Usually chihauhaus do fine if you give them enough Greenies to clean their plaque. It could be your dogfood. Greenies are much better than dentastix and combined with a good dry dogfood like Nature's Recipe may do better for you. Be sure not to use Human toothpaste as it can not only be poisonous to a dog but can also cause bad breath. And definitely avoid any treats containing propylene glycol because it's a toxic alcohol similar to antifreeze which, though it only gradually shortens the life of a large dog, often kills small dogs, and can also cause bad breath. You may also want to look at some books which give recipes for cooking your own dogfood. Several brand names conceal that they're just Diamond dogfood out of Gaston, SC which, under a variety of names, kills about twenty or more dogs in an average year.