Critics say the perceived benefits are a placebo.
Homeopathy is said to work, including using herbal medicine, to help the body heal itself.
But the Royal Veterinary College London reviewed major studies over three decades and concluded there were no robust studies which showed a benefit of homeopathy in animals.
Professor Peter Lees, who led the study, said his research had examined more than 50 trials or trial reviews, which had implications for humans.
He said homeopathy was “unethical and outrageous”, adding: “It is scientifically implausible that homeopathy has any effect. It cannot have any benefit and by giving homeopathy you may be withholding alternative drug based products which may have some benefit.
“Ultimately homeopathy is quackery and using it may prolong suffering and shorten lives.”
The study comes as a book No Way To Treat A Friend claims alternative medicine is a trick of the mind.
Arlo Guthrie, editor of VetSurgeon. org, added: “Animals can’t choose which treatment they take, nor can they experience placebo.”