Whether you choose to do the in house ZOOM or an at home system there is always upkeep afterwards. Teeth are porous and constanlty taking up stain. Some teeth are more porous than others. How much coffee, tea wine etc you drink can affect the need for more frequent treatments as well. Drink all staining liquids through a straw, use an electric toothbrush and go for your twice yearly dental cleaning to help upkeep your whitening. The ZOOM should not be done more than once every 8 to 12 mos and in between you can use some good over the counter products to touch up.