Teeth whitening: what to know before you start

Teeth whitening is one of the most requested cosmetic treatments, and one of the simplest. But between the ads, the drugstore kits and the home remedies, it's hard to know what to expect. Here's how it works, who it suits, and how to handle the sensitivity that sometimes comes with it.

How it actually works

Every legitimate method relies on the same principle: a peroxide gel (hydrogen or carbamide peroxide) penetrates the enamel and breaks down the coloured molecules that have built up inside the tooth over the years — coffee, tea, red wine, tobacco, natural aging. Peroxide doesn't "paint" the tooth white; it lightens its natural shade.

What varies is the concentration and the contact time. In the clinic, a stronger gel is applied under supervision in a single session, often an hour or a bit more, after the gums have been protected. At home, you wear custom-made trays with a milder gel every day for one to three weeks. In the end, the results of the two approaches are comparable; the clinic is faster, home is more gradual and easier to touch up later. Many dentists combine the two.

So-called whitening toothpastes mostly remove surface stains through abrasion. They make teeth a little cleaner, not lighter. As for home remedies — charcoal, baking soda, lemon — they wear down enamel without changing the deeper colour, and what's lost doesn't grow back.

Sensitivity and precautions

Sensitivity is the most common side effect: brief twinges with cold, sometimes out of nowhere, during treatment and for a day or two afterwards. It's temporary and doesn't mean the tooth is damaged. A few measures keep it in check:

• Have an exam before starting: a cavity or a receded gum under peroxide gel hurts, and whitening waits until that's fixed.
• Use a toothpaste for sensitive teeth for the two weeks before treatment, and keep using it afterwards.
• Space out home sessions, or shorten wear time, if sensitivity sets in.
• Avoid very cold or very hot food and drinks for the first few days.
• Don't exceed the recommended duration and frequency: longer doesn't mean whiter.

Gums can also get irritated if the gel overflows. That's one reason to prefer fitted trays over the one-size-fits-all kind found in generic kits.

Who isn't a good candidate, and what to expect

Whitening isn't recommended during pregnancy or breastfeeding, for lack of data, or before the late teens, while the teeth are still developing. It's postponed when there are untreated cavities, gum disease, cracked enamel or already significant sensitivity.

It also helps to know what it doesn't do. Peroxide only works on natural teeth: fillings, crowns and veneers keep their shade. If you have any on your front teeth, they may look darker afterwards, and may need to be redone to match. Grey staining from certain antibiotics taken in childhood, or a tooth that darkened after a root canal, responds little or not at all to standard whitening; other options exist.

Finally, results vary from one person to the next. Yellow shades lighten better than grey ones; some teeth change several shades, others one or two. The effect lasts from a few months to a few years depending on your habits, and an occasional touch-up is usually enough to maintain it. A dentist who shows you your starting shade, explains what's realistic for your teeth and suggests the right option is the best starting point for a smile that looks like you — with no unpleasant surprises.