Teens who try electronic cigarettes may be more than twice as likely to progress to traditional cigarettes as their peers who haven't used the devices, a recent U.S. study finds.

The results from surveys following more than 2,300 high school students in Hawaii mirror findings from a separate analysis of smoking habits among about 2,500 Los Angeles teens published in JAMA last August, adding to evidence that using e-cigarettes may be a gateway to smoking tobacco.

"The question of whether e-cigarette use will operate to prevent smoking or promote smoking is the number one public health question of our time," said lead study author Thomas Wills, interim director of the cancer control program at the University of Hawaii Cancer Center in Honolulu.

"Now we have evidence from longitudinal studies that e-cigarette use increases the likelihood of starting to smoke among adolescents," Wills added by email. "This matters because increased smoking could lead to increased rates of cardiovascular disease and cancer - not just lung cancer but also the 16 other cancers smoking has been linked to."

About 2 million middle- and high-school students tried e-cigarettes in 2014, triple the number of teen users in 2013, the Centers for Disease Control and Prevention reported last April.

The data sparked alarm among tobacco control advocates who fear e-cigarettes will create a new generation of nicotine addicts who may eventually switch to conventional cigarettes.

Big tobacco companies, including Altria Group Inc, Lorillard Tobacco Co and Reynolds American Inc, are all developing e-cigarettes. The battery-powered devices feature a glowing tip and a heating element that turns liquid nicotine and other flavorings into a cloud of vapor that users inhale.

An international review of published research by the Cochrane Review in December concluded that the devices could help smokers quit but said much of the existing evidence on e-cigarettes was thin.

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In the latest study, Wills and colleagues surveyed teens once when they were about 15 years old and again a year later.

At the start of the study, 31% of the adolescents had tried e-cigarettes at least once, and 15% had smoked regular cigarettes. A year later, 38% of the teens admitted using e-cigarettes and 21% smoked.

Among adolescents who had never smoked at the start of the study, those who had used e-cigarettes at that point in time were about 2.9 times more likely to report smoking traditional cigarettes in the second survey.

For teens who confessed to smoking cigarettes at the start, using e-cigarettes didn't appear to influence their smoking habits by the end of the study.

Limitations of the study include a lack of data on other forms of tobacco products and the possibility that some types of e-cigarettes now available may not have been covered by the survey, the authors note in their January 25 online report in Tobacco Control.

Even so, the results solidify the evidence connecting e-cigarettes and future smoking found in previous studies of adolescents and young adults, noted Adam Leventhal, director of the Health, Emotion and Addiction Laboratory at the University of Southern California in Los Angeles.

"I am increasingly convinced that vaping's link with later smoking is a general phenomenon that is relevant to many youths," Leventhal, who wasn't involved in the study, said by email.

Nicotine addiction may fuel this transition from vaping to smoking, said Dr. Brian Primack, assistant vice president for research on health and society at the University of Pittsburgh.

"One property of highly addictive drugs like this is that the body gets used to them and needs more over time," Primack, who wasn't involved in the study, said by email. "After someone has gotten used to some nicotine in the form of an e-cigarette, they may ultimately transition to traditional cigarettes to get nicotine more efficiently."