More than a thousand businesses across the U.S. are receiving letters from the 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 the FDA calls an epidemic.

The warning provided data showing a sharp rise in underage use of the devices, including Juul, Vuse and others.

It marks a shift in the agency’s tone on e-cigarettes.

“We’ve seen this blossom into a $15 billion industry in the United States,” Thoracic surgeon Dr. Vita McCabe says. “We also know that a third of our middle schoolers have tried it.”

E-cigarettes are generally considered a less dangerous alternative to regular cigarettes, but health officials have warned that the nicotine in them is harmful to developing brains.