A school-based survey shows nearly one in 11 U.S. students have used marijuana in electronic cigarettes, heightening concern about the new popularity of vaping among teens.

E-cigarettes typically contain nicotine, but results published Monday in the journal JAMA Pediatrics mean a little more than two million middle and high school students have used the devices for pot. U.S. health regulators recently gave the five largest e-cigarette makers 60 days to produce plans to stop underage use of their products.

Nearly nine percent of students surveyed in 2016 said they used an e-cigarette device with marijuana. It's unclear whether marijuana vaping is increasing among teens or holding steady. It was the first time a question about marijuana vaping was asked on this particular survey.