The most popular method of whitening teeth is the at-home, dentist-supervised option. During your visit to the dentist, he or she takes a mold of your teeth and creates a comfortable, customized mouthtray. The mouthtray holds a mild solution whose main ingredient is carbamide peroxide, a substance that reacts with water to release a nontoxic bleaching element (hydrogen peroxide). The mouthtray is placed inside the mouth and over the teeth. When the mouthtray is worn nightly or 30 minutes twice daily, the treatment usually lasts one or two weeks. Patients with more sensitive teeth may choose to limit the time that the whitening solution is on the teeth to one session per day. Used in this way the whitening treatment may take longer.