Diet sodas and other sugar-free drinks are usually highly acidic, which weakens the enamel on your teeth and makes them more susceptible to cavities and dental erosion. The level of phosphoric acid, citric acid, and/or tartaric acid is usually high in sugar-free drinks so it's best to avoid them
When you drink soda, the sugars it contains interact with bacteria in your mouth to form acid. This acid attacks your teeth. Both regular and sugar-free sodas also contain their own acids, and these attack the teeth too. With each swig of soda, you're starting a damaging reaction that lasts for about 20 minutes.