The study of nearly 40,000 youth found that e-cigarette use among the adolescents doubled between 2011 and 2012, from 3.1 percent to 6.5 percent.

The study, published in the U.S. journal JAMA Pediatrics, showed that adolescents who used the devices were more likely to smoke cigarettes and less likely to quit smoking.

The study's cross-sectional nature didn't allow the researchers to identify whether most youths initiated with conventional cigarettes or e-cigarettes.

"Despite claims that e-cigarettes are helping people quit smoking, we found that e-cigarettes were associated with more, not less, cigarette smoking among adolescents," said lead author Lauren Dutra, a postdoctoral fellow at the UCSF Center for Tobacco Control Research and Education.

"E-cigarettes are likely to be gateway devices for nicotine addiction among youth, opening up a whole new market for tobacco," she said.

E-cigarettes are battery-powered devices that look like cigarettes and deliver an aerosol of nicotine and other chemicals.

Promoted as safer alternatives to cigarettes and smoking cessation aids, the devices are rapidly gaining popularity among adults and youth in the U.S. and around the world.

Unregulated by the U.S. Food and Drug Administration, e- cigarettes have been widely promoted by their manufacturers as a way for people to quit smoking conventional cigarettes. They are sold in flavors such as chocolate and strawberry that are banned in conventional cigarettes because of their appeal to youth.

"It looks to me like the wild west marketing of e-cigarettes is not only encouraging youth to smoke them, but also it is promoting regular cigarette smoking among youth," said senior author Stanton Glantz, UCSF professor of medicine and director of the Center for Tobacco Control Research and Education.

The new results are consistent with a similar study of 75,000 Korean adolescents published last year by UCSF researchers, which also found that adolescents who used e-cigarettes were less likely to have stopped smoking conventional cigarettes.

In combination, the two studies suggest that "e-cigarettes may contribute to nicotine addiction and are unlikely to discourage conventional cigarette smoking among youths," the researchers wrote in their paper.

In a related editorial, Frank Chaloupka of the University of Illinois at Chicago, said the rapid rise in the use of e- cigarettes has stimulated a vigorous debate in the tobacco control community over the devices' potential public health impact and about how best to regulate them.

"While much remains to be learned about the public health benefits and/or consequences of ENDS (electronic nicotine delivery systems) use, their exponential growth in recent years, including their rapid uptake among youths, makes it clear that policy makers need to act quickly," Chaloupka said.

Xinhuanet