Charcoal is the new favorite cleanser among white-teeth fanatics. Social media is jammed with videos of YouTubers spreading charcoal powders and pastes on toothbrushes, and laughing about the resulting messy black smiles before a rinse reveals what they say are more-pearly whites.

“It goes with tattoos and edginess and a feeling of ‘I’m using this because I’m in the know,’ ” says Lu Ann Williams, director of innovation for Innova Market Insights, which tracks new food and consumer products. There were 131 food, beverage and personal-care products containing charcoal introduced last year, up 39% from two years earlier, she says.

Hello Products asks consumers to “take a brush on the wild side” with its $5 activated charcoal toothpaste. A My Magic Mud ad featuring a tattooed singer says to “rock that smile” with its charcoal-infused dental floss, toothbrushes and mouthwash. Procter & Gamble says it is launching a charcoal toothpaste next February as part of its Crest 3D White brand. Charcoal products often use black packaging to stand out.

“Everyone wants to try something new, but it has to be something that looks cool. It has to be more than just white,” says Susan Trumpbour, a product development consultant for beauty and personal-care companies in Bedford, N.Y.

Lauren Chouinard, a 27-year-old digital marketing manager in Capitola, Calif., has been brushing twice a day for three months with a charcoal toothpaste she bought online for $15. “It does make a mess,” she says. The black residue rinses from lips and gums, but it adds an extra step to morning routines to clean the sink. 

Even so, she thinks her teeth seem whiter. “It’s counterintuitive, that you are going to get dirtier before you get cleaner,” she says.

Dentists are grinding their teeth at the use of the abrasive product. “It may whiten teeth in the short run, but will eventually wear into the dentin,” the next layer below the enamel, which is yellow, says Matthew Messina, a dentist in Columbus, Ohio, and a consumer adviser for the American Dental Association.

A 2017 review in the Journal of the American Dental Association says there is insufficient evidence to show dental products with charcoal are safe or effective for your teeth. A big concern is that there aren’t any well-controlled human studies to show their safety, says John K. Brooks, a professor at the University of Maryland School of Dentistry and lead author on the paper, although he says the first recorded use of charcoal in oral hygiene has been credited to Hippocrates in ancient Greece.

“Activated” charcoal, which has been treated to become more porous, has long been used as a remedy in poisoning cases, since it binds to ingested toxins before they are absorbed by the body.

That reputation as a detoxifier has given rise to many consumer products. Procter & Gamble last month launched charcoal-based shampoos and conditioners across hair-care lines including Pantene, Head & Shoulders and Herbal Essences. Shiseido ’s BareMinerals makeup line launched a $28 makeup brush with “activated charcoal infused into the fibers of the brush.” A black tube of Lavanila underarm detox mask features charcoal.

Charcoal is getting a boost from social media, which has fueled beauty trends from unicorn-inspired makeup and hair colors to facial masks made of glitter and gold.

Ms. Funchess says her charcoal toothbrushing video quickly rose to become her third-most popular from over 60 videos she has posted in the past two years, behind a clothing review and a tutorial on do-it-yourself vanity mirrors.

Beth Manos Brickey, a 38-year-old food and wellness blogger in Long Beach, Calif., says that on days when she is in “selfie mode,” she uses her charcoal toothpaste and posts pictures and videos on her Instagram account. Those images “are going to get more attention with the black mouth,” she says. “It encourages dialogue.”

Write to Anne Marie Chaker at anne-marie.chaker@wsj.com