Whitening strips have been shown in clinical studies to be effective in whitening teeth. They contain hydrogen peroxide that is held against the teeth by the strip. The hydrogen peroxide oxides and breaks down stains. Crest whitening strips are intended to be worn for 30 minutes a day.
Yellow-tinged teeth are more likely to become whiter in response to whitening strips. Gray or brown stained teeth don't usually whiten much, if at all, after exposure to whitening strips. Whitening strips are generally considered to be safe when used as directed. However, overuse can damage the enamel of the teeth and cause sensitivity. Severely over-whitened teeth can become translucent, and the enamel can become porous and break down.