous system. ¹¹⁶

Spinosad is not very toxic to people and other mammals, and is useful in organic gardening.¹¹⁷¹¹⁸¹¹⁹¹²⁰¹²¹It can cause some irritation and redness if it gets on skin or in eyes.

T-Z

Tetrachlorvinphos (TCVP) (organophosphate)

lab tech working with petrie dish - vet medicine; chemical ingredients in flea products

An organophosphate insecticide for animals and their living areas. It kills fleas, ticks, flies, lice and larvae.

Organophosphates are chemical ingredients processed in a specific way differently from other insecticides.

Tetramethrin (Pyrethroid Family)

tick - black, brown; family of flea & tick chemical ingredients

A powerful broad-spectrum, first-generation synthetic insecticide.¹²²¹²³¹²⁴¹²⁵

Discover more about your cat’s health and flea treatments at “Cat Health“.

Sources

Sources used on this website are either primary or secondary.

Primary sources are always preferable and have the most reliable information because they’re original and directly referenced.

Scientific abstracts and data are good examples of primary sources.

Secondary sources are weaker because they usually consist of opinions or articles that give no sources of their own, although sometimes they refer to primary sources.

When I use secondary sources, most are those with some authority, such as veterinarian or cat behaviorist books and articles.

List of Sources

See here for the list of all sources used on this page…

Updated July 10, 2023