By Stephanie Stahl

PHILADELPHIA (CBS) — There is a renewed warning from a Philadelphia dentist about dangers linked to a popular type of toothpaste. Charcoal toothpaste is the hot new thing — all-natural whitening — and it’s especially big with millennials and on social media.

Now comes payback time and the unwanted, surprising dangers for many.

Black toothpaste made from activated charcoal turns your teeth temporarily black. It leaves a big mess in the sink, but it’s a popular trend.

“It was like the thing to have,” Hannah Abou-Harb said.

The 24-year-old says she and her friends at Rowan University were excited to use charcoal toothpaste.

“At first, it was fun. I would always videotape, Snapchat my friends, ‘Oh look, my teeth are black, but now they’re white,” Abou-Harb said. “It worked, looks-wise. Yes, my teeth were very white and they felt clean.”

But her teeth quickly became sensitive to cold drinks and she developed new cavities.

“I was shocked because it does say it’s all-natural and I was like how can an all-natural product hurt your teeth?” Abou-Harb said.

Activated charcoal is highly absorbent and used medically to absorb and remove toxins so it can theoretically remove surface stains on the teeth.

But here’s what else it can do.

“Abrasion of the enamel, which will make you more prone to decay and then, of course, it’s absorbed in the gums,” Dr. Shireen Malik said. “It goes into your fillings. It can even go into your tonsils if left longer.”

Malik, a Philadelphia-based dentist, says the abrasiveness of charcoal toothpaste can also make teeth more discolored and it doesn’t contain fluoride, which prevents cavities.

“I don’t know why people are using it. I have no idea,” Malik said. “Instead of making them stronger, it’s making them weaker.”

The American Dental Association says there isn’t enough data to “substantiate the safety and efficacy claims” linked to charcoal toothpaste.

“I think my teeth suffered a lot of damage from it,” Abou-Harb said.

Abou-Harb’s teeth look nice and white now that she’s stopped using the charcoal toothpaste.

Dentists say the safest whiting comes with the professional treatments and that consumers should always use toothpaste that contains fluoride, the best way to keep teeth strong and cavity-free.

Stephanie Stahl