Why Is My Cat Acting Weird After Flea Treatment?

Cats generally don’t appreciate flea treatments, but they do generally tolerate them.

Some will react in a weird way and this is essentially because they have been poisoned.

One thing you have to remember is that flea treatments are chemical solutions to a flea infestation.

Fleas are extremely resilient creatures.

why is my cat acting wierd after flea treatment

They’ve been around since time began and so they are hard to kill off.

Because they are so hard to kill the chemicals used are very toxic.

Supposedly only toxic to the fleas, but that isn’t always the case.

Flea treatments come in all sorts of preparations, such as sprays, dips, shampoos, powders and collars.

Used correctly, they shouldn’t cause any harm to your cat.

If your cat reacts in a weird way, you need to understand why. 

Did you know: In some circumstances, fleas can kill cats!

What is “weird”?

Key symptoms of toxicity are hypersalivation, trembling, twitching, and seizures.

cat acting wierd after flea treatment

Sometimes these can happen shortly after application of the flea treatment, but the symptoms can also take some time to manifest as the level of toxins ingested build up.

Bad Reactions

These supposedly safe products can poison your cat, but it isn’t as straightforward as that.

As stated before, flea treatments have to be toxic to kill the parasites.

There are many brands on the market, and all have different strengths and chemical components.

Your cat may react badly to any one of those chemicals.

Just like humans, cats have their individual tolerances and sensitivities.

It is known that many cats react badly to a type of chemical called pyrethroids, which most flea treatments contain.


It is unlikely that your cat will be poisoned by absorbing these chemicals through the skin if you follow the application instructions correctly, but if swallowed and they reach the internal organs, they can make your cat very sick.

flea treatment spray

The effects can range from mild to severe, depending on the product and how much is taken internally.

Unfortunately, the toxins in flea treatments may affect the kidneys, liver, or even the brain of your cat.

No cat lover will deliberately give these treatments to their cats to ingest, but that is the most common cause of flea treatment poisoning.

Ingesting Flea Treatment Residue While Grooming

Your cat may well be the cause of its own sickness.

A cat’s instinctive need for cleanliness may be the culprit.

While grooming their fur, they may ingest enough of the flea treatment residue to harm themselves.

Even other cats that haven’t been given treatment may fall ill because they have licked the fur of one that has during play or mutual grooming.

That is why some flea treatments have been developed to apply to a spot on the back of the neck.

It is one place that cats cannot reach themselves.

To stop other cats from licking the medication, the treated cat should be isolated.


Don’t mix up dog and cat flea treatments.

If you have both dogs and cats make sure that you use the correct treatment on the correct animal.

cat getting flea treatment

This advice may sound obvious, but people can get distracted and make mistakes.

Dog flea treatments are a bit different and stronger than the ones used on cats and will probably contain a higher dosage of pyrethroids.

Long term use of chemical flea treatments can damage a cat’s immune system which will make them more prone to diseases and may well shorten their lifespans.


If your cat’s behaviour has changed and there are symptoms of sickness shortly after flea treatments you can try bathing it with soap and water.

Make sure your cat has completely dried afterwards and hopefully that will get rid of whatever treatment residue is left.

If your cat continues with signs of sickness or strange behaviour, then take it to a vet.

Whatever the cat is suffering from, it may well be something entirely unrelated to the flea treatment and coincidental, but it is better to be safe than sorry.

Concerns over flea treatments are valid worries and the recommended course of action is to consult a vet before you buy and apply one. 

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