Tooth whitening is not harmful when used as recommended, but a portion of the population will experience tooth or gum sensitivity. I experience cold sensitivity for about 48 hours after whitening. This is because the whitening solution (a peroxide) opens microscopic channels in the teeth. The peroxide can then penetrate these channels to break down any stain, but in doing so causes your tooth to feel temperature variations more intensely. These channels close with minerals from your saliva and the sensitivity decreases; because these channels are open you must avoid heavily staining foods (coffee, red wine, etc.) for 48 hours after whitening. Your dentist can guide you to an ideal product for your needs.