E-cigarettes and their tempting flavours are attracting teens who might not otherwise have smoked tobacco, new research has revealed.
The study disproves previous theories that e-cigarettes are responsible for a decline in youth cigarette smoking.
Combined e-cigarette and cigarette use among adolescents in 2014 was higher than total cigarette use in 2009, according to the University of California, San Francisco.
The authors said the low-risk youths in the study, who went on to smoke regular cigarettes, may not have used nicotine at all if e-cigarettes did not exist.
Lead author Lauren Dutra said: "We didn't find any evidence that e-cigarettes are causing youth smoking to decline.
"While some of the kids using e-cigarettes were also smoking cigarettes, we found that kids who were at low risk of starting nicotine with cigarettes were using e-cigarettes.
"Recent declines in youth smoking are likely due to tobacco control efforts, not to e-cigarettes."
The analysis builds on several previous studies that have reported that adolescents who start with e-cigarettes are more likely to subsequently smoke traditional cigarettes.
The US Food and Drug Administration (FDA) restricted e-cigarette purchases to adults ages 18 and older in August last year.
The FDA will also require a warning label on e-cigarettes, starting in August 2018, about the addictive nature of nicotine.
But the ruling does not regulate advertising or flavours and e-cigarettes continue to be sold in flavours that appeal to youth.
Researchers examined survey data from more than 140,000 middle and high school students.
Cigarette smoking among US adolescents declined during that decade - but did not decline faster after the advent of e-cigarettes in the US between 2007 and 2009.
Senior author Stanton A. Glantz said: "E-cigarettes are encouraging - not discouraging - youth to smoke and to consume nicotine, and are expanding the tobacco market."
The study was published in science journal Pediatrics on January 23.