IN THE quest for the perfect Hollywood smile, they provide an instant and cheap makeover for the mouth.

But the teeth-whitening kits used by thousands of Britons who want polished molars can cause permanent damage, according to dentists.

Super-strength whitening kits that promise to create brighter smiles by bleaching teeth with high levels of hydrogen peroxide dye can lead to chemical burns and aggravated gum disease, the British Academy of Cosmetic Dentistry (BACD) announced yesterday.

The do-it-yourself kits, on sale over the counter in Britain for as little as £10, are in demand as an alternative to surgical bleaching treatments offered by dental clinics at a cost of between £300 and £1,300.

Yet some kits available online are 250 times more concentrated than the legal limit and can cause damage or over-bleaching, resulting in so-called “fridge door teeth”, clinicians claim. At worst, they may exacerbate gum disease, leading to tooth loss.

Private dental clinics are burgeoning, with the market for cosmetic dentistry in Britain now said to be worth £1 billion.

More than a quarter of Britons have had cosmetic dental work, including caps and braces. However, whitening treatments are the most popular, according to a recent survey.

James Goolnik, a dentist and board member of the BACD, said that the kits offering a “cheap, quick fix” were ineffective and offered a false economy.

“Whitening is a bit like a facial in that it helps to unlock pores in your tooth so that stains are gently removed leaving teeth cleaner and brighter,” he said. “All whitening is based on a hydrogen peroxide solution; the only difference in the hundreds of systems out there is the concentration and the way the solution is applied to your teeth. Not all of them are safe.

“By law, shop-bought kits in the UK can’t have a hydrogen peroxide concentration level more than 0.1 per cent. But other countries have no regulations at all.”

Dr Goolnik said that take-home kits should be used with caution since they involved using poorly moulded mouth guards, often worn overnight, allowing leakage of gel or solutions containing hydrogen peroxide, a chemical typically associated with hair colourings, into the mouth.

“If the gel goes on to the gums, it can cause blistering. In someone who has got irritation or decay, it can accelerate the process,” he said.

Dr Goolnik said that the kits with high levels of hydrogen peroxide were designed to be used by professional dentists, painted directly on to the teeth while the gums were protected.

He said: “We are seeing more and more people coming in with damage caused by whitening kits . . . They go to the supermarket and see whitening toothpastes. There is no evidence they can actually whiten teeth and people might not see a difference, so they want the next thing up. At best they get no result, and at worst they get permanent sensitivity.

“It is essential to invest, at a bare minimum, in going to see your dentist before you use one of these kits, but you don’t want to spend money on whitening for no effect and only a dentist can get your teeth to the maximum whiteness.”

A spokesman for the General Dental Council, which regulates dentistry in Britain, said: “Tooth-whitening products contain bleach and need to be handled with caution. Only registered dentists are permitted to apply materials and carry out procedures designed to improve the aesthetic appearance of teeth.”

Carmel McHenry, a spokeswoman for the British Dental Association said: “Liquids that discolour teeth will darken them again over time.” She recommended a full dental assessment before the procedure.

A spokesman for Boots the Chemist, which sells its own brand of whitening kits, said: “People with sensitive teeth or gums are advised to talk to a dentist before using whitening kits, and all users are advised to pay careful attention to the detailed instructions on the packaging. Our own-brand kit uses a non-peroxide formula and has been extensively tested and passed as safe to use.”

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