The most recent study of the effectiveness of teeth bleaching was by the Cochrane Collaboration, an international non-profit group that reviews health care treatments.

In the study, researchers from the University of Michigan analysed 25 clinical studies that looked at over-the-counter and dentist-dispensed whitening treatments. It found that, after two weeks of use, tooth-whitening products worked no better than using no whitening product, but the degree of effectiveness varied depending on the bleaching agent used.

The reviewers warned that mild tooth sensitivity was common, but usually was only temporary. The findings, however, didn't identify any longer-term studies that could map out potential dangers or long-term side effects.