Teenagers who smoke e-cigarettes are more likely to take up smoking tobacco in later life, new research has warned.
The electronic fags "normalise" smoking and lead to young adults taking up smoking after continuous use after a year.
Researcher Dr David Hammond from the School of Public Health and Health Systems at the University of Waterloo, Ontario, Canada said: "While our study provides strong evidence that e-cigarettes are associated with smoking initiation among youth, the association is unclear.
"E-cigarettes may help to re-normalise smoking; however, the association between e-cigarettes and smoking may simply reflect common factors rather than a causal effect: the same individual and social risk factors that increase e-cigarette use may also increase the likelihood of youth smoking."
In the study researchers looked at 44,163 students in more than 80 schools in Canadian provinces Albert and Ontario in two phases, between 2013/14 and a follow up between 2014/25.
They split students up into six categories, current daily smokers, current occasional smokers, former smokers, experimental smokers, puffers and those who had never tried it.
In the 30 days prior to the study youths who used e-cigarettes were more likely to begin smoking tobacco and to continue smoking after one year.
Dr Hammond said: "Youth may be trying e-cigarettes before smoking because they are easier to access.
"Until recently, youth could legally purchase e-cigarettes without nicotine, whereas regular cigarettes cannot be sold to young people under 18 years of age."
But the prevalence of smoking has decreased slightly over time leading the authors to conclude that if e-cigarettes are promoting youth smoking, the overall impact has been modest.
The scientists have recommended that further research should be conducted on the link between smoking initiation and nicotine e-cigarettes, compared with non-nicotine cigarettes.
Dr Hammond added: "The findings from our study provide support for both sides of the debate. It is highly plausible that 'common factors' account for a substantial proportion of increased cigarette-smoking initiation among e-cigarette users.
"At the same time, it would be foolhardy to dismiss the likelihood that early exposure to nicotine via e-cigarettes increases smoking uptake.
"Attributing the relative importance of these two factors will not be straightforward, and represents a critical challenge to the research community."
The research was published in the Canadian Medical Association Journal.