Teenagers who vape are two and a half times as likely to become regular smokers within a year of trying e-cigs, according to new research.
A study of more than 10,000 youngsters found the battery-powered devices really are a gateway to the real thing - rather than helping people quit.
And other alternatives including hookahs, non-cigarette combustible tobacco such as cigarillos or smokeless tobacco trigger conventional smoking just as quickly.
It is the first time scientists have compared the trendy products simultaneously with subsequent cigarette use.
At the outset none of the 12 to 17 year old male and female participants had smoked a cigarette. They were followed up 12 months later.
Dr Benjamin Chaffee said: "We estimated that 'ever use' of e-cigarettes was associated with 2.53 times greater odds of subsequent cigarette use."
He added: "Among youths who had never smoked a cigarette at baseline, adjusted odds of any cigarette use initiation were approximately double for ever users of e-cigarettes, hookah, non-cigarette combustible tobacco and smokeless tobacco compared with never users.
"Odds of past 30-day cigarette use at follow-up were also approximately double for ever users of e-cigarettes, hookah, non-cigarette combustible tobacco and smokeless tobacco compared with never users."
Dr Chaffee, of the Centre for Tobacco Control Research and Education at the University of California, San Francisco, added use of more than one product increased the risk even more.
He said non-cigarette tobacco use among teenagers increased between 2011 and 2015, with e-cigs and hookahs becoming increasingly popular.
This has coincided with the rate of decrease in youth smoking slowing over the past decade.
New product risks
Dr Chaffee warned: "In light of these observed associations between non-cigarette tobacco use and future smoking, novel tobacco products have the potential to undermine public health gains in combating the smoking epidemic.
The study published in JAMA Pediatrics was based on a national questionnaire of 10,384 US adolescents who reported never having smoked a conventional cigarette when first asked in 2013 or 2014 and were then surveyed again in 2014 or 2015.
Dr Chaffee and colleagues said it is important to understand whether non cigarette tobacco use encourages conventional smoking.
He said: "We report three central findings. First, youths who initiated tobacco use with non-cigarette products were more likely to have smoked cigarettes one year later than were youths who had never used tobacco.
"Second, the odds ratios were of similar magnitude across products and between ever use and former and current use, suggesting that any use of non-cigarette tobacco, whether former or current, is similarly associated with future smoking.
"Ever users of multiple tobacco products were more likely to initiate smoking than were ever users of a single product, and product specific associations with future smoking were essentially independent, suggesting the risk of progressing to conventional cigarette smoking is increased with use of multiple forms of non-cigarette tobacco."
Dr Chaffee said a number of factors could explain the findings, inclkuding e-cigs and other non-tobacco products inducing nicotine dependence, symptoms of which have been reported by youths who use cigars and smokeless tobacco, as little as once a month.
Teenagers who use the products may also find conventional cigarettes to be more convenient and effective in satisfying nicotine cravings.
Dr Chaffee said: "Use of non-cigarette tobacco could change how youths perceive cigarettes. Of all tobacco products, adolescents generally perceive cigarettes to convey the most health risks."
He added: "This study's findings provide evidence that despite their differences, disparate alternative cigarette products contribute to a similar process that leads to cigarette use initiation.
"In policy terms, the findings provide a rationale to treat alternative cigarette products as a group and potentially extend policies that work for one product to the others, such as a ban on flavouring.
"Even if youths do not progress to smoking cigarettes, any tobacco use is harmful. The estimated health risks of noncigarette tobacco products should include the additional health consequences of future cigarette use."
Last year a study of more than 44,000 school children in Canada found those who used e-cigs were nearly twice as likely to go on to become regular tobacco smokers,
Health experts in Britain view them as a crucial tool in the fight against tobacco. Public Health England controversially endorsed the devices in its Stoptober campaign.
Schoolchildren in England are now more likely to have tried e-cigarettes than traditional cigarettes, with more than one in three 15-year-olds having used the devices, despite the fact it has been illegal to sell them to under-18s since October 2015.
E-cigarettes contain a liquid form of nicotine that is heated into vapour to be inhaled, avoiding the harm caused by tobacco smoke.
Around three million adults in Britain have used e-cigarettes in the decade or so that they have been on the market.
Health experts agree that the devices are much safer than smoking tobacco - and the gadgets are thought to have helped 22,000 people quit smoking each year.
But the Commons Science and Technology committee has launched an inquiry into their use, warning of 'significant gaps' in knowledge over the impact that they have on health.