A popular beauty product marketed as a natural tooth whitener could actually damage people's teeth and has even been linked to cancer, a Kiwi dentist warns.
Brushing your teeth with activated charcoal powder became a trend after a social media campaign featuring beauty bloggers went viral.
Since then scores of online retailers - including Auckland-based companies Pure Nature and SmileStore.co.nz - have started stocking it.
Activated charcoal, also known as activated carbon, is a fine black powder often made from coconut shell particles heated until they become porous, which allows them to bind to toxins.
It has been used in emergency departments for decades to treat people who have been poisoned.
Consumer websites selling activated charcoal powder say it "naturally cleans, polishes, and whitens your teeth, and assists in removing toxins and hindering bad breath".
However, Auckland cosmetic dentist Dr Clarence Tam, who has tried activated charcoal dental products and has treated many people who use them, is sceptical of such claims.
"The risks outweigh the benefits in my opinion," she told the Herald on Sunday.
Tam said she would not recommend that her patients use activated charcoal powder to whiten their teeth because it was abrasive and could damage the enamel - the outer layer that protects teeth. Once enamel is damaged it cannot be repaired.
Professor Damien Walmsley, British Dental Association scientific adviser, shared Tam's concerns, telling the Guardian in an interview last year activated charcoal could damage tooth enamel.
Tam added that people using activated charcoal to whiten their teeth would likely brush very hard to try to get their pearlies as white as possible and this could cause even more damage.
Activated charcoal powder could also darken gums, which gave the illusion of whiter teeth even if it didn't change their colour, Tam said.
The United States Centre for Disease Control has also warned it was highly likely that inhaling large amounts of activated carbon powder could cause cancer.
Tam said it would be easy to accidentally breathe in small amounts of powder while mixing it up to form a tooth-whitening paste.
She said research had shown activated carbon powder was as carcinogenic as cigarette smoke.
So if people were going to use activated charcoal to try to whiten their teeth they should use a pre-mixed paste instead of powder, to eliminate the risk of inhalation, and use a soft toothbrush and light brushing technique, Tam said.
Pure Nature general manager Emmett Bowker said his store sold activated charcoal powder for a variety of uses - including teeth whitening - and he was yet to see any empirical evidence that using it to brush your teeth could damage enamel.
"I have heard that. [But] there's no direct warning from any dental association that I'm aware of - as soon as we hear that I'm sure we would advise people not to do it."
SmileStore.co.nz marketing manager Matt Heath said an international system used to measure how much toothpastes may erode dentin, underneath the enamel, had ranked activated charcoal powder as being less damaging than some whitening toothpastes.
The Relative Dentin Abrasion (RDA) index scored the substance between 70 to 90, classifying it as a medium abrasive. The original Colgate Total scored 70 and Colgate 2-in-1 Tartar Control/Whitening was ranked at 200.
The FDA recommended a score of 200 or less and regarded 250 as harmful.
However, Tam said she would not recommend that her patients use a product with a score higher than 70.