Teeth-whitening kits available on online marketplaces have been found to contain illegally high levels of hydrogen peroxide which can burn gums and cause dental damage.
An investigation carried out by Which? tested 36 kits and 21 of them exceeded the legal amount of the chemical for home use while one had 300 times too much of the active ingredient.
Online marketplaces removed the unsafe products that went above legal limits from sale when they were flagged.
The consumer group said it was concerned that social media users may be encouraging others to use products with dangerous levels of the chemical.
Sue Davies, Head of Consumer Protection Policy at Which?, said: “It’s worrying that our tests have revealed so many of these products sold on online marketplaces – and often hyped on social media – are breaking legal limits for hydrogen peroxide and putting the health of users at risk.
“It’s clear that self-regulation is not working, leaving people exposed to a flood of unsafe products online.”
Teeth-whitening products for use at home can contain up to 0.1% hydrogen peroxide, while the legal limit for application by professional dentists is 6%.
The worst offender found by Which was a bleaching gel kit containing 30.7% hydrogen peroxide, from the Oral Orthodontic Materials store on AliExpress.
The company said it “found the mistake, corrected it in time and that the product has been removed from sale”, while AliExpress told the group: “We take product safety very seriously and after being notified by Which? of its findings, we took prompt action and removed the third-party product listings identified to be in violation of our listing policy.”
Dr Paul Woodhouse from the British Dental Association said: “Hydrogen peroxide is a seriously strong chemical and not to be messed with.
“Dentists are trained in its usage and they also know what whitening products are effective to use and safe for teeth and gums.
“If you destroy gum tissue, you are never going to get it back and you lose your teeth. If it penetrates the surface of your tooth, which is likely, it’s probably going to lead to the death of that tooth.”
