As the number of U.S. teenagers using electronic cigarettes increases sharply, a new study has revealed another use for the device: vaping marijuana.

Roughly one-fifth of high-school students who have tried e-cigarettes said they also put cannabis, hash oils or THC-infused wax into the device, according to a study published in the journal Pediatrics on Monday.

“This is a relatively novel way of using marijuana, and kids are using it at a fairly high rate,” said Meghan Morean, co-author of the Yale University study and now assistant professor at Oberlin College, according to Yale News.

“The smell of vaping marijuana isn’t as strong as smoking it, plus the similarity in appearance of hash oil and nicotine solutions make this a really inconspicuous way of using marijuana,” Morean said. The authors note that vaporizing cannabis in liquid form can produce a much stronger effect than smoking the dried leaves.

Researchers say it’s the first confirmation that teens are using the device for something other than vaporizing nicotine. Among the 3,847 students surveyed at five high schools in Connecticut, males and younger students were more likely to vaporize cannabis using e-cigarettes.

The number of middle and high-school students who have engaged in vaping tripled to 2 million in 2014 from the previous year, the Centers for Disease Control and Prevention reported.

In 2014, a study in the journal Nicotine and Tobacco Research found that teens who used e-cigarettes were more inclined to smoke regular tobacco compared with teens who hadn’t tried the electronic device.

The authors of the study emphasize that the conclusions do not provide a link between e-cigarette accessibility and increased marijuana use, according to Yale News. Dr. Suchitra Krishnan-Sarin, a professor at Yale University who oversaw the study, told NBC News that she believes more research must be done to deliver a more accurate picture.

“We also need evidence, similar evidence from other states in the U.S., especially states in which marijuana is legal. It would be interesting to see if rates differ in terms of how teens are using this product,” said Krishnan-Sarin.