Cranberries, and particularly cranberry juice, have been used for decades as a means of preventing or treating urinary tract infections in people. The mechanism of action is unsure. One theory is that molecules in the juice may make it harder for bacteria such as E. coli to stick to surfaces, and therefore make it difficult for an infection to build up. This is likely the case in animals too, however, the amount they would have to drink would be impractical and if there is a known infection, antibiotics are a much better treatment. If you think your puppy has a urinary tract infection, see your veterinarian because these infections are often very uncomfortable and can ascend to become kidney infections.