The side effects of methoprene on cats

Written by jennifer gittins
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The side effects of methoprene on cats
Methoprene can cause issues in some cats. (the cat image by Norbert Tuske from

According to the Environmental Protection Agency, or EPA, methoprene is classified as an IGR, or insect growth regulator. IGRs work by preventing the target insect from ever reaching maturity and, therefore, halt any potential reproduction. IGR is effective against several different insects, including, but not limited to, fleas, horn flies, mosquitoes and fire ants. When used specifically in insecticides for pets, it might go by the brand names of Biospot, Zodiac or Frontline. Like any medication, methoprene-based products can have several potential side effects in felines.

Fur Appearance

Following application, the cat's fur might take on a wet or oily appearance. This can last up to 24 hours following the application of the medication.

Temporary Irritation

Some cats can experience temporary irritation at the application site If the irritation is persistent, the owner should bring the cat into a veterinarian immediately, as this could indicate an allergic reaction.

Constant Itching

Mild to severe itching can occur in some cats following application of methoprene. Severe itching or itching that is persistent should be brought to the attention of a veterinarian. (Reference 4)


There might be redness at the application site―especially if the cat is itching at the area.


Itching at the application site can also cause inflammation for some cats.

Hair Loss

Methoprene products can cause hair loss at the application site, which could be temporary or permanent. Hair loss should be brought to the attention of a veterinarian as soon as possible.


Rashes could occur in some cats, especially around the site of application. Rashes should be reported to a veterinarian, as they could indicate a sensitivity to the product.


Drowsiness is also a potential side effect of methoprene-based products. Drowsiness that does not subside should be brought to the attention of a veterinarian.

Fur Discoloration

Though unusual, it is possible for the cat's fur to become discoloured from the product, especially around the site of application. Fur discolouration can be permanent, though will likely disappear thanks to shedding and hair regrowth.

Allergic Reactions

Certain products containing methoprene can cause allergic reactions. An allergic reaction will vary from cat to cat and may cause issues such as hives, a rash or swelling. Some cats could even suffer from anaphylactic shock, which requires emergency veterinary attention or the cat can die.

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