The European Commission has looked into the safety and potential risks of using tooth whitening products. They found that toothpastes, mouth rinses and tooth whiteners containing up to 0.1% hydrogen peroxide can safely be used by consumers, but higher concentrations entail potential risks.

Hydrogen peroxide is a highly reactive chemical, and swallowing a large dose can be lethal. When highly diluted, however, it is non-irritant or mildly irritant. Hydrogen peroxide is used in cosmetics, mainly as bleach in dental products such as tooth whiteners and some hair products. Low concentrations may also be used in toothpastes and mouth rinses as a disinfectant to prevent plaque and inflammation of the gums.

People with certain genetic disorders are more vulnerable to hydrogen peroxide because their bodies cannot break it down effectively. The two most common side effects of using tooth whitening products containing hydrogen peroxide are mouth irritation and increased tooth sensitivity to temperature changes.

Although most studies agree that bleaching does not harm the enamel, some investigations report that it does harm the surface of the teeth and may affect the surface of fillings and other restorations. Also, hydrogen peroxide can contribute to the development of existing tumors.

See more info about the risks here.