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Dog's fur black and smelly after flea medicine. What should I do?

Updated on September 23rd, 2025

Pet's info: Dog | Mixed Breed Large (61lb +) | Male | neutered | 1 year old | 66 lbs

hi, i just gave my dog his flea medicine (vectra-3D) and right after i put it on his fur started getting black on his neck, now theres a big black spot and it smells really weird. ive used that flea medicine for months now and nothing like this ever happened, i tried calling my vet but there closed until friday. It looks like the spot is getting bigger and im not even sure if its from the medicine or if he rubbed on something. should i give him a bath? what should i do?

1 Answer

Most Helpful Answer

Kevin Sanada, DVM

Answered By Kevin Sanada, DVM

Veterinarian

Published on November 22nd, 2017

It would be rare for this product to cause discoloration like this, but is not unheard of. Dogs can have local reactions to this product, which can cause irritation to the skin and discoloration of the hair.

As long as Cass is otherwise acting normal, this is not an emergency and can wait a day or two for treatment. If Cass is having tremors or seizures, then this would be an emergency and require an immediate visit to the vet.

Washing the area is a good idea. Use cool water and dog shampoo to wash it off. Rinse the area for at-least five minutes after washing.

I hope that helps and Cass gets better soon.
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