Teenagers who use electronic cigarettes are nearly twice as likely to go on to smoke tobacco, a major study shows.
Researchers say e-cigs get children hooked on nicotine after they become attracted by their flavours and lower perceived harm.
The study of 10,000 American teenagers will fuel growing concerns that vaping acts as a gateway to smoking.
Researchers questioned 12 to 17-year-olds twice, with a year between the interviews.
Of those who had never smoked before, teens were 87 per cent more likely to try traditional cigarettes in the intervening year if they had ever used e-cigarettes.
The scientists, from the University of California, San Francisco, also found that other forms of tobacco increased the chances the children would try cigarettes.
They said: ‘Any use of e-cigarettes, hookah, non-cigarette combustible tobacco, or smokeless tobacco was independently associated with cigarette smoking one year later.’
The researchers believe that e-cigarette use provides a ‘catalyst’ which triggers a ‘causal pathway from initial use of e-cigarettes to tobacco smoking among youths’.
Writing in the JAMA medical journal, they said e-cigs are ‘initially favoured by youths’ because of the ‘flavours, social acceptability and lower perceived harm’.
But then there is a ‘transition to smoking through nicotine dependence, increasing accessibility and other pathways’.
The scientists admitted that people who try e-cigarettes may also be more prone to rebellion.
But they found that even when they took risk-taking into account, e-cig users were still more likely to take up tobacco. Health experts in Britain view e-cigarettes as a crucial tool in the fight against tobacco, and last year Public Health England even endorsed the devices.
But others are concerned about unresolved safety claims and are worried about their use among young people.
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.
A study last year found Canadian teenagers who had used e-cigarettes were 79 per cent more likely to become daily tobacco smokers a year later.
E-cigs contain liquid 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.
Experts agree that the devices are much safer than smoking tobacco – and the gadgets are thought to have helped 22,000 people a year quit.
But the Commons science and technology committee last year launched an inquiry into their use, warning of ‘significant gaps’ in knowledge over the impact that they have on health.