States that have made it illegal for minors to buy electronic cigarettes, like Connecticut, have seen an uptick in the number of adolescents smoking conventional cigarettes, according to Yale researchers.

The researchers, from Yale School of Public Health, analyzed data from the National Survey on Drug Use and Health and found that state bans on e-cigarettes to minors resulted in a 0.9 percentage point increase in the rate of conventional cigarette use among young people ages 12 to 17.

E-cigarettes don’t burn tobacco or produce smoke, instead delivering nicotine by producing a vapor.

The use of e-cigarettes is commonly referred to as “vaping,” and some e-cigarette manufacturers have been criticized for making flavors that appeal to young consumers, such as bubble gum. E-cigarettes also tend to cost less than conventional cigarettes.

“Conventional cigarette use has been falling somewhat steadily among this (12-17) age group since the start of the 21st Century,” Abigail Friedman, assistant professor of public health and the study’s author, said in a statement.

“This paper shows that bans on e-cigarettes sales to minors appear to have slowed this decline by about 70 percent in the states that implemented them,” she said. “As a result of these bans, more teenagers are using conventional cigarettes than otherwise would have done so.”

The study, which was published this week in the Journal of Health Economics. The National Survey on Drug Use and Health, from which researchers drew data, is conducted by the National Institute on Drug Abuse, a federal agency.

More than 40 states have banned the sale of e-cigarettes to minors under 18, including Connecticut where such a ban took effect in October 2014.

Additionally, earlier this year, state lawmakers moved to prohibit the use of e-cigarettes in all places where tobacco cigarettes are banned. That means vaping is banned in buildings owned, operated or leased by the state; all health care institutions; restaurants and bars; schools; elevators; public and private college dorms; dog race tracks, and places that simulcast off-track betting race programs or jai alai games.

Regulation of e-cigarette use has drawn vocal supporters and critics in Connecticut. During public hearings earlier this year, vaping proponents testified before lawmakers that e-cigarettes were the only things that helped them kick long-standing tobacco cigarette habits. Opponents of vaping worried about e-cigarettes’ potential health risks and their appeal to younger smokers.

The use of e-cigarettes among middle and high school students tripled between 2013 and 2014, according to Yale researchers citing U.S. Centers for Disease Control data. Therefore, Friedman suggests that banning e-cigarette sales may be more effective in reducing teen smoking in states if the ban was for those under 16 rather than those under 18.

Habitual use of conventional cigarettes typically spikes at age 16, she said.

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