Teeth whitening involves bleaching your teeth to make them lighter. Teeth whitening can't make your teeth brilliant white, but it can lighten the existing colour by several shades. Teeth whitening is a form of dentistry and should only be performed by a dentist or another regulated dental professional, such as a dental hygienist or dental therapist, on the prescription of a dentist.
Some home kits don't contain enough of the whitening product to be effective. More generally, if a dental professional is not doing the whitening, the mouthguard provided may not fit properly so some of the bleaching gel may leak out onto your gums and into your mouth, causing blistering and sensitivity.