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Why is my dog still scratching after Trifexis for fleas?

Updated On September 23rd, 2025

Pet's info: Dog | Russell Terrier | Male | neutered | 1 year and 7 months old

My dog just started trifexis for his fleas. They said within 1 hour it should kill them. But he still continues to scratch a lot and it's been almost 3 days. Why could this be and how does trifexis actually work?

2 Answers

Most Helpful Answer

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Answered By Dr. Melanie, BVSc MS

Veterinarian

Published on December 15th, 2016

Trifexis works by killing fleas when they bite your dog, but they don't repel fleas. If Dallas is still scratching a lot, fleas could be getting on him from the home, or he could have developed a flea allergy from a previously heavy burden of fleas. It is important to disinfect your home when your pet has had fleas. Wash his bedding in hot soapy water, and vacuum the furniture and carpet surfaces. You can use a spray to kill fleas in the house, such as Indorex, or a flea bomb would work well to kill the fleas in the home. I would also take Dallas into the vet to discuss the continued scratching, as he may benefit from medications such as steroids if he has a flea allergy.

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Other Answers

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    Answered By Allison Newth, VMD

    Veterinarian

    Published on July 13th, 2017

    This is a good product for fleas and ticks, however, if they did have fleas before you applied the product there could be fleas in the environment that are still getting on them. Flea infestations require heavy cleaning (all pet bedding should be washed and dried on hot once a week for a month, the whole house should be vacuumed daily for 1 week and then weekly for three weeks, this includes vacuuming furniture and anywhere fleas could have jumped off your dog and laid eggs. You have to get those eggs out of the environment). Additionally, just like when humans get bug bites, the itch doesn't go away as soon as the bug flies off of us. Your dogs could be itching from flea bites they received days ago. There are also many other reasons dogs itch including food allergies, seasonal allergies, mites, and contact irritants such as harsh shampoos, just to name a few. Your veterinarian would be able to help you determine whether Baby and your other dog may have something other than fleas causing them to itch.

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