Side Effects of Using Imidacloprid on Cats for Fleas

By Erin Ringwald

scratching cat image by Lucid_Exposure from

Imidacloprid, the active ingredient in several topical flea treatments, works by killing fleas on your pet within 12 hours of application. In general, topical flea treatments are a safe method of killing and preventing fleas on your cat; however, like any chemical, your cat may develop side effects. Knowing the possible side effects will help you know when medical attention is necessary for your cat.

Skin Irritation

Side effects can range from minor to more serious. The most common one is irritation at the application site. Your cat may begin to scratch or chew at the location where you applied the topical treatment. Upon physical inspection you may notice the skin is red and irritated. According to, if your cat is scratching and chewing at the application site, she may spread the medication to other parts of her body, causing irritation in other locations as well.


Some cats may become listless after an application of a flea treatment containing imidacloprid. It may be difficult to determine this side effect in cats, since they often sleep most of the day anyway. However, if your cat is listless, you will notice that you have trouble waking your cat when sleeping. Depression can also develop along with the listlessness. This is also defined by long periods of sleep, but you may also notice when your cat is awake, he has no interest in things he usually enjoys. For example, if your cat normally enjoys watching birds out the window, he may choose to lie down away from the window and pay no attention to the birds.

Vomiting and Drooling

Digestive problems are easier to identify, since there is something you can physically observe. Your cat may begin vomiting or develop diarrhea. Vomiting may be frequent or occasional, as may diarrhea. These problems may be caused by the topical application of the medication or from oral ingestion. Your cat may have licked the application site, bringing the imidacloprid into his system. This can also lead to excessive drooling, according to, because of the chemical’s bitter taste.


Another serious side effect is seizures. Some cats will have a more extreme side effect to the topical application and begin to shake or have tremors. If the problem progresses, you may notice your cat is having a seizure. Often, a cat with a seizure will drop to the ground and his limbs will go stiff, while his entire body convulses. According to Pawprints and Purrs, your cat may also pass out, be unresponsive or lose control of bodily functions during a seizure.

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