Toronto, Oct 30 (IANS): Use of e-cigarettes by high school students is strongly associated with cigarette smoking later, results of a large study show.

"E-cigarettes may help to re-normalise smoking," said one of the researchers, David Hammond of the University of Waterloo in Ontario.

The study, published in CMAJ (Canadian Medical Association Journal), included 44,163 students from two Canadian provinces.

The researchers looked at e-cigarette use at the start of the study in phase-1 (2013-14) and at follow-up (2014-15, with 87 schools).

They classified students into six categories -- current daily smokers, current occasional smokers, former smokers, experimental smokers, puffers and those who had never tried smoking.

Among students in both study phases, youth who used e-cigarettes in the 30 days prior to the start of the study were more likely to start smoking cigarettes and to continue smoking after one year.

At the same time, the prevalence of smoking decreased slightly over time. Therefore, if e-cigarettes are promoting youth smoking, the overall impact has been modest to date, suggests the finding.

The study only looked at smoking initiation related to e-cigarettes and not the possible impact of e-cigarettes on smoking cessation.

"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," Hammond added.

While it is highly plausible that "common factors" account for a substantial proportion of increased cigarette-smoking initiation among e-cigarette users, the authors also noted that it would be foolhardy to dismiss the likelihood that early exposure to nicotine via e-cigarettes increases smoking uptake.