Tooth whitening

Tooth whitening is the one piece of dentistry most often bought in the wrong place. It is also, in the United Kingdom, one of the few that is regulated to the point where the law is the most useful thing to know about it: who may legally carry it out, and what may legally be sold to anyone else.

Why teeth darken

Two different things are at work, and almost every confusion about whitening comes from mixing them up.

Surface stain settles on the outside of the enamel from tea, coffee, red wine, dark fruit and juices, tobacco, some mouthwashes and iron supplements. Enamel is slightly porous, so these accumulate in the surface rather than sitting loose on it, which is why brushing stops shifting them after a while.

The tooth's own colour lies deeper. Enamel is translucent rather than white, and the dentine beneath it is yellow; the shade of a tooth is mostly the dentine showing through. Enamel thins with age and dentine thickens, so teeth darken over a lifetime whatever anyone does. Some tetracycline antibiotics given in childhood cause grey or banded discolouration, excess fluoride in childhood can leave mottling, injury can darken a single tooth, and a tooth that has had root canal treatment often goes grey over the years that follow.

Cleaning removes surface stain. It does not and cannot change the tooth's own colour. Bleaching, with peroxide, changes the shade. The two are routinely sold as the same thing, and they are not.

What the law allows

Tooth whitening is legally the practice of dentistry in the UK, confirmed by the High Court. That has specific consequences:

• Only a registered dentist may provide it, or a registered dental hygienist or dental therapist working to a dentist's prescription. Registration is the test, not the product used or the training certificate on the wall.
• Products containing or releasing more than 0.1% hydrogen peroxide may only be supplied to dental practitioners. Up to 6% may be used in cosmetic whitening.
• The patient must be examined first, for decay, gum disease and anything else that would make whitening unwise, and the first application in each cycle of treatment must be carried out by the dentist, or by a hygienist or therapist under direct supervision. The rest of the cycle may then be completed at home.
• The patient must be over eighteen for cosmetic whitening.
• Anything sold to the public — on a high street, in a supermarket or online — may legally contain no more than 0.1% hydrogen peroxide. That is far too weak to change the colour of a tooth, which is the honest answer to why home kits bought over the counter do little.

So whitening offered by a beauty salon, a mobile technician, a stall, a hairdresser or anyone else not on the dental register is a criminal offence under the Dentists Act, however the treatment is described and however gentle the product is said to be. Prosecutions follow. Concerns can be reported to the General Dental Council, and complaints about illegal peroxide levels in products go to Trading Standards.

One label deserves suspicion. "Peroxide-free" does not always mean what it appears to: sodium perborate releases hydrogen peroxide and was banned from cosmetic products in 2010, and other compounds break down to peroxide in the mouth. A product bought from outside the UK may contain a concentration that cannot lawfully be sold here, at a strength nobody has checked.

What whitening at the dentist involves

Any decay is filled and gum disease treated first, because peroxide reaching a cavity or an inflamed gum hurts and because whitening a mouth that needs treatment is treating the wrong thing.

The usual route is a home kit under supervision. Impressions are taken and thin trays made to fit the individual's teeth closely, which matters more than it sounds: a tray that fits holds the gel against the enamel and keeps it off the gums, where leaked gel causes most of the discomfort people report. The gel is supplied at a prescribed strength, the first application is done at the practice, and the remainder is worn at home for a set time each day over a period of days to a few weeks.

The alternative is whitening in a single appointment at the surgery, with a stronger gel, the gums carefully isolated, and the gel renewed two or three times in the session. A lamp is sometimes used; the evidence that light adds much to the result is thin, and the gel does the work. Some practices combine a surgery session with trays to take home.

Cosmetic whitening is not NHS treatment, which covers what is clinically necessary to keep teeth and gums healthy. Whitening carried out as part of treating a specific problem can be a different matter, and is worth asking about rather than assuming.

What whitening will not do

This is where disappointment usually comes from, and none of it is obvious in advance.

It does not touch anything artificial. Crowns, veneers, bridges, white fillings and dentures stay exactly the shade they were made. A front tooth with a large white filling will whiten around the filling and leave it standing out. Where there is a lot of visible dental work, whitening the natural teeth first and then replacing the work to match is the order that gives an even result — and it costs more, which is why it should be discussed before anything starts rather than after.

It works poorly on some kinds of discolouration. Grey or banded staining from childhood antibiotics responds slowly and incompletely. Mottling from excess fluoride may become more noticeable rather than less, because the white patches lighten differently from the rest. A single dark root-filled tooth needs a different procedure, bleaching from inside the tooth rather than outside it.

It needs enamel to work on. Where gum recession has exposed root surface there is no enamel, and that part of the tooth will not lighten.

It is not permanent. The result fades, slowly or quickly depending on habits, and is topped up periodically rather than done once. Teeth are also more absorbent for a day or two afterwards, so that is the worst moment for red wine and strong tea.

Side effects and the risk of overdoing it

Sensitivity to cold during treatment and for a few days after is common and settles. A desensitising toothpaste used for a couple of weeks beforehand reduces it, and a dentist can prescribe something stronger. Gum irritation and white patches where gel has leaked are usually a sign the tray does not fit well enough or that too much gel has been loaded.

The real hazard is not a single supervised course but repeated unsupervised use at unknown strength. Too much peroxide, too often, wears and roughens enamel, which leaves teeth more sensitive, more prone to picking up stain and more prone to decay. Results can come out patchy, chalky or uneven. The appeal of a strong kit bought online worn overnight is precisely what makes it a poor idea, and there is nobody checking the mouth it is going into.

Whitening is also avoided during pregnancy and breastfeeding, not because harm has been shown but because it has not been studied and the treatment can wait.

Toothpastes, charcoal and the rest

"Whitening" toothpaste works by abrasion, polishing surface stain off the enamel. That can make a visible difference to a stained tooth and makes no difference at all to the tooth's own colour. Used heavily, the more abrasive ones wear enamel, which eventually makes teeth look darker as more dentine shows through.

Charcoal toothpastes have been reviewed by the Oral Health Foundation, which found insufficient clinical and laboratory evidence behind their whitening and safety claims. Any effect is superficial stain removal. Some are over-abrasive, and the more important problem is that many contain far less fluoride than the 1,350 to 1,500 parts per million that protects against decay — so the trade is a slightly cleaner-looking tooth for a less protected one.

Strips, pens and one-size trays sold to the public fall into two groups. Those legally sold here are at or below 0.1% peroxide and will not change a tooth's colour. Those bought from abroad at higher strengths are unlawful to supply here, come without an examination, and are applied in a mouth nobody has looked at.

A scale and polish by a dentist or hygienist removes surface stain that brushing cannot reach, and for a great many people whose complaint is stain rather than shade, that and giving up smoking is the whole answer.

When whitening is not the right treatment

Where the problem is the shape of a tooth, a chip, a gap or discolouration that will not bleach, the options are composite bonding, veneers, or in some cases removing a very thin surface layer of enamel to take out superficial defects. These are worth knowing about mainly because they are irreversible in a way whitening is not: enamel removed does not grow back, and a veneered tooth stays a veneered tooth. That makes them a conversation with a dentist rather than a purchase.

Keeping teeth lighter without treatment

Most of what darkens teeth is habit, and most of it is modifiable. Tobacco stains worse than anything else and giving it up improves far more than tooth colour. Tea, coffee, red wine and dark juices do most of the rest; drinking them with a meal rather than sipped through the day, and following with water, reduces the contact time. Brushing twice a day with fluoride toothpaste, cleaning between the teeth, and regular appointments keep stain from building into the enamel in the first place.

See also cleaning teeth and gums, dental check-ups and fillings, crowns and implants.