Drinks that are free from sugar will not decay your tooth enamel at anything like the rate of sugary ones, especially when they are fizzy. Some sugar free alternatives, such as xylitol, can even be of benefit to teeth by helping to destroy the sort of oral bacteria that commonly feed on sugars in the mouth. However, just because a drink does not contain sugar, it does not mean it is good for your teeth. Acidic drinks may still cause harm, for example, and staining may still occur. That said, they tend to be better, generally speaking.