(CNN) – More than 1,000 businesses across the U.S. are getting letters from the U.S. Food and Drug Administration, warning them to stop selling e-cigarettes to minors.
It's part of the largest enforcement effort in agency history to crack down on what it calls an "epidemic."
The most recent National Youth Tobacco Survey finds one in five high school students uses tobacco – and by far the most common method of consumption is e-cigarettes, with 2.1 million middle and high school users.
"It's really addictive," teenager Emma Hudd said.
The FDA has warned kids against the products in social media campaigns.
"Vaping can deliver nicotine to your brain, reprogramming you to crave more and more. Don't get hacked," one PSA said.
On Wednesday, the FDA magnified its message, sending five major e-cigarette manufacturers and more than 1,100 businesses warning letters – and, in some cases, fines for selling to minors.
The agency warned companies about labeling and flavors that attract or target kids, and about distributing products in easy-to-hide packages that could pass for candy.
"Which is why I'm sure teens like it, because in school they're just kind of everywhere. They can easily take a hit when no one's looking," teen Andrew Paulus said.
Earlier this year, the FDA asked manufacturer Juul Labs to submit documents to understand why their products are so popular with youth.
The company said in a statement that "appropriate flavors" help adult smokers switch, while adding it will work with the FDA "to keep e-cigarettes out of the hands of young people."
The FDA is giving Juul and other manufacturers 60 days to share plans to reduce youth sales or face possible criminal or civil action.