Vaping is a "one-way bridge" to cigarette smoking among teenagers, a new study suggests.
A team of US researchers wanted to examine whether vaping was a predictor of future cigarette smoking among 17 and 18-year-olds.
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 12th grade (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.
Meanwhile, those who had vaped in the past month were also more likely to move away from the perception of cigarettes as posing a "great risk" of harm, the authors added.
They said this finding was "consistent with a desensitisation process".
"These results contribute to the growing body of evidence supporting vaping as a one-way bridge to cigarette smoking among youth," the authors said.
"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.
Commenting on the latest study, Professor Peter Hajek, director of the Tobacco Dependence Research Unit at Queen Mary University of London (QMUL), said: "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.
"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."
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.
"We know e-cigarettes are far less harmful than smoking and we also know that teenagers are experimenting with these products.
"While we don't want to encourage that, the key public health priority is to prevent young people from starting to smoke, a habit that eventually kills one in two regular smokers.
"If trying an e-cigarette causes regular smoking, then we should be alarmed. However, this study and previous American studies which have made similar assertions have not found this, and so we must be very cautious about jumping to such a conclusion on the basis of this study."
The study comes after health experts gave vaping an emphatic thumbs up after the first long-term study of its effects in ex-smokers.
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, scientists found.
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.
Experts said they hoped the findings will reassure would-be quitters who have been confused by mixed messages about the safety of e-cigarettes.
Some previous studies suggesting that vaping is as harmful as smoking have little in common with real-world experience, it has been claimed.