I stopped using Crest when my teeth started turning mysteriously brown at the base within a week of using one of their whitening pastes. I couldn't understand why that was happening so suddenly until I read the label and saw a disclaimer that the ingredients may cause harmless, superficial brown marks on your teeth. Others online had the same experience. It was the most ludicrous thing I've ever read. Threw it away and my teeth went back to normal.
If someone went on Shark Tank and pitched a teeth whitening system that has the side-effect of making your teeth brown they'd be laughed out of the room.
Currently playing: F-Zero GX (GC), Sands of Destruction (DS), Dark Alliance 2 (PS2)
Training log: https://powpowpunishment.blogspot.com