What whitening actually does
A peroxide gel sits against the tooth and breaks down the staining held inside the enamel and the layer beneath it. The tooth is not coated, bleached in the laundry sense, or resurfaced. Its own colour is lightened.
This is the entire difference between whitening and veneers. Whitening changes colour and takes nothing away. Veneers change colour and shape and remove enamel permanently.
The two ways it is done
In the clinic. A protective barrier on the gums, a stronger gel, roughly an hour. You leave lighter the same day. It suits a deadline, and it is the option people book before a wedding.
At home, in custom trays. We take an impression, the trays are made to fit your teeth exactly, and you wear them with a milder gel for a couple of hours a day or overnight, for about two weeks. Slower, and usually the better result, because total contact time is what does the work.
The trays last for years, which is what makes topping up cheap afterwards. That is the reason we lean towards this one for most people.
What it will not change
Crowns, veneers and white fillings. These do not lighten at all. If you have one on a front tooth, whitening everything around it will leave it looking darker than before, and matching it again means replacing it. We look for this at the examination and tell you before you start, not after.
Stains from inside the tooth. A tooth that darkened after an injury or a root treatment has its discolouration in the wrong layer for external whitening. There are other approaches, including whitening from inside a root treated tooth, and they are a separate conversation.
Shape, position and length. Colour is one of three complaints people bring in. Whitening addresses exactly one of them.
What it costs you in risk
Sensitivity, usually. Sharp, brief twinges at cold, for a few days, settling on its own. Starting with a milder gel and shorter sessions reduces it, and there are pastes that help.
Gum irritation if the gel sits on the gum, which is what a custom tray is for and what shop bought one size trays are bad at.
The one genuine danger is whitening over untreated decay, a cracked tooth or an exposed root surface, where the gel reaches places it should not. That is not a risk of whitening. It is a risk of whitening without an examination first.
Why we push it before porcelain
It is reversible in the only sense that counts: if you do nothing more, your teeth are still your teeth.
For a real share of the people who come in asking for a Hollywood smile, colour was the actual complaint. Whitening answers it for a fraction of the cost, and the ones who still want veneers afterwards go ahead knowing it was not just the shade they were unhappy with.