Urinary crystals have a number of causes but when they are caused by an irreversible metabolic disorder in the excretion of uric acid the problem needs to be treated by a long term change to a special diet royal canin urinary uc low purine is designed for dogs age 1 to help to maintain health and prevent relapse in dogs with metabolic disorders causing urinary stones the recipe includes proteins with a low purine content and also helps to prevent the build up of cysteine stones fluid loss