While the purveyors of e-cigarettes talk up the technology as a harmless substitute to traditional smoking, there's a fast-mounting body of research saying that's not the whole story. Not only are some e-cigarette substances even more dangerous than tobacco, but there's increasing evidence that vaping encourages people to take up smoking.

Now a new study has found that a significant portion of high-school e-cigarette users have also used their devices to vaporise marijuana. According to researchers at Yale University in the US, nearly one in five high school students is looking beyond the standard nicotine and water-based liquids sold for use with the products.

The researchers surveyed 3,847 high-school students in Connecticut and discovered that 18.7 percent of high-school e-cigarette users reported using e-cigarettes to vape cannabis or byproducts like hash oil.

"This is a relatively novel way of using marijuana, and kids are using it at a fairly high rate," said Meghan E. Morean, lead author of the study, in a press release.

Compared to smoking marijuana via conventional means, e-cigarettes – some of which are intentionally designed for use with cannabis-based liquids – offer teenagers a comparatively discreet means of imbibing the substance.

"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," said Morean.

The researchers also found that e-cigarette use among teenagers is on the rise, with over a quarter (27.9 percent) of students reporting they vaped.

The findings are published in Pediatrics.