By the next year, 38 percent of the e-cig users had started smoking traditional cigarettes, according to the study. Just 10 percent of the young people who didn't use e-cigs had started smoking tobacco cigarettes, researchers found.
"The differences remained statistically significant and robust even when we controlled for multiple known risk factors for initiating cigarette smoking, such as age, sex, race, ethnicity, socio-economic status, sensation seeking, parental smoking and friend smoking," said Dr. Brian Primack, lead author of the study.
Primack and study co-author Dr. James Sargent offered several theories in their analysis about why e-cigarettes could be a gateway device to tobacco cigarettes.
One theory is that the e-cigs deliver nicotine slower than traditional cigarettes, which allows an e-cig users to become more tolerant of nicotine's side effects and progress to tobacco.
Primack and Sargent speculated that because e-cigarettes mimic the behavioral and sensory sensation of traditional smoking, the user is more accustomed to the practice of using traditional cigarettes. And they suggested that smoking, which has become increasingly banned in a variety of settings in recent decades, may have become somewhat "renormalized" by electronic cigarettes.
Sargent pointed out that electronic cigarettes can be marketed on television, unlike cigarettes, which had been banned from TV ads for more than four decades.
Sargent said the study should serve as a warning for policymakers at a time when "more youth than ever are using e-cigarettes and that as many as half of these adolescents are not smoking traditional cigarettes."
"It is important to continue surveillance of both e-cigarettes and tobacco products among young people so policymakers can establish research-informed regulations to help prevent e-cigarettes from becoming gateway products on the road to youth smoking," Sargent said.
Primack noted that e-cigs currently aren't subject to many of the laws that regulate traditional cigarettes, such as age limits and taxes, and that "they also come in youth-oriented flavorings ... such as apple bubble gum and chocolate candy cane" that are barred for regular smokes.
The study comes less than a month after another one in the Journal of American Medical Association also suggested that e-cigs could serve as a gateway to traditional tobacco for young users.
In that study, which looked at about 2,500 high school students in Los Angeles, 31 percent of e-cig users said they had ended up using combustible tobacco products, including cigarettes, cigars and hookah pipes, in the following six months, compared with just 8 percent of students who had never used e-cigs before.