Why teeth darken — and what is actually whitened
There are two kinds of discolouration, and the difference matters.
Extrinsic staining is a layer on the surface: from coffee, tea, red wine, tobacco. It does not need whitening — it comes off with a professional cleaning for 10,000-15,000 AMD. People often come in for whitening when all they actually need is a cleaning — and they are happy with the result.
Intrinsic discolouration is the tooth's own shade of enamel and dentine. It changes with age (dentine yellows), from medication, fluorosis or a dead nerve. This is where whitening works: the gel penetrates the enamel and breaks down the pigment molecules. The enamel is not scratched or thinned — only the colour changes.