The BBC is currently running a story about and independent lab in the U.K. that tested 16 do it yourself teeth whitening kits. They found that 14(!) of them contain illegal levels of Hydrogen Peroxide. Like, really illegal levels.
In one case the legal limit was exceeded by a factor of 230 times.
Hydrogen peroxide, commonly used as a hair dye, can aggravate gum disease and cause sensitive teeth – or even chemical burns to the mouth.