A school-based survey shows nearly 1 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 mean 2.1 million middle and high school students have used them to get high.
The devices are generally considered a less dangerous alternative to regular cigarettes, despite little research on their long-term effects including whether they help smokers quit.
The rise in teenagers using them has alarmed health officials. Last week, the Food and Drug Administration gave the five largest e-cigarette makers 60 days to produce plans to stop underage use of their products.
Nearly 9 percent of students surveyed in 2016 said they used an e-cigarette device with marijuana, according to Monday's report in the journal JAMA Pediatrics. That included one-third of those who ever used e-cigarettes.
The number is worrying "because cannabis use among youth can adversely affect learning and memory and may impair later academic achievement and education," said lead researcher Katrina Trivers of the Centers for Disease Control and Prevention.
Students who said they lived with a tobacco user were more likely than others to report vaping marijuana.
It's unclear whether marijuana vaping is increasing among teens or holding steady. The devices have grown into a multi-billion industry, but they are relatively new.
It was the first time a question about marijuana vaping was asked on this particular survey, which uses a nationally representative sample of students in public and private schools. More than 20,000 students took the survey in 2016.
A different survey from the University of Michigan in December found similar results when it asked for the first time about marijuana vaping. In that study, 8 percent of 10th graders said they vaped marijuana in the past year.
"The health risks of vaping reside not only in the vaping devices, but in the social environment that comes with it," said University of Michigan researcher Richard Miech. Kids who vape are more likely to become known as drug users and make friends with drug users, he said, adding that "hanging out with drug users is a substantial risk factor for future drug use."