The bleaching gel is usually hydrogen or carbamide peroxide that breaks down into oxygen molecules. These molecules go into the tiny pores of the enamel and dentin, and break up the stains that block out the light. As the stains are broken into smaller and smaller pieces, more light passes through the teeth and makes them look lighter. Stains that are accumulated over time that are yellow to slightly brown produce the best results. Teeth that are dark brown to blue-gray are the most difficult to bleach because the stain is deeply embedded into the tooth’s structure. These types of stains are usually caused by medications taken during the development of the teeth such as tetracycline, and can take a minimum of three weeks to see results, and as much as six months for complete results. Depending on the bleaching gel used and the length of time it is worn, results can be seen within an hour, overnight or within three to four days. Complete results can be achieved in as little as a week or as much as several months depending on the source of the stain and the ability of the teeth or tooth to be bleached. The whitening procedure may last for up to three years -- even longer in some cases, depending on personal habits. To brighten the color back up, a simple process of bleaching for one or two nights with a single syringe is usually sufficient.