Activated charcoal can be used for whitening teeth. The carbon particles absorb stains and toxins so that it whitens teeth.
Activated charcoal does not damage your health unless ingested in extremely large amounts. When used in the mouth it is perfectly safe and in fact will absorb bad toxins and bacteria from the mouth improving your gum health and breath!
Just do not attempt to make it yourself from charcoal at home as it is not the same stuff!