Researchers didn't find any evidence that e-cigarettes have led to a decline in smoking.
E-cigarettes - thought to be responsible for a decline in youth cigarette smoking - are actually attracting a new population of adolescents who might not otherwise have smoked tobacco products, a new US study has warned.
Researchers at University of California, San Francisco (UCSF) did not find any evidence that e-cigarettes have caused youth smoking to decline in the US.
In fact, combined e-cigarette and cigarette use among adolescents in 2014 was higher than total cigarette use in 2009, according to the study.
The researchers concluded that the low-risk youth in the study, who went on to smoke regular cigarettes, may not have used nicotine at all if e-cigarettes did not exist.
"We did not find any evidence that e-cigarettes are causing youth smoking to decline," said Lauren Dutra, a former postdoctoral fellow at the UCSF Centre for Tobacco Control Research and Education.
"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," Dutra said.
"Recent declines in youth smoking are likely due to tobacco control efforts, not to e-cigarettes," said Dutra.
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.
In the new study, the researchers examined survey data from more than 140,000 middle and high school students who completed the Centres for Disease Control and Prevention (CDC)'s National Youth Tobacco Survey between 2004 and 2014.
The researchers found that 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.
The researchers also performed an in-depth analysis of the psychosocial characteristics of e-cigarette users.
Research has established that smokers tend to display certain characteristics that non-smokers are less likely to show, such as a tendency to live with a smoker or to wear clothing that displays a tobacco product logo.
The smokers in the national youth survey displayed these characteristics, but the adolescents who were only using e-cigarettes displayed few of these qualities.
"E-cigarettes are encouraging - not discouraging - youth to smoke and to consume nicotine, and are expanding the tobacco market," said Stanton A Glantz, professor of medicine at UCSF.
The study was published in the journal Pediatrics.