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A 15-year-old uses an e-cigarette in Peekskill, N.Y., in April, 2015. A large new study, published Monday in the Canadian Medical Association Journal, found teenagers who used e-cigarettes were more likely to start smoking a year later.BRYAN THOMAS/The New York Times

As the federal government moves toward regulating vaping products in Canada, public-health experts across the country have been divided over the potential benefits and harms of e-cigarettes. A particular point of contention is whether e-cigarettes encourage young people to take up smoking.

A large new study, published Monday in the Canadian Medical Association Journal, found teenagers who used e-cigarettes were more likely to start smoking a year later. But rather than providing support for those who think e-cigarettes are a gateway to smoking, the University of Waterloo researchers were quick to point out it's unclear whether one causes the other.

"It's a strong, it's a consistent and it's a dramatic effect. The question is: Is it causal?" says lead author, Dr. David Hammond, a professor and Canadian Institutes of Health Research chair in applied public health at University of Waterloo's School of Public Health and Health Systems.

Dr. Hammond says the link between e-cigarettes and smoking could largely be explained by a shared common risk factor: "The type of kids that are likely to try e-cigarettes are also likely to try other things because they're risk-takers."

While vaping products, including e-cigarettes and vaping liquids, are widely available in Canada, the market operates in a regulatory grey area. Nicotine-containing e-cigarettes can be readily found in vape shops and online, but none has been approved for sale in Canada. The federal government introduced proposed legislation last year that would regulate vaping products and make them less accessible to young people. In the meantime, most provinces have created their own patchwork of rules.

At the time of the study, starting in 2013, the researchers noted non-nicotine e-cigarettes could be legally purchased by minors and accounted for a large proportion of the e-cigarette market.

The researchers examined data from participants of The COMPASS study, an ongoing cohort study of students from Grades 9 to 12 in Alberta and Ontario. They analyzed the responses of 44,163 students surveyed about their e-cigarette and cigarette use at baseline in 2013-2014, and 41,262 surveyed in a follow-up a year later. The researchers also zeroed in on a longitudinal sample of 19,130 students.

Among the longitudinal sample, those who said they used e-cigarettes at baseline were twice as likely to try smoking, and almost twice as likely to be daily smokers by the following year.

Dr. Hammond says he anticipates these findings will be seized upon by those who believe e-cigarettes increase smoking rates. But on the flip side, he says, the researchers also found the overall prevalence of smoking declined between baseline and follow-up – a trend that's consistent with what has been happening more broadly in Canada.

"As e-cigarette use has gone up among kids, smoking prevalence has continued to go down," he says. "So we conclude by saying if there is any causal effect, it's small enough that it hasn't really stalled the decline in smoking in Canada."

Dr. Tim Stockwell, director of the Centre for Addictions Research of BC and professor in the department of psychology at the University of Victoria, said he was happy the researchers provided cautious and balanced interpretations of their findings.

While he says young people should be discouraged from using nicotine, Dr. Stockwell, who was not involved in the study, believes e-cigarettes should be subsidized and people should receive prescription nicotine devices if they're chronic smokers. If e-cigarettes are adequately regulated, they are much safer than tobacco, he says. And since tobacco use in young people and adults has declined since the advent of e-cigarettes, he says, "I think they are net positive for public health and safety and should be treated accordingly."

One of the world's largest tobacco companies, British American Tobacco, has forecast revenue from its vapour products will double to $1.3-billion next year as other firms expand past traditional cigarettes