Vaping - widely touted as a way to quit smoking - may instead act as a gateway for teenagers in school to pick their first cigarette, a study has warned.
The research, published in the Tobacco Control journal, identifies a 'robust association' between e-cigarette use and the increased probability of smoking a cigarette within year.
Researchers, led by those from University of Leeds in the UK, surveyed 2,836 adolescents from 20 schools in England. Some had tried tobacco but the vast majority were non-smokers. A third had used an e-cigarette.
They were surveyed again a year later and asked if they had tried a conventional cigarette, and how often. Among the adolescents who had never smoked but had tried an e-cigarette, 118 out of 343 reported smoking at least one cigarette (34 per cent) over the year.
Among the group who had not smoked and never used an e- cigarette, the figure was 124 out of 1383 (just under nine per cent).
"The findings suggest that among the teenagers who had never smoked, the use of e-cigarettes was a strong predicator that within 12 months they would have tried a conventional cigarette," said Mark Conner, from the University of Leeds.
The survey data revealed that those who used e-cigarettes and had no friends who smoked were five and a half times more likely to start smoking.
"Adolescents who have used e-cigarettes and who initially have no friends who smoke may be at particular risk of starting to smoke cigarettes," said Sarah Grogan, from Manchester Metropolitan University.
"This is particularly interesting as it runs contrary to the suggestion that adolescents who try e-cigarettes would have been likely to try smoking anyway due to factors such as peer pressure from friends who smoke," said Grogan.
The researchers also looked at the teenagers who had already smoked at least one cigarette at the start of the study. They wanted to see if there was an association between e-cigarette use and an increase in tobacco use.
Among those who had tried an e-cigarette, 24 per cent increased their cigarette smoking, whilst among those who had not tried an e-cigarette, 13 per cent increased their smoking.
It is also plausible that the use of e-cigarettes 'normalises' smoking or leads to nicotine addiction, researchers said. It could also be that the use of e-cigarettes creates friendship networks with smokers.