Teeth whitening: what is safe, what is hype

Tooth colour comes from two places: surface stains from coffee, tea, tobacco and pan, and the intrinsic colour of the dentine underneath. Only the first responds to abrasives. Only bleaching agents change the second.

Charcoal powders and 'whitening' toothpastes work purely by abrasion. They may remove some surface film, but used aggressively they wear enamel and expose more yellow dentine, making teeth look darker over time.

Over-the-counter kits contain low-concentration carbamide peroxide with generic trays. They can work slowly, but ill-fitting trays leak gel onto the gums and cause chemical burns.

In-office whitening uses a higher-concentration hydrogen peroxide gel with a physical gum barrier placed by a clinician, activated by an LED lamp over two or three cycles. Results are typically three to five shades in one visit.

Sensitivity is the main side effect and it is transient - usually gone in 24 to 48 hours. A desensitising paste for two weeks before treatment greatly reduces it. Whitening does not lighten crowns, veneers or composite fillings, so those may need replacing to match afterwards.