A method and apparatus for preventing tooth decay by treating the tooth surface, including the occlusal surface and unexposed surfaces such as subgingival, interproximal, and contact areas, with a laser with a coherent or noncoherent light source are described. This process makes the tooth more resistant to acid and better able to bond fluoride, thus requiring a lower concentration of fluoride. The method allows the treatment to penetrate deeper into the tooth then previously accomplished with other methods and can be used in a dental office or at home.