Whitening formulas work thanks to hydrogen peroxide—it’s oxygen molecules hit the stains on your teeth and breaks them up. Then you wipe them off. It can be irritating to some people, so be careful.With in-office whitening at your dentist, a high-intensity light’s used to activate the oxygen. Over-the-counter methods require longer period of contact time to deliver the oxygen. This can take half an hour for 10 days to two weeks.QUIZ: Face SatisfactionHere are some various methods of whitening.In-office whitening: Makes teeth up to ten shades lighter, with the fastest and strongest effects. This costs $500 to $1,200.Brush-ons: Best over-the-counter choice. You can get up to four shades lighter. You can reach back teeth better than with the strips.Strips: Get you four to six shades lighter, but only on the front six teeth.Whitening toothpastes: They remove stains by chemical removal or polishing.The ideal combo: In-office whitening followed up with brush-ons and toothpastes. Repetitive whitening is safe, at low concentrations. Be sure not to do more than once every two weeks or you’ll thin the enamel. Home whitening strips don’t appear to damage the enamel. A five-minute fluoride treatment before bleaching will allow the teeth to fully recover in two weeks.WATCH VIDEO:MORE: The Mouth-Health Connection