Hydrogen peroxide is safe for teeth and gums, and it is commonly used as a whitening agent in toothpaste or on its own, according to HowStuffWorks. People also use a combination of baking soda and hydrogen peroxide as a form of toothpaste.
People have been using hydrogen peroxide in the mouth for centuries, but its use is not without side effects, explains HowStuffWorks. High concentrations or prolonged use of hydrogen peroxide damages the tongue, gums or the roof of the mouth. If swallowed, the hydrogen peroxide also damages the esophagus. Because of these side effects, dentists recommend that patients only use 3 percent hydrogen peroxide and only for a short period of time.