Hello, Thanks for writing to us Hydrogen peroxide has a whitening effect because it can pass easily into the tooth and break down these complex molecules. Less complex molecules that reflect less light lead to a reduction or elimination of the discoloration of both the enamel and the dentin. The length of treatment depends on the level of discoloration and on the whitening product used. It ranges from short interventions in the dental office to applications at home lasting minutes/hours per day and repeated over a number of days. Single dark teeth can also be bleached successfully. Heavy tobacco stains may need as much as three months of treatment. Whitened teeth generally do not darken after treatment or darken only slightly, and when they do, retreating the teeth can easily be done. Hope this helps you. Wishing you good health... Regards.