One of our cats developed what appeared to be a food allergy. We tried several different brands, but she would vomit shortly after eating any of them. After her trip to the vet's for her regular checkup, she was put on the z/d as a trial, and has had no problems since. Since we have four other cats that have access to each other's food, they are all getting the same. We will get the random hairball, now and again, but that's it.