Flea Treatment: Is It Poisonous to Your Cat?

Choosing a flea treatment can be scary!

Buckle your seatbelts guys and gals.   The topic of flea control is controversial and complicated. Since cats are especially sensitive to any treatment or medication, understanding how various treatments work is important.

Are flea treatment poisonous?

We share our love and our fleas!

There are horror stories out there in regards to people who have applied some form of treatment to their cat and had awful adverse reactions.  The topic is so stressful, many people avoid treating their cats year-round, or avoid treating their indoor kitties altogether.   Since fleas can hitch a ride on your shoes and clothes (or your dog!) to infect your indoor cats, this often leads to household flea infestations. So how do you tell what’s true and what’s bunk?

Commonly, there are several methods of applying flea treatment to your kitty:

  • Topical drops – This is the standard between-the-shoulderblades method for monthly prevention.
  • Shampoos – These are designed to bathe the fleas off as well as give monthly prevention (in some cases).
  • Dusts – Typically comprised of an insect killer, they go on dry and are brushed into the fur.
  • Sprays – The moist form of dusts, these come in flea treatments to spray directly on the cat, and formulas to spray onto your furniture (knowing the difference between the two is important).
  • Dips – Somewhere between a shampoo and a spray.  You completely soak your animal, but then you do not rinse it off.
  • Collars – Localized flea control that’s applied directly to the collar instead of directly on your cat.
  • Orally – Monthly preventatives given in pill form.

The main difference between these methods is how they are given to your cat.  In many cases, the active ingredients or pesticides used in these products are the same.

So, what are these active ingredients?

The active ingredients in any flea control product are the ingredients that work directly on the fleas.

Active ingredients

My active ingredients are fipronil and methoprene

The active ingredients of a flea product fall into two types:

  • Adulticides – This is a fancy name that just means it directly kills adult fleas.
    • Common adulticides:
      • Fipronil – Found in Frontline.  May cause some mild skin irritation if a cat is allergic or sensitive to fipronil, and may cause some mild tummy upset and drooling if swallowed.
      • Pyrethrins – These chemicals are a naturally produced from chrysanthemums. The adverse effects are similar to fipronil, but can cause more severe effects like tremors and seizures if your cat comes into contact with a highly concentrated form (such as a spray for the yard or a spray used on farms).
      • Pyrethroids – Chemicals that are synthetically produced from chrysanthemum plants.  The ingredient that is extremely toxic to cats, permethrin, is a member of this category. Other pyrethroids (Like Etofenprox) may cause a tingling sensation that causes your cat’s fur to twitch and odd behavioral changes if they are sensitive.
      • Imidacloprid – Found in Advantage.  The side-effects are extremely similar to fipronil.
  • Insect Growth Regulators (IGRs) – Another fancy name that means the chemical prevents the fleas from being able to reproduce.
    • Common IGRs: Methoprene (Precor), fenoxycarb, and pyriproxyfen (Nylar)
      • All of these directly affect the fleas, and have very little to no effect on your kitty.

Okay, so what about the inactive ingredients?

Since the chemicals aimed to kill and regulate a flea population aren’t very well-absorbed into the skin or through the stomach, the inactive ingredients in these products are simply carriers. These are your mild alcohols, oils, soap ingredients, water, dusts, etc. that help the flea control soak into the skin of your cat.  These also include the scents and fragrances, if there are any. On the whole, these are fairly safe ingredients.  Again, if your cat is sensitive or swallows some, it may cause some mild skin irritation or tummy upset.

Let’s talk about toxicity

Now that we’ve covered the basics, we can really dig into the four things you should never, ever put on your cat:

1. Permethrin

Permethrin is a chemical that is synthetically produced from the chrysanthemum plant.  It is a very common ingredient in flea and tick formulas for dogs, but is poisonous to cats.  Kitties are usually exposed to permethrin either through an owner applying a dog product on accident, a cat lounging around on the dog’s bedding after application,  or through a cat cuddling with or grooming the family dog.

This chemical is also frequently found in sprays and bombs used for furniture/carpets and the outside yard.  The simplest method of prevention is to check your labels, and if you have a cat, keep this chemical completely out of your home, even when applying something to your dog.

The common signs of a cat exposed to permethrin are severe:

  • Constant drooling
  • Stumbling
  • Twitching
  • Muscle tremors
  • Dilate pupils
  • Seizures
  • Hyperthermia (an increase in body temperature from all of the muscle movement)
  • Death

An owner’s first instinct upon discovering that their kitty came into contact with permethrin is to bathe the product off.  Never bathe a cat exposed to permethrin without the explicit permission of your vet. While we do want the product removed, permethrin causes tremors and seizures.  The stress and rapid temperature changes that happens while bathing a cat can prematurely trigger a seizures.


Don't make me take a bath!

Unless your vet feels safe directing you to bathe at home, always head into a vet clinic and let your vet bathe your cat.  They are better equipped to handle the effects of a permethrin poisoning.

2. Amitraz

Amitraz is a medication commonly found in dog tick collars. Many owners believe that collars are generally safe, and take less care to check the labels than they would with flea drops. Again, cats who cuddle with or groom a dog wearing an amitraz collar can expose themselves.

Signs of amitraz poisoning in a cat are just as severe:

  • Depression
  • Vomiting
  • Weakness
  • Stumbling
  • Unresponsiveness
  • Hypothermia ( a severe decrease in body temperature
  • Slow heart rate
  • Death

Like with permethrin, if you have a cat in the home, you should completely avoid using this product on your dogs.

3. Concentrated Essential Oils (The label will say 100% oil on it)

In response to the fear of causing a reaction in their cat, many owners have switched to trying natural solutions.  The problem is that some essential oils can be as toxic or even more so than the flea products.  Kitties lack the appropriate enzymes in their livers to metabolize essential oils (and many other things).  This makes them extremely sensitive to natural remedies. Since essential oils are another large topic of their own, I won’t go into too much detail.  All essential oils are potentially toxic (tea tree oil being the most well-known), so avoid putting them on your cat completely unless they are formulated or recommended by a vet.

4. Using any product that is not labeled for use on cats

This one should go without saying, but we’ve all been in that place while looking at the price tag on a cat flea control product and wishing we could find a cheap generic.  The dog product or furniture/yard product you already have at home starts to look appealing.

But the products labelled for use in other creatures do not always mention feline safety.  Chemicals that are toxic to cats are not always toxic to other creatures, so they may contain ingredients that will kill your kitty.  Paying the extra $15.00 will save you a lot more on vet bills later.

got fleas

One side-effect of fleas is that your cat will eat their own feet.

What about all those other side-effects?

I cannot count the times I have heard “but I read on the web that [insert any flea product name here] will kill my cat!” from nervous kitty owners. Let’s take a moment to delve into these claims and ease your mind.

Topical drops are generally very safe for a few reasons:

  • The concentration of the active ingredients is very low – The dose needed to kill tiny fleas on a 10 pound kitty is not all that much. It’s much weaker than the dose that is needed to kill your cat, so the vial doesn’t contain enough chemical to even be lethal. They’re mostly comprised of inactive ingredients.
  • The skin acts as a barrier to foreign chemicals – Your cat’s skin naturally prevents toxins from seeping into the body. While the carriers will help the chemicals reach the oil-secreting glands of the skin to help them in working properly, the amount that soaks into the bloodstream is so small as to be negligible.  Unlike essential oils, it’s just not going to reach your cat’s liver or kidneys.
  • Many products have been on the market for decades and are very well-tested – In the case of pyrethrins, we’ve been using them as pesticides for 100s of years (though not always on cats).  The side-effects of these are extremely well documented. Even the reported cases where cats have accidentally been given the topical drops in their mouths, they do not show life-threatening signs.


In the vast majority of cases where people report deaths in association with a particular flea treatment, the death of the cat can be traced down to one of two causes: another health issue that caused death around the time the owner applied the treatment, or the cat coming into contact with a dog that had used one of the toxic ingredients above.

Toxicity is directly related to dose.  The effects of any poison will depend solely on the amount of the poison that has been eaten/absorbed.  Flea drops like Frontline just don’t contain enough fipronil to kill your cat.

What mild side-effects can they cause?  Any application of any feline-approved flea treatment has the potential to cause the following:

  • Redness or irritation at the site of application (this can include loss of hair) – this is not a burn, as these chemicals are neither acidic nor alkaline.  These reactions occur in cats with sensitivities or allergies to the active or inactive ingredients. Cats that show redness after application of one product should switch to another.
  • Drooling – Cats groom, and they have an extremely sensitive nose.  Foaming at the mouth after application is an extremely common (and harmless) reaction to the taste or the smell of the flea treatment.  The drool will go away after flushing their mouth out with a tasty cat treat or some wet food.
  • Vomiting – This is also related to the taste and smell.  If they lick a little off, or they really don’t like the smell, it might cause a cat to vomit.
  • Fur Twitching – The pyrethroid chemicals cause a tingling sensation in sensitive cats that has been likened to pins and needles.  The sensation is purely annoying, but does not do damage to the cat. If a sensitive kitty is bothered by a tingly flea product, it can be bathed off with Dawn, and a drop of vitamin E oil can ease the tingling.
  • Behavior changes like rolling, rubbing, crying, and hiding – This is also related to the tingling sensation, and is treated the same way as above.

Never find me

I'm hiding.  He'll never find me.

With the exception of the toxic ingredients, topical flea treatments are actually very safe for your cat. If you’re still frightened about using a chemical on your kitty, talk to your vet about oral medications as an alternative.  Working with your veterinarian is the safest course of action you can take when trying to get rid of fleas. Before choosing a treatment, have them check your labels for you and verify everything there is safe to use in a home with cats.

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