A study published on Thursday found an association between smoking and e-cigarette use among adolescents but didn't answer a pressing public-health question on whether e-cigarettes acted as a gateway to smoking.
Published in JAMA Pediatrics, the study found that among those who have smoked, adolescents who also used e-cigarettes were less likely to have given up smoking than those who did not use e-cigarettes.
The authors of the study, Lauren Dutra and Stanton Glantz, a prominent opponent of e-cigarettes, concluded that the "use of e-cigarettes does not discourage, and may encourage, conventional cigarette use among U.S. adolescents."
Critics say the results do not support such a conclusion.
Dr. Michael Siegel, a professor of community health sciences at Boston University School of Public Health who has spoken publicly in favor of e-cigarettes, said that while the study draws a correlation between smoking and e-cigarette use, there was no evidence to prove e-cigarettes led to smoking.
"The authors seem to have an axe to grind," he said. "I could equally argue that what this study shows is that people who are heavy smokers are attracted to e-cigarettes because they are looking to quit."
The study was funded by the National Cancer Institute and conducted by the University of California San Francisco's Center for Tobacco Control Research and Education.
It comes as the U.S. Food and Drug Administration prepares to gain regulatory control over e-cigarettes, which generated sales of nearly $2 billion last year, and which some analysts believe could eventually exceed the $80-billion tobacco market.
The aim of the study was to further understand the relationship between e-cigarette use, conventional cigarette use and quitting among U.S. adolescents.
It relied on data from some 40,000 adolescents who completed the 2011 and 2012 National Youth Tobacco Surveys carried out by the Centers for Disease Control and Prevention.
The authors said that since the study did not follow its subjects over time, they couldn't determine whether most youths began smoking conventional cigarettes before moving to e-cigarettes, or vice versa.
Adult smoking rates have fallen to 18 percent from 43 percent in 1965. Even so, more than 3,200 young people a day under the age of 18 try their first cigarette, a recent government report found, and the use of e-cigarettes by young people doubled between 2011 and 2012.
E-cigarettes are battery-powered cartridges that look like cigarettes and contain a nicotine liquid that, when heated, creates an inhalable vapor. This vapor, advocates say, is less dangerous than traditional cigarette smoke since it does not contain lung-damaging tar.
Nicotine itself is considered relatively benign compared with cigarettes, but data on the long-term safety of e-cigarettes, which contain a variety of chemicals, is limited.
That uncertainty has led a number of cities, including New York, Chicago, Boston and, most recently, Los Angeles, to restrict the use of e-cigarettes in restaurants, bars, nightclubs and other public spaces.