Vaping is a "one-way bridge" to cigarette smoking among teenagers a new study suggests, days after a separate study suggested vaping is safer than cigarettes.
The study, published in the journal Tobacco Control, examined data from American pupils in their final year of high school in 2014 and again a year later.
The authors found that among teens who had never smoked a cigarette by the final year of high school (17-18-year-olds), those who had vaped in the previous 30 days were more than four times more likely to report past-year cigarette smoking when questioned at follow-up.
They said this finding was "consistent with a desensitisation process".
"Vaping as a risk factor for future smoking is a strong, scientifically-based rationale for restricting youth access to e-cigarettes."
However two academics in Britain disputed the findings with one saying the result is "trivial".
"This paper just shows that teenagers who try cigarettes are more likely to also try e-cigarettes (and the other way round) compared to teenagers who do not do such things. This is trivial," director of the Tobacco Dependence Research Unit at Queen Mary University of London (QMUL) Professor Peter Hajek said.
"People who read sci-fi novels are also more likely to watch sci-fi movies than people who do not like sci-fi. There is no reason why these activities should be performed in one order only," Hajek said.
Linda Bauld, professor of health policy at the University of Stirling, added: "This article is the latest American study to claim that using an e-cigarette can lead to tobacco smoking in teenagers - in fact the authors go as far as to describe it as a 'one way bridge' to smoking."
"If this were true it would be very worrying," Bauld said.
"We know e-cigarettes are far less harmful than smoking and we also know that teenagers are experimenting with these products."
The research comes after the first long-term study of vaping effects on ex-smokers which found after six months, people who switched from real to e-cigarettes had far fewer toxins and cancer-causing substances in their bodies than continual smokers.
Nicotine patches also appeared to be far safer than tobacco products, according to the analysis of saliva and urine samples which was published in the journal Annals of Internal Medicine.
