The safety level of hydrogen peroxide depends on the concentration level of the solution. Hydrogen peroxide in a pure form is explosive and in highly concentrated forms is dangerous to the environment and the human body, according to Wikipedia. However, low concentrations of hydrogen peroxide are safe to use.
Hydrogen peroxide is commonly used to clean small wounds and kill bacteria and as a mouthwash. It's also a key ingredient in tooth whitening products and toothpastes. In these applications, the concentration of hydrogen peroxide ranges from approximately 3 to 10 percent, levels at which it is safe for use, as HowStuffWorks explains. Consult a dentist before using straight hydrogen peroxide in the mouth.