i think most use the chihuahua formula because it is formulated specifically for chis. it has things in there that cater to the breed, for example, l-carnitine for their hearts and other things that help with common problems in the breed. my chi is still on puppy food, but in a few weeks i will switch her to royal canin. i think most people will agree it is a good food and a lot of people on here use it.