Whitening works by peroxide diffusing through enamel into the dentine and breaking down the pigmented molecules held there. That single sentence explains most of what people find surprising about it: it only works on natural tooth structure, it takes time rather than effect, and it does not remove anything physical from the surface.

Active ingredient
Hydrogen peroxide, or carbamide peroxide which breaks down into it
Works on
Natural enamel and dentine
Does not work on
Crowns, veneers, composite fillings, bridges
Sensitivity
Common, usually temporary, related to concentration and duration

Why the order of treatment matters

Ceramic and composite do not whiten. If you have a crown on a front tooth and you whiten the teeth beside it, that crown stays exactly the shade it was and will now be the darkest thing in your smile.

So whitening comes first and restorations are matched afterwards. Reversing that order is one of the most common and most expensive mistakes in cosmetic dentistry, because the fix is remaking the restoration.

In-practice and at-home are the same chemistry

The difference is concentration and supervision. In-practice whitening uses a higher concentration with the gums protected, and produces a visible change in one or two appointments. Home whitening with custom trays uses a lower concentration over one to two weeks and commonly reaches a similar endpoint.

Many plans combine them: an in-practice session for the initial change and trays for maintenance, since whitening is not permanent and colour returns gradually.

The stains it cannot lift

  • Tetracycline staining from antibiotics taken in childhood — deep, often banded, and usually resistant.
  • Fluorosis, which is white mottling rather than yellowing, and can look more obvious after whitening.
  • A tooth that has darkened after root canal treatment, which needs whitening from the inside rather than the outside.
  • Grey tones generally, which respond less well than yellow ones.

None of these means nothing can be done; they mean whitening alone is not the answer, and a consultation that identifies which kind of discolouration you have is worth more than any product.

Does whitening damage enamel?

Supervised whitening at clinical concentrations is not shown to cause meaningful enamel loss. The common side effect is sensitivity, which is usually temporary. Unsupervised high concentrations and acidic products are a different matter.

Why are my teeth sensitive afterwards?

Peroxide passes through enamel into the dentine, and that transiently increases nerve sensitivity. It typically settles within days. Desensitising gels and lower concentrations over longer periods both reduce it.

How long does it last?

Commonly a year or two, varying widely with diet and smoking. Occasional top-up whitening with trays is the usual way it is maintained.

Is there a shade limit?

Yes. Each tooth has a point beyond which it will not lighten further, and that point differs between people. Anybody promising a specific shade before seeing your teeth is guessing.