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My Cat Is Acting Weird After Flea Treatment

Josie F. Turner
By Josie F. Turner, Journalist specialized in Animal Welfare. Updated: November 2, 2023
My Cat Is Acting Weird After Flea Treatment

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Cats can act weird after flea treatment due to many reasons, including simple confusion at the application. There are some reasons why nervous, agitated or weird behavior can be a sign of a problem. The most significant is when it is a symptom of flea treatment poisoning in cats. Flea treatment is toxic if ingested internally, but this should not occur when administered correctly. It can be possible that certain cats may have an allergy to topical flea treatment applications. The extent of the reaction will depend on their level of sensitivity.

While deworming and flea treatment is relatively very safe, you may wonder why my cat is acting weird after flea treatment. AnimalWised looks into the possible reasons for this behavior, especially when it is one of the symptoms of flea treatment poisoning in cats.

You may also be interested in: How to Know if My Cat is Sick
  1. What is flea treatment for cats?
  2. Flea medicine poisoning in cats
  3. Symptoms of flea treatment poisoning in cats
  4. Treatment of flea medicine poisoning in cats
  5. Other reasons your cat acts weird after flea treatment

What is flea treatment for cats?

Even in indoor environments, parasites are a concern for your cat. If your cat is an outdoor cat, then the risk is even greater. An infestation can occur via many methods. If your cat ingests something they shouldn't, then eggs or larvae of parasites can pass into the gastrointestinal system. Interaction with feces or any close contact with an affected individual can be vectors for infestation. Some of the types of parasite known to infest cats include:

  • Hookworm
  • Roundworm
  • Lungworm
  • Tapeworms
  • Whipworm
  • Heartworm
  • Giardia
  • Coccidia
  • Ticks
  • Fleas

Many of these are gastrointestinal parasites in cats which make their way into the digestive system and leach nutrients. The results of an infestation vary with some causing serious discomfort and others being almost asymptomatic. Fleas are an external parasite and pass disease on to the feline host through biting. This occurs either by an adult flea jumping on to the coat of the cat from another animal or person, or by the larvae developing in the environment and attaching themselves to the cat.

Fleas cause their own physical problems in the form of itching, which leads to scratching and discomfort. This scratching can result in hair loss, painful sores and even infections when the wounds come in contact with bacteria. Fleas also can carry other diseases and even further parasites.

Fleas ‘play an important role as intermittent hosts in the development of at least three species of tapeworms’[1]. This occurs because flea larvae eat the proglottids of tapeworms and then carry with them during the pupal stage. These are then ingested by cats and the tapeworm can complete its cycle. For example, a cat can get worms from eating adult flies infested with tapeworms.

There are many other possible diseases which can be caused by flea infestation. A report from 2017 even claims that “the prevalence of flea-borne diseases has been greatly underestimated by health practitioners and agencies”[1]. The same report reveals some of these diseases which include:

  • Rickettesial disease (caused by the bacteria Rickettesia)
  • Bartonellosis
  • Plague
  • Tapeworms
  • Flea allergic dermatitis
  • Feline leukemia virus (FLV)
  • Calicivirus

Since these infestations can lead to potentially life threatening illnesses, prevention is incredibly important. This is where deworming and anti-flea treatment comes in. It works by applying the serum onto the nape of the cat's neck. Most applicators will come in a prepackaged pipette which gives your cat enough protection for a certain amount of time, usually around 3 to 6 months.

The hair is parted at the back of the cats neck and the contents are squeezed out. The area is then gently massaged into this part of the cat. The treatment then spreads across the cat's fur and prevents infestation. While cats are known for their lengthy grooming sessions, the cat should not be able to lick the treatment in any amount which is going to cause particular problems. If the pipette has been misapplied, then this nay not be the case.

Flea medicine poisoning in cats

The contents of the flea treatment pipette are adsorbed into the skin. This means the flea medicine spread out over the cat's coat to provide full-body protection. If we see our cat acting weird after flea treatment, it could be due to intoxication. Intoxication can occur when the components are too strong for the individual cat's system. Perhaps they have a weakened immune system or are an otherwise vulnerable cat.

Flea medicine poisoning can cause the cat to act weird after treatment. It often occurs when the product is too strong for its intended usage. This could be when a cheap product is used or when an adult flea medicine is used on a kitten. Different products are marketed in different regions, so it is important to ask your veterinarian for advice on which cat flea treatment you should use.

Cat flea treatment poisoning can also occur when it is used on another animal in the house and your cat licks the treatment. Some anti-flea treatments for dogs come in aerosol form, so you need to ensure you don't intoxicate the cat when using it in the same environment. A cat could even lick the used pipette if we are no careful after application.

It is important to emphasize that the deworming treatment should be discussed with the vet first. They will assess the cat by giving them an examination and recording their vital statistics. If there are any factors which would make a certain type of anti-flea treatment unsuitable for an individual cat, the vet will be able to determine this. We should note that deworming should be administered alongside a concurrent vaccination schedule for cats.

Some of the most common flea treatment medication brands include the following:

  • Advantage
  • Frontline
  • Seresto (by Bayer)
  • Revolution (also known as Stronghold)
  • Cheristin
  • PetArmor
  • Sentry Fiproguard
  • Hartz UltraGuard
  • ZoGuard Plus
  • Bio Spot Active Care
My Cat Is Acting Weird After Flea Treatment - Flea medicine poisoning in cats

Symptoms of flea treatment poisoning in cats

If your cat is acting weird after flea treatment application, you will need to consider the possibility of flea treatment poisoning. It is possible the cat will have ingested a small amount and will react disgustedly. However, if they have ingested enough it can pose a serious risk to their health. For this reason, it is important to look out for possible symptoms of flea treatment poisoning in cats:

  • Hypersensitivity to light
  • Hypersensitivity to noise
  • Weakness in hind legs
  • Constant ear tremors
  • Nerve contractions
  • Mandible crepitus (grating jaw)
  • Lack of muscle coordination
  • Seizures in severe cases
  • Hypersalivation
  • Dilated pupils
  • Tics
  • Temperature increase
  • Loss of appetite
  • Apathy and discouragement
  • Nervous spasms

One of the most common symptoms of flea medicine poisoning is when the cat is not eating after flea treatment. If it occurs for a short period after application, it won't be a problem. If it persists for more than 24 hours, you will need to take the cat to a veterinarian.

Treatment of flea medicine poisoning in cats

The adsorption of these anti-flea products can cause poisoning in the cat, even in relatively small amounts. These poisonings can be very mild or severe, the latter being potentially fatal if not treated in time. The most common products leading to flea-treatment poisoning in cats are fipronil and pyrethin. These are the group terms for different types of insecticides and pesticides. The exact amounts of pyrethin and fipronil in a product will depend on the brand.

Different brands will have different active ingredients. For example, fipronil can be found in flea medicines such as Frontline Plus. However, the only active ingredient in Adavntage flea medicine is imidacloprid. Your cat may act weird after a certain flea treatment if they have a sensitivity to it, but be perfectly fine with another type.

When you discover poisoning, you will need to go to the vet. They will make an assessment and will determine if hospitalization is required. You will need to bring the product with you, especially if you have used an oral flea medication for cats. They will be able to provide the right antidote which will likely be applied via intravenous fluids.

The veterinarian will wash the cat to remove any remains of the treatment if applied externally. This will help to prevent seizures. Once this has occurred, the vet will apply the treatment necessary for treating subsequent symptoms. These symptoms may alleviate themselves on their own, but there is always a risk it will have affected the digestive system. This includes vital organs such as liver, kidney and brain. The vet may carry out tests to see if these organs have lost any functionality and treat accordingly.

In serious cases, the intoxication can drastically harm the functionality of vital organs. Find out more with our article on life expectancy for cats with kidney failure.

My Cat Is Acting Weird After Flea Treatment - Treatment of flea medicine poisoning in cats

Other reasons your cat acts weird after flea treatment

While poisoning is the most worrying reason your cat might act weird after flea treatment, it is not the only one. If your flea already has a flea infestation, then they might be acting irritable or annoyed because of the fleas. It might take a little time for the treatment to become effective.

Another reason your cat might be acting weird is that you have had to take them to the veterinarian. Even the most gentle vet might have to restrain a cat which is already nervous due to being taken out of their home. This anxiety might not stop after they get home. They will show signs of irritation which may include vocalizations and even slight aggression. However, these behaviors should stop eventually once they have calmed down.

This is why it is very important to know the symptoms of flea treatment poisoning in cats. You will need to determine the difference between being annoyed and toxic shock from poisoning.

This article is purely informative. AnimalWised does not have the authority to prescribe any veterinary treatment or create a diagnosis. We invite you to take your pet to the veterinarian if they are suffering from any condition or pain.

If you want to read similar articles to My Cat Is Acting Weird After Flea Treatment, we recommend you visit our First aid category.


1. Dobler, G., & Pfeffer, M. (2011). Fleas as parasites of the family Canidae. Parasites & vectors, 4, 139.

2. Rust M. K. (2017). The Biology and Ecology of Cat Fleas and Advancements in Their Pest Management: A Review. Insects, 8(4), 118.

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Bonita, I’m sorry I didn’t mention that your description of your cats behavior matches that exactly of FHS. We also call them “fits”. Whatever is going on, you can only get help from a veterinarian for symptoms as serious as these!❤️
Bonita Pope Alford
I used revolution on my cat, about a week later he started having these fits it horrible. His eyes dilate his ears go back his tail starts whipping then he will scream and take off. He done this for 24 hrs straight. Then off and on ever since he tries to run from his tail and you can tell he is in pain. I neednto know if any other cats have done this . How long it last and so on. I can be holding him when it happens and he will bite me claw me jump down run from himself. When it is over he will come back and try to make up to me but you can tell when it going to happen again you see his skin crawl. I really hope someone can shed some light on this for me and him. I am all clawed and bit up and that's just like him. I don't know how to help him . When he is like that you can't catch him and you dare not trust him. Please help get to the bottom of this. This was his 1st treatment he is a little over a year old.
Thank you
Both of my cats had the same thing. You are describing an illness called feline hypersensitivity syndrome (FHS). The treatment is a. medication called Gabapentin. It can be administered on the inner skin of the ear flap, if your cat doesn’t like pills or liquids. Your cat will need a prescription from your veterinarian. I get ours from a pet pharmacy called Wedgewood Pharmacy, who ships directly to you. Take care, Kim
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My Cat Is Acting Weird After Flea Treatment