Simply put, other processes do not work effectively. Teeth whitening systems found in mass retail markets do not use Carbamide Peroxide, the exact ingredient used in professional dental offices. Some over the counter teeth whitening trays are not moldable to your teeth and do not hold the teeth whitening gel in the proper position. Many over the counter teeth whitening gels are not thick enough to stay in place and will leak out of the teeth whitening tray. Most over the counter teeth whitening gels typically have less effective active ingredients or a lower concentration of active ingredients.