Whitening strips penetrate the enamel to temporarily bleach the teeth. Smoking, wine and coffee will do the opposite.
Dr. Denis Kinane, dean of the University of Pennsylvania School of Dental Medicine, believes there to be no harm with long-term, frequent use of whitening strips.
The main ingredient is usually hydrogen peroxide, which is readily degraded in the body and excreted as urine.
Dr. Edmond Hewlett, from the American Dental Association, warns that if people are worried about damaging their teeth, they should be sure to avoid aggressive brushing, which can damage tooth fibers and even cause cavities.