Why is it not legal? Tooth whitening seems so simple! Dental professionals train for many years to understand the strictures of the mouth, to recognise dental disease and to prescribe the correct treatment for each individual patient. The chemicals used to whiten the teeth could permanently damage the teeth and gums. To protect the public against this type of damage, the person providing the treatment must be fully dentally qualified and registered with the General Dental Council (GDC) .