Teens who use e-cigarettes are more likely also to use conventional tobacco products, according to a new study. The news is sure to bolster those calling for stricter regulations on e-cigarettes, devices that deliver a puff of scented water vapor laced with nicotine. But the study does not prove a causal relationship -- it’s not clear if e-cigarettes are a gateway to conventional cigarettes, or another habit that teens who smoke pick up.

For their study, published Thursday in the journal JAMA Pediatrics, University of California, San Francisco, researchers Lauren Dutra and Stanton Glantz examined data on nearly 40,000 middle school and high school students who completed the National Youth Tobacco Survey in 2011 and 2012. The survey, conducted by the U.S. Centers for Disease Control and Prevention, polls youngsters about their tobacco use, attitudes toward tobacco and exposure to tobacco-related influences.

In 2011, just 3.1 percent of the minors surveyed had tried e-cigarettes at least once; by 2012, that had more than doubled to 6.5 percent in the surveyed population. Kids who used e-cigarettes were more likely to have tried conventional cigarettes and to be current smokers. In 2011, teen smokers who used e-cigarettes were more likely to say they intended to quit smoking within the year, though e-cigarette use was also associated with “lower abstinence from cigarettes.”

“While the cross-sectional nature of our study does not allow us to identify whether most youths are initiating smoking with conventional cigarettes and then moving on to (usually dual use of) e-cigarettes or vice versa, our results suggest that e-cigarettes are not discouraging use of conventional cigarettes,” the authors wrote.

Boston University public health researcher Michael Siegel criticized the authors’ conclusions in a post on his blog. He took particular issue with the part of the Dutra and Glantz’s conclusion where they opined that e-cigarettes are aggravating youth smoking.

”Despite acknowledging that they cannot tell from their study whether e-cigarette use precedes smoking or whether smoking precedes e-cigarette use, they nonetheless draw the conclusion that e-cigarette use precedes smoking,” Siegel wrote.

The study adds another piece to a still-scanty patchwork of medical literature on e-cigarettes, which first appeared on the commercial market in the early 2000s but have really taken off only within the past few years. Some smokers say e-cigarettes are a good way to help them drop regular tobacco products, and some research appears to back them up. One study, published in the Lancet last September, found that e-cigarettes were as effective as nicotine patches in helping smokers quit.

E-cigarettes also seem like a safer option for both smokers and those around them. The chief ingredients of an e-cigarette puff are propylene glycol, vegetable glycerin and usually nicotine. Nicotine by itself is addictive, but hasn’t been shown to pose a serious health risk. The e-cigarette contains no tar, and appears to be largely free of the dangerous chemicals delivered in conventional cigarettes. A study published this past December compared e-cigarette vapor and cigarette smoke. While e-cigarette vapor contained higher concentrations of nicotine, the researchers couldn’t detect traces of some of the most toxic elements of cigarette smoke (though they didn’t test for all of the more than 5,000 chemicals contained in a puff).

Still, many anti-tobacco advocates are worried that e-cigarettes make smoking seem normal, even cool, in a way that will lead to more people puffing smoke along with their e-cigarette. Just this week, the Los Angeles City Council voted to ban e-cigarettes from restaurants, bars, beaches and other public spaces.

“I will not support anything — anything — that might attract one new smoker,” City Council President Herb Wesson said, according to the Los Angeles Times.

New York has banned e-cigarette use within 100 feet of schools, and prohibited e-cigarette sales to minors. Several other states have also moved to enact restrictions.

“The panic over nicotine addiction in and of itself seems circular: Nicotine addiction is bad because addiction is bad,” Amanda Marcotte wrote at Slate on Wednesday. “Addiction is bad because don't you know that addiction is bad? I'll explain it to you just as soon as I finish this cup of coffee.”

In an editorial accompanying the study on JAMA Pediatrics, University of Illinois at Chicago health researcher Frank Chaloupka said it was high time to bring e-cigarette regulation under the jurisdiction of the Food and Drug Administration’s tobacco control powers. In the future, the FDA might try to reduce e-cigarette appeal to kids by banning flavored products and restrict marketing. Less clear, Chaloupka says, is whether e-cigarettes should be taxed, or by how much.

“While much remains to be learned about the public health benefits and/or consequences of [e-cigarette] use, their exponential growth in recent years, including their rapid uptake among youths, makes it clear that policy makers need to act quickly,” Chaloupka wrote in JAMA Pediatrics. “Adopting the right mix of policies will be critical to minimizing potential risks to public health while maximizing the potential benefits.”

SOURCE: Dutra, Lauren and Stanton Glantz. “Electronic Cigarettes and Conventional Cigarette Use among US Adolescents: A Cross-sectional Study.” JAMA Pediatrics 6 March 2014.