This is so because in-office bleaching methods have more bleaching concentration than that which is offered over the counter thereby producing faster results. In-office tooth bleaching is also the safest. Unlike tooth whitening done at home, in-office whitening is conducted and monitored by a professional practitioner. While hydrogen peroxide acts faster, carbamide peroxide when left in the mouth for a time divides into urea and hydrogen peroxide that acts on the stains by the oxidation process. It gets into the spaces of the rods in the enamel of the tooth, lifts the stains and drains the stain out. Several repetitions of this and the teeth become as white as its original color. When using hydrogen peroxide for example, the concentration of the mix that is recommended at the back of the bottle should be adhered strictly if not less than what is recommended just to test it out. While studies have shown (Kugel et al) that the effect of peroxides at decreasing the tooth enamel is very insignificant, it nonetheless tends to make the teeth more sensitive for some time and when the proportion of the mix is on the strong side, the mix could result to painful gums and cause irritation to the surrounding tissues inside the mouth. Because of this, the market is being flooded with different brands every time and because it is one of those procedures that are fast and effective without the pain (mostly), it is so very easy to ride the bandwagon; yet, side effects could happen even adversely if unbeknown to the customer, the products are not used as indicated. Extrinsic staining and intrinsic staining are the two basic types at which the teeth develop stains and discoloration. Extrinsic stains are developed in the surface of the teeth that resulted from the accumulation of food debris and color. While superficial stains and discoloration could be removed by prophylactic cleaning and brushing, there are stains that are a little more stubborn and could accumulate between the spaces of the tiny cracks in the tooth that would need bleaching to remove the stains. Since teeth whitening and bleaching are among the simplest cosmetic procedures and provided you do not have cracks crowns porcelain veneers, special teeth and gum conditions, home kits are advisable provided you start with a low mixture of bleaching agents and water. Peroxides that are sold over the counter have indications and recommendations for the mixture which you could use.