As per Harvard Health, using charcoal toothpaste on teeth is safe in small doses, but people shouldn't use it every day. "It's simply too abrasive...risking damage to tooth enamel...[it] may even stain teeth inadvertently when charcoal particles get caught in tiny tooth cracks," Harvard Health said. "Most charcoal toothpastes don't contain fluoride, which protects teeth against decay," it added.