Tooth whitening uses an oxidising agent that passes through the enamel and breaks down pigment held in the dentine. It lightens the natural tooth. It does not change the colour of a crown, a veneer or a composite filling, which stay as they are and can become noticeable once the teeth around them lighten.
The two ways it is done
In-office whitening is done at a single appointment with a higher concentration and the gums protected. The dentist-prescribed alternative uses custom-made trays worn at home over a period of two to six weeks with a weaker gel. The two are often combined, with the trays then used to maintain the result.
How long it lasts
The effect is not permanent. Results may last around three years, with wide variation depending on smoking, coffee, tea, red wine and oral hygiene. Whitening is better understood as something reviewed and topped up than as a one-off event.
Side effects and who should do it
The common effects are sensitivity to cold or sweet things, a sore gum or throat, and temporary white patches on the gums. These are usually temporary. No reliable percentage exists for how often sensitivity occurs, so treat any reassuring figure with suspicion. The non-negotiable point is that only registered dental professionals should carry out whitening. Untreated decay, inflamed gums or a cracked tooth should be dealt with first, otherwise sensitivity will be considerably worse and the underlying problem will simply have been painted over. It is also worth agreeing the target shade against a shade guide before starting, and asking how any existing crowns or fillings at the front of the mouth will be matched afterwards, since those will not lighten with the rest.