(Meredith/CNN) -- Nearly 1 in 11 middle and high school students in the US has used cannabis in an e-cigarette, according to a new report.
That breaks down to 12.4% of high schoolers and 4.5% of middle schoolers -- 8.9% combined -- who have ever vaped marijuana, which is on par with or higher than what previous studies have found. The data come from the 2016 National Youth Tobacco Survey, which includes more than 20,000 students nationwide.
Health experts say this finding adds to evidence that a growing vaping trend is affecting kids' developing brains and acting as a gateway to other drugs.
The report also comes just after Dr. Scott Gottlieb, head of the US Food and Drug Administration, took aim at e-cigarette manufacturers Wednesday, warning that they must show in the next two months how they'll keep the devices out of the hands of young people.
"Kids need to be made aware that this isn't just a harmless fad," said professor Richard Miech, who studies drug use trends at the University of Michigan. He was not involved in the new report, published Monday in the journal JAMA Pediatrics.
"This high rate of cannabis use in e-cigarettes is a public health concern," study author Katrina Trivers, an epidemiologist at the US Centers for Disease Control and Prevention, said in an e-mail.
"Any form of tobacco product use is unsafe among youth, irrespective of whether it's smoked, smokeless, or electronic," added Trivers, lead epidemiologist in the agency's Office on Smoking and Health.
The rapid spread of e-cigarettes, which work by heating a liquid until it vaporizes, was flagged in a 2016 report by the US surgeon general, which cited a 900% increase in e-cigarette use by high school students from 2011 to 2015. E-cigarette use declined for the first time in 2016 but held steady the following year. Trivers said these trends are "comparable to a rollercoaster ride."
The surgeon general's report warned that e-cigs were a possible vehicle for drugs like marijuana, but at the time, it was "unclear and understudied" how often that happened.
One of the studies that did exist at the time was an anonymous survey of high school students in Connecticut. It found that 5.4% of them had vaped cannabis -- less than half of the 12.4% cited in the new report.
Perhaps this has to do with the types of students surveyed and where they lived. But the numbers "could simply reflect the fact that vaping cannabis is more popular now than it was in the past," said the author of the Connecticut study, Meghan Morean, an assistant professor of psychology at Oberlin College and an adjunct assistant professor of psychiatry at the Yale University School of Medicine. She was not involved in the latest report.
Morean is collecting new data on Connecticut students vaping cannabis, writing in an e-mail that she expects these numbers to be "considerably higher" than they were a few years ago.
Why the rise? It may have something to do with more relaxed laws around marijuana, a perception that the drug is less harmful than in the past, the increasing popularity of vaping products and how discreet some of these devices are, Morean said.
One of the most popular vapes, Juul, "looks like a flash drive ... and the kids can just tuck it away when they're done. So, how're you going to know?" Francis Thompson, the principal of Jonathan Law High School in Milford, Connecticut, previously told CNN.
Even if teachers and parents know that it's an e-cig, they may not know what's inside.
"It is very difficult (if not impossible) to tell whether a vaping device contains nicotine e-liquid or cannabis just from looking at the device or the cartridge," Morean wrote. It may not even have a smell that's detectable to others, she added.
Miech said, "I get calls every other day about Juuls. Everybody's talking about Juuls, and there seems to be lots of anecdotal evidence that Juul use has really accelerated just in this particular year.
"If that's true ... then I would expect to see a very big increase in vaping in the coming years," he added. "As vaping in general goes up, then vaping nicotine will go up, and vaping marijuana will go up. I think they're all grouped together."
A study published in August also found that teens who used e-cigarettes and hookah were up to four times more likely to use marijuana later on. However, "it's not possible to conclusively determine whether the increased popularity of e-cigarettes in recent years has led to more kids trying cannabis," Trivers said.
Other studies have drawn a link between vaping and cigarettes, suggesting that vaping is a "first step towards becoming an established cigarette user," Miech said.
Miech is also a lead investigator for a separate nationwide survey, similar to the one cited in Monday's report, called Monitoring the Future. According to its 2017 findings, 4% of eighth-graders, 9.8% of 10th-graders and 11.9% of 12th-graders have ever vaped marijuana. Miech said the new report's similar numbers bolster his findings.
His work also found 24% of students had used marijuana over the previous year, up 1.3% from 2016.
Some students are more likely than others to put cannabis in their e-cigs, according to the new report: males, tobacco users and students who live in the same household as a tobacco user. Among students who had ever used an e-cigarette, nearly a third had used one to vape marijuana.
E-cigarettes have been the most popular tobacco product among both middle and high school students since 2014, according to a CDC report.
"This whole vaping thing has come on the scene pretty recently," Miech said. With limited data, it can be hard to predict where this trend is going and what's driving it.
"That's the billion-dollar question," he said.
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