Teens who used e-cigarettes and hookah were up to four times more likely to use marijuana later, according to a study published Monday in the journal Pediatrics.
Researchers from the University of Pennsylvania and the University of Southern California surveyed 2,668 students at 10 public high schools in Los Angeles beginning in fall 2013, when they were 14 years old and in ninth grade.
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The students answered a paper-and-pencil, phone or internet survey that asked whether they had ever used (or had used in the past 30 days) e-cigarettes, combustible cigarettes or a hookah water pipe. They were also asked whether they had used any type of marijuana product. The use of less popular tobacco products such as smokeless tobacco and cigars was not studied.
In a followup survey in fall 2015, when the students were 16 years old and in 11th grade, the survey asked whether they had used three types of marijuana products: combustible, vaped or edible.
The researchers found that the students who had tried e-cigarettes when they were freshmen had a more than three-fold greater likelihood of ever using marijuana and using marijuana in the past 30 days than students who hadn't tried e-cigs.
"When we were thinking about this topic, we were kind of just reflecting on the fact that we have more tobacco products on the market now than ever," said Janet Audrain-McGovern, an associate professor in the Department of Psychiatry at the University of Pennsylvania's Perelman School of Medicine and lead author of the study. "And we've seen cigarette smoking decline among young people, but we've seen increase in use of these other tobacco products. At the same time, we've also seen in many areas of the country a lessening of the restrictions surrounding marijuana use."
The researchers controlled for factors that could be associated with an increased risk of marijuana use, including family history of both tobacco and marijuana use, peer use, depression and impulsivity.
"That doesn't mean necessarily that the association is causal," cautioned Dr. Ana Navas-Acien, a professor of environmental health sciences at Columbia University's Mailman School of Public Health, who was not involved with the study. "I think we need to be very careful with interpreting these relationships. But it seems that the use of these tobacco products, including combustible cigarettes and e-cigarettes, seems to precede the use of marijuana somehow."
Students who had used hookah at the time of the first survey had a more than three-fold increase in the odds of having tried marijuana two years later and over a fourfold increase in the odds of using marijuana in the previous 30 days.
One potential reason for the strong tie between hookah and marijuana use is that hookahs allow the mixing of multiple combustible products and could be used with marijuana itself, said Navas-Acien, who also studies e-cigarettes and hookah usage.
Along with the potential of addiction to nicotine and the substances in marijuana, there are toxicants in hookah that can lead to increased health concerns, she said. Smoking hookah can result in 1.7 times more nicotine exposure than a regular cigarette, the researchers said.
Nicotine changes the brain and enhances the pleasure experienced from subsequent drug exposures, Audrain-McGovern said. The changes in the airway from hookah smoking and e-cigarette vaping could also make smoking and vaping marijuana easier because of decreased sensitivity to irritation.
"Once you start vaping, I think you become known as the person who vapes, who's cool," said Richard Miech, a professor at the University of Michigan and principal investigator of Monitoring the Future, an ongoing study of adolescent drug use. He was not involved with the new study. "You get invited to parties where people are going to smoke cigarettes and smoke marijuana because you vape already. So there's that social component."
Students probably develop peer groups that they wouldn't otherwise, and that is a possible link between vaping and future cigarette and marijuana smoking, Miech said.
"And I think there are probably other social pathways as well, other than just friendship networks," he added. "It probably changes your attitude. You probably vaped for a while and say 'I don't see any problem. I'm not dropping dead. I guess it's not as dangerous as they're telling me it is.' "
More than 11% of high schoolers use e-cigarettes, and between 5% and 11% smoke hookah, according to the authors of the study.
"The brain is still developing during the teen years," Hongying Dai, an associate professor at Children's Mercy Hospital and the University of Nebraska Medical Center, who researches tobacco use and health disparities, wrote in an email. She was not involved in the study.
"Nicotine exposure might lead to changes in the central nervous system that predisposes teens to dependence on other drugs of abuse. Experimenting with e-cigarettes might also increase youth's curiosity about marijuana, reduce perceived harm of marijuana use, and increase the social access to marijuana from peers and friends."
Teens had about 3.5 times greater likelihood of using marijuana in the previous two years for each additional tobacco product used, the study found.
"And we know that these two drugs sort of go together," Audrain-McGovern said. "So if you smoke cigarettes and then you later try marijuana, you're more likely to quickly progress to using marijuana again."
The surveys did not measure the frequency of marijuana use, so whether e-cigarette or hookah use aligns with specific levels of marijuana use is unknown, researchers said. More data demonstrating a relationship between e-cigarettes and hookah use and marijuana use would be useful in determining policies to protect teen health, they said.