The Complete Guide to Teeth Whitening: How It Works, What Is Legal and What Results Are Realistic

Teeth darken for two broad reasons: stain builds up on the surface, or the colour inside the tooth changes. Dentists call these extrinsic and intrinsic stains, and the difference shapes what whitening can do.

An extrinsic stain sits on the outer surface. Coloured compounds from food, drink and tobacco attach to the thin protein film that covers enamel, or become held in plaque and tartar. NHS guidance names coffee, tea, red wine and smoking as common causes of stained or darkened teeth. Some mouth rinses, chlorhexidine in particular, can also stain teeth with long-term use. Abrasive cleaning can remove some surface stain, but it does not change the colour inside the tooth.

An intrinsic stain sits inside the tooth, in the enamel or in the dentine beneath it. Recognised causes include:

• Ageing: enamel wears gradually, so the dentine beneath shows through more and the tooth looks darker overall.
• Injury: after a knock, blood products from the pulp can enter the tiny channels in the dentine and darken a single tooth.
• Dental treatment: some filling materials and root canal medicines can discolour the tooth they sit in.
• Tetracycline antibiotics, a recognised cause of intrinsic staining.
• Developmental conditions that affect how enamel or dentine forms, such as fluorosis.

Internalised staining, a third type, starts on the surface and works its way in through worn enamel, receding gums, decay or the edges of old fillings. Understanding how a tooth is built makes the pattern clearer: enamel is the hard outer layer, dentine forms the bulk of the tooth beneath it, and the pulp sits at the centre.

The cause matters because peroxide acts on organic colour compounds. Review evidence notes that some intrinsic stains, fluorosis among them, are not fully removed by bleaching. A single dark tooth also needs its cause checked, because injury or an old root filling may be behind it.

Whitening gels work by oxidation: peroxide breaks down coloured molecules inside the tooth so they absorb less light and the tooth looks lighter.

The coloured compounds in teeth are called chromophores. They absorb part of visible light and reflect mainly the complementary colour, which in teeth is typically yellow or brownish. Some chromophores are small organic molecules, such as tannins; others are coloured metal ions such as iron, copper or manganese. Reviews describe peroxide bleaching as oxidising the organic ones.

Hydrogen peroxide is the active ingredient. It is an oxidising agent that produces highly reactive free radicals, and these oxidise the chromophores. Carbamide peroxide is hydrogen peroxide bound to urea. In use it separates into hydrogen peroxide and urea, and by weight it is about 36% hydrogen peroxide, so a 10% carbamide peroxide gel releases about 3.6% hydrogen peroxide.

For the peroxide to work, it has to reach the stain. Evidence suggests it penetrates the enamel structure, and one review describes the success of bleaching as directly related to that ability to penetrate. The reaction acts on natural tooth tissue only, so it has no effect on the colour of crowns, veneers, fillings or dentures. The deeper science, including what the gel does to enamel and dentine, is set out in the science pages in this section.

Peroxide chemistry also explains the strength limits in UK law. Gels are regulated by the hydrogen peroxide they contain or release, so a carbamide peroxide gel is judged by its hydrogen peroxide equivalent. Gels of up to 16% carbamide peroxide stay within the 6% hydrogen peroxide limit. The home gels used at Deepcar Dental Care are 16% carbamide peroxide and 6% hydrogen peroxide, both within that limit.

In general, a dentist-led home whitening course begins with the clinical examination that UK rules require, and ends with tray wear at home.

In general terms, NHS guidance describes home whitening in this order:

• A dentist checks your teeth and gums.
• The dentist prescribes the gel and has trays made to fit your teeth.
• You wear the trays at home over several weeks.

The table gives the details of a whitening course at Deepcar Dental Care:

Realistic whitening results mean natural teeth can become lighter by an amount that varies between people and cannot be promised in advance.

How much lighter teeth become depends on the individual, including the starting colour and what caused it, since some intrinsic stains are not fully removed by bleaching.

Whitening also has fixed limits, which is why it helps to know which front teeth are natural and which are restored. Crowns, veneers and fillings will not change colour, so existing work on the front teeth stays as it is and may stand out against lighter natural teeth. Matching a restoration to the colour of the teeth around it is a separate process, and dental shade matching explains how it is done.

Whitening changes colour only. It does not change the shape, size or position of teeth, close gaps or repair chips, and what whitening can and cannot change sets out those limits in more detail.

You can have teeth whitening at Deepcar Dental Care, 334 Manchester Road, Deepcar, Sheffield, S36 2RH.

There is free parking directly outside, and the practice is on the ground floor and wheelchair accessible. It is open 8am to 8pm, Monday to Sunday. You can read what patients say on our patient reviews page.