(Reuters Health) - Adolescents who have smoked e-cigarettes are more than three times more likely to move on to marijuana than teens who have never tried vaping, a U.S. study suggests.

At 10 high schools in Los Angeles, a total of 2,668 students participated in two surveys - one in the fall of ninth grade and another in the fall of eleventh grade. None of the ninth graders had used marijuana, but 43 percent had smoked cigarettes or sampled hookah, and 48 percent had tried vaping.

By eleventh grade, almost nine percent of the students were using marijuana and roughly 18 percent said they had tried it at some point in the previous two years.

Among teens who hadn’t tried e-cigarettes by ninth grade, about 16 percent tried marijuana by eleventh grade, the study found. By contrast, 40 percent of students who had tried e-cigarettes by ninth grade had tried marijuana two years later.

Kids who were using any tobacco products in ninth grade - whether e-cigarettes, traditional cigarettes, or hookah - were more than twice as likely to try pot over the next two years as ninth graders who didn’t have any history of tobacco use, the study also found.

“These tobacco products may give an adolescent experience inhaling such that they are more willing to try other substances that also require inhaling hot smoke or vapor,” said lead study author Janet Audrain-McGovern of the Perelman School of Medicine at the University of Pennsylvania in Philadelphia.

“The airway may get used to inhaling hot substances, making the transition to inhaling marijuana smoke less irritating,” Audrain-McGovern said by email.

Beyond just making it easier to inhale, sampling various tobacco products also exposes teens to nicotine, which can alter executive function and the reward system in the brain, Audrain-McGovern added. Nicotine exposure may enhance the level of pleasure experienced by subsequent drugs like marijuana.

While teen smoking has long been linked to an increased risk of drug use, U.S. adolescents today are more apt to try vaping than smoking traditional cigarettes - and less is known about how e-cigarettes impact future substance use.

Big tobacco companies are all developing e-cigarettes. The battery-powered devices feature a glowing tip and a heating element that turns liquid nicotine and other flavorings into a cloud of vapor that users inhale.

In the study, 55 percent of female students and 45 percent of male students hadn’t tried any tobacco products by ninth grade, researchers report in Pediatrics.

One limitation of the study is that researchers lacked data on the frequency of marijuana use and the exact type of liquid nicotine or flavorings used with e-cigarettes or hookah. It also wasn’t a controlled experiment designed to prove whether or how using these tobacco products might directly cause teens to use marijuana.

Still, the findings underscore the potential for tobacco products to make marijuana more appealing to teens, said Hongying Dai, a researcher at the University of Nebraska Medical Center in Omaha who wasn’t involved the study.

“Experimenting with e-cigarettes or hookah might increase youth’s curiosity about marijuana, reduce perceived harm of marijuana use, and increase the social access to marijuana from peers and friends,” Dai said by email.

It’s also possible that some teens who became marijuana users in the study would have tried it regardless of whether they had used tobacco first, said Dr. Benjamin Chaffee, a tobacco researcher at the University of California San Francisco who wasn’t involved in the study.

“They could be high-risk youth with any underlying tendency to engage in use of multiple substances,” Chaffee said by email. However, “the authors demonstrated a strong association after using methods to account for well known risk factors that would lead youth to use both tobacco and marijuana, like family history of smoking and tendency for impulsive behavior.”

SOURCE: bit.ly/2M6o6Ju Pediatrics, online August 6, 2018.