I have been taking activated charcoal and brushing my teeth with it once a day. I sprinkle a bit on my toothbrush with my toothpaste and brush for 2-3 minutes. It will make your mouth black but rinses right away. It can be a bit messy but I dab some on while holding my toothbrush over my trash barrel to minimize the mess. My teeth feel super clean and look super white after brushing! I love this.
This is the brand I use: