More American teenagers are choosing e-cigarettes as their introduction to smoking than traditional tobacco products, a study has shown.
The trend has worried experts who warn that nicotine may have harmful effects on the brains of adolescents. They also fear that e-cigarettes might act as a "gateway" to tobacco.
Scientists made the discovery after analysing data on the smoking habits of 40,000 to 50,000 students in around 400 secondary schools in the US.
They found that among 14 to16-year-olds, 9% reported using an e-cigarette in the past 30 days while only 4% had smoked a tobacco cigarette.
In the older 17-18 age group 16% reported using an e-cigarette and 14% a cigarette containing tobacco.
Dr Wilson Compton, deputy director of the US National Institute on Drug Abuse, Bethesda, which funded the research, said: " I was really surprised that among the eighth graders over a third of users of e-cigarettes hadn't previously tried tobacco cigarettes
"In a lot of kids, their first exposure to nicotine is now with these e-products."
E-cigarettes, widely used as an aid to quitting smoking, contain a heating element that produces a vapour containing nicotine but are free of many of the harmful substances associated with tobacco.
The vapour is often given sweet flavours - examples include bubble gum and milk chocolate cream - that might be attractive to younger teens, the researchers point out.
"E-cigarettes have made rapid inroads into the lives of American adolescents," said senior study investigator Dr Richard Meich from the University of Michigan.
Speaking at the American Association for the Advancement of Science (AAAS) annual meeting in San Jose, California, Dr Compton stressed that in the hands of young teenagers, e-cigarettes were far from harmless.
He said: "There's a suggestion that for adolescents this is not a safe substitute. There are changes in the brain that are unique to nicotine exposure in adolescents.
"Recent work in mice has shown significant brain effects when exposure to nicotine takes place in puberty."
Mice exposed to nicotine became hooked on cocaine much more rapidly than if the drug was administered by itself, supporting the "gateway" hypothesis that suggests the tobacco chemical can help foster dependency on other substances.
Dr Compton added: " The other potential risk is that you start with e-cigarettes, that gets you into a nicotine habit, and then you switch over to the real thing. That's speculation but what we don't want to do is look back and say I wish we'd done something.
"J ust like tobacco becomes a habit that's life long and hard to break, e-cigarettes might start you on the same pathway.
"They could not be described as harmless. Nicotine itself is not a harmless substance. It produces changes in heart rate and blood pressure, can be dangerous in high doses, and certainly wouldn't be recommended for the adolescent brain."
He said it was important parents did not fall into the trap of thinking that "vaping" e-cigarettes was harmless.
"Parents are our most important tool for shaping teenager behaviour and they may not see this as risky," said Dr Compton.
The same investigation, now in its 40th year, revealed a long-term decline in the use of tobacco cigarettes by American teenagers.
In 2014, the prevalence of smoking tobacco cigarettes among 8th, 10th and 12th grade students combined was 8% compared with 10% the year before, and 28% in 1998.
"Part of the reason for the popularity of e-cigarettes is the perception among teens that they do not harm health," said Dr Miech.
Deborah Arnott, chief executive of Action on Smoking and Health, said, "Nicotine can be harmful to the growing brain so it's best if young people avoid it. But if they're going to experiment it's better to use e-cigarettes as vaping is far less dangerous than smoking and much less addictive.
"We need to keep track but so far in the UK and the US, smoking rates are going down more than e-cigarette use is growing. This would not be the case if vaping really were a gateway into smoking. If anything it looks like e-cigarette use among young never smokers may protect them from taking up smoking."