Whitening works
on enamel only.

What it will not lighten

This matters for planning. If you have a white filling in a front tooth and you whiten, the filling stays the shade it was and now stands out. The order is whiten first, then replace the filling to match. Reversed, you pay twice.

• Crowns, veneers, bonding and white fillings -- ceramic and composite do not respond at all
• Grey discolouration from tetracycline, which responds poorly and unevenly
• A single dark tooth that has had a root canal -- that needs a different technique from the inside
• Staining that is really surface build-up, which is a cleaning, not a whitening

In-office and take-home

In-office uses a stronger gel with the gums protected, in about an hour, with a visible change the same day. Take-home uses custom trays and a milder gel over one to two weeks. The end point is similar; the difference is speed and how much sensitivity you get at once.

Custom trays are worth the difference over a boil-and-bite kit. A tray that fits holds gel against the teeth and off the gums, which is both more effective and considerably more comfortable.

Sensitivity

Common, temporary, and manageable. Peroxide moves through enamel into the dentine beneath, and that is what produces the zinging sensation on cold.

It settles within a day or two of stopping. A sensitivity toothpaste for a fortnight beforehand helps, as does shortening each session rather than pushing through. Sensitivity is not damage, but it is a reason to slow down rather than to grit your teeth.

How long it lasts

A year or two, depending mostly on coffee, tea, red wine and tobacco. Most people top up with a few nights of tray wear rather than repeating the whole course.

There is a ceiling. Teeth have a natural maximum brightness and past that point more gel achieves nothing except sensitivity. Anyone selling you an endless course is selling sensitivity.