The charcoal products, which are increasingly popular, often contain no fluoride to help protect the teeth.
And there is no scientific evidence to back up the claims they make, the authors say.
Excessive brushing with them can do more harm than good, they add.
They advise people to go to their dentist for advice on bleaching, or whitening, their teeth.
And they say it is better to stick to using a regular fluoride-based toothpaste.
Charcoal was first used for oral hygiene purposes in ancient Greece, as a way of removing stains from teeth and disguising unpleasant odours from diseased gums.
Dr Joseph Greenwall-Cohen, co-author of the study from the University of Manchester Dental School, said "more and more shops are selling charcoal-based toothpastes and powders", including Superdrug, Boots and Tesco, after celebrities had started talking about using them.
But he said the claims they made had been found to be unproven by a 2017 US review of 50 products.
Some said they were "anti-bacterial" or "anti-fungal", that they helped with "tooth whitening" and would "reduce tooth decay".