The only ingredient that actually bleaches teeth lighter. Works in strips and trays; in toothpaste and rinses contact time is too short to do much.
Hydrogen peroxide diffuses into enamel and dentine and breaks down the pigment molecules that make teeth look yellow. It changes the tooth's own colour, unlike abrasives, which only remove surface stain. Carbamide peroxide releases roughly a third of its weight as hydrogen peroxide.
Results depend on strength and contact time. Strips and trays hold gel on teeth for 30 minutes or more; a toothpaste or mouthwash is rinsed off within minutes. In the EU, products sold freely may contain at most 0.1% hydrogen peroxide; 0.1-6% is dentist-supervised only and not for under-18s.
A Cochrane review found home peroxide strips and tray gels whiten teeth versus placebo, with stronger products working better. All included trials were manufacturer-sponsored, short-term and mostly at high risk of bias, and long-term harms have not been studied well. Evidence for low-level peroxide in toothpaste and rinses is thinner.
25 short-term trials, all manufacturer-sponsored and mostly at high risk of bias: peroxide strips and tray gels whiten teeth versus placebo, with differences driven by peroxide strength. Mild-to-moderate tooth sensitivity and gum irritation were the common side effects.