More than 40 states have banned the sale of electronic cigarettes to minors, but a new study out of the Yale School of Public Health indicates that these measures have an unintended and dangerous consequence: increasing adolescents’ use of conventional cigarettes.

Using data from the National Survey on Drug Use and Health, the research finds that state bans on e-cigarette sales to minors yield a 0.9 percentage point increase in rates of recent conventional cigarette use by 12 to 17 year olds, relative to states without these bans.

“Conventional cigarette use has been falling somewhat steadily among this age group since the start of the 21st century. This paper shows that bans on e-cigarette sales to minors appear to have slowed this decline by about 70 percent in the states that implemented them,” said Abigail Friedman, assistant professor of public health and the study’s author. “In other words, as a result of these bans, more teenagers are using conventional cigarettes than otherwise would have done so.”

This research is in press at the Journal of Health Economics.

Electronic cigarettes, which entered the U.S. market in 2007, generally cost less per use than conventional cigarettes, are perceived to be safer and offer a wide variety of flavors, making the product particularly popular among youths. According to the Centers for Disease Control and Prevention, rates of e-cigarette use by middle and high school students tripled between 2013 and 2014. “To understand the public health implications of this increase in e-cigarette use,” said Friedman, “we need to know how e-cigarettes impact conventional cigarette use. This paper provides key evidence on that question.”

Guided by her findings and the fact that habitual use of conventional cigarettes first spikes at age 16, Friedman suggests that bans on e-cigarette sales may be more effective in reducing teenage smoking if they were limited to those under 16, rather than those under 18. This middle ground solution may provide a way to reduce teen smoking while the long-term effects of vaping, still largely unknown, are being determined.

“Policy makers have been assuming that banning e-cigarette sales to minors will improve public health. This paper’s finding, that these bans increase conventional cigarette smoking among teens, suggests that we may need to rethink this conclusion,” she said.