In the UK, over-the-counter (OTC) whitening strips can only have as much as 0.1 percent of hydrogen peroxide, while Crest Whitestrips have more — a lot more. In the US, the American Dental Association says whitening strips can have as much as 10 percent hydrogen peroxide, which is what Crest Whitestrips have. The maximum strength of hydrogen peroxide allowed in whitening strips in the UK is 6 percent, and these are only available with a prescription from the dentist (via Electric Teeth).
British dentists are wary of the potential damage hydrogen peroxide could cause in your mouth. If the chemical is used for too long or if the concentration is too high, hydrogen peroxide could burn the soft tissues in your mouth like your gums, tongue, and cheeks. The chemical could aggravate any current problems your teeth might have like cavities, and it could cause inflammations that could trigger infections. But even if you use hydrogen peroxide correctly, it can still cause sensitivity (via Electric Teeth). Since teeth whitening treatments work best after your teeth have been professionally cleaned anyway, you may be better off going to the dentist before you use any whitening strips to get a professional opinion, simply because your teeth might not tolerate hydrogen peroxide as well as you hope they will.
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