6 Signs Your Flea Medication Isn’t Working

By PetMD Editorial on Jun. 13, 2017
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How to Tell if Your Pet’s Flea Preventive is Not Working

By John Gilpatrick


Fleas are extremely unwelcome intruders for pet and owner alike. When these pesky little biters make their way onto your pet and into your home, it takes some heavy duty stuff to get them gone.


But what happens if your pet’s flea medication proves fruitless? Or worse, what if a flea medication causes your pet to experience unfortunate side effects?


What are Flea Medications Made From?


There are more than a dozen “active” ingredients across the spectrum of flea medications. These are the ingredients that give each medicine its potency. Also in each respective formula are numerous “non-active” or “inert” ingredients, which are basically vehicles that help make the active ingredient more effective or palatable.


“The vast majority of these ingredients should be non-toxic to dogs and cats, but every once in a while an individual animal’s metabolism or skin will have a reaction to something that most animals don’t,” says Dr. Maria Verbrugge, a clinical instructor at the University of Wisconsin School of Veterinary Medicine.



This article was verified and edited for accuracy by Dr. Katie Grzyb, DVM

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Presence of Fleas

This sounds like a no-brainer – your flea medication isn’t working if there are still fleas around – but the reality is a little more complicated than that.


“By the time you see fleas on your pet, they have generally been laying thousands and thousands of eggs in your house,” Verbrugge said. She adds that it takes between six and eight weeks for those eggs to hatch, so using flea medication as prescribed doesn’t necessarily mean they’ll be gone at the end of the prescription. “It may be working great, but the last few fleas are still hatching.”


This means flea medication requires patience beyond what you might be prepared for or are expecting, says Verbrugge, but at a certain point, proper application of a flea medication should make the problem go away.


If it doesn’t, the fleas may have found another unwilling but vulnerable host in the house. “All animals in the house need to be treated,” Verbrugge says. “If you treat your dog and not your cat, the fleas will keep on breeding, keep on laying eggs, and you will continue to see them in the house.”

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Redness and Itching

Some negative reactions to flea medications are topical, meaning they’re limited specifically to the area on the skin where the medication is applied.


Of all the negative side effects related to a topical reaction to flea medication, redness and itching are perhaps the least serious, said Dr. Cherie Pucheu-Haston, an associate professor of veterinary dermatology at the Louisiana State University School of Veterinary Medicine. She says reactions like these may be treated at home by simply washing the applicate site to remove the topical product.

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Loss of Hair

Of all topical side effects, loss of hair is the most common, said Dr. Milivoj Milosevic, a veterinary dermatologist from Seattle and adjunct professor of dermatology at Washington State University. Milosevic adds that hair loss is particularly common in cats. Like redness of the skin, hair loss is not considered an emergency, but you and your vet should find a new medication for your afflicted pet.

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Biting Around the Base of the Tail

While this might suggest a topical problem, the tail base is, in fact, a hot spot for flea allergies. If an animal is being treated with something that’s not effective, the first place this might manifest is at the tail base, says Milosevic.


A lot of times, this occurs because a product isn’t everything it promises to be. “Many of the products that are labeled for use every 30 days begin to lose their efficacy within three weeks,” Milosevic says.


If you think the cause of your pet’s discomfort is your medication, consult with your veterinarian before selecting a new one. “Some patients may require a longer acting – three-month or eight-month – product,” said Milosevic. “Or in some cases, veterinarians may suggest multiple products for those highly sensitive animals.”

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Vomiting and Lethargy

You might think these signs are emblematic of a problem with an oral medication, however, Verbrugge says, these reactions are most common when an animal licks off the topical flea medication that’s been applied to the hair and skin. Other signs that your animal has licked off the topical medication are drooling and foaming at the mouth. Some medications can cause mild oral irritation or ulceration in the mouth if licked in large quantities.


Occasionally, these symptoms can also occur as the result of oral medications, said Milosevic. That’s because animal proteins—pork, beef, etc.—are sometimes used as non-active ingredients in order to make medications more appealing to the animal. Your dog or cat may experience a flare-up of symptoms if it has a food allergy to any of these proteins.

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Muscle Tremors and Seizures

By far the most reactions to flea medication occur when cats are exposed to certain flea medications that are meant for dogs, which might happen because a pet owner mistakenly buys the wrong product. Pucheu-Haston says she has also seen it in cases where cats rub against or lick off a topical medication that is on the skin or hair of a dog in the household.


The active ingredients that cause this reaction, which can be fatal in some cats, belong to a family called pyrethroids and include chemicals such as permethrin and cyphenothrin, Pucheu-Haston says.


“In general, pyrethroids should be used with extreme caution in households containing cats. If the pet owner is in any doubt, they should check with their veterinarian before using the product,” she says, adding that suspected ingestion should be treated as an emergency.