(CNN) - Young people who vape are more likely to use marijuana, according to a study published Monday. The findings, researchers say, support the theory that nicotine rewires the developing brain, changing how people respond to and crave addictive substances.
"Adolescents have a brain that's still changing and developing," said Dr. Nicholas Chadi, the lead author on the study, who conducted the research as a fellow in pediatric addiction medicine at Boston Children's Hospital and Harvard Medical School.
When a young brain is exposed to an addictive substance such as nicotine, it "tends to be sensitized to other substances; it tends to seek a thrilling, rewarding sensation," said Chadi. "And so other substances like marijuana become more appealing."
The research, published in the medical journal JAMA Pediatrics, analyzed more than 20 pre-existing studies of people ages 10 to 24. The review found that the odds of marijuana use were 3.5 times higher in people who vaped compared to those who didn't.
That link suggests that "e-cigarettes really need to be considered in the broad category of addictive and harmful substances," said Chadi, who is now an assistant professor at the University of Montreal.
"We can't think of e-cigarettes as a less-harmful alternative to cigarettes with adolescents," he said, in part because "just like cigarettes, e-cigarettes increase your risk of using marijuana, and marijuana, we know, has several implications and negative health consequences in adolescents."
Teenage use of the drug is associated with declines in intelligence and mental function that persist into adulthood, according to some studies, and marijuana has been linked to psychosis, which can include hallucinations and delusions.
Review looked at correlation, not causation
Studies included in the new review looked for correlation, not causation, meaning that researchers couldn't prove that e-cigarettes directly caused increases in cannabis use. But when it comes to figuring out why marijuana use consistently increased in those who vaped, Chadi said "it's hard for me to find a better explanation."
"A reasonable hypothesis," he said, "is that e-cigarette use might cause at least a part of that increased risk of using marijuana."
Most of the 21 studies included in the review adjusted for gender, age, race and socioeconomic status, said Chadi. Research works best "by adjusting all of those factors, and repeating the same study in different parts of the world and using different designs," he said.
"The more times you do that, the less chances you have of finding an association that's not there, that's not true," he added.
Compared to young adults ages 18 to 24, teens younger than 17 were also more likely to use marijuana if they vaped, the review found. "The relationship seemed to be stronger in younger people versus older people," said Chadi. That makes sense, he said, because "a younger brain is more vulnerable to the effects of substances in general."
If a younger teen is exposed to e-cigarettes, "you would expect that there would be a higher risk of them using marijuana, versus an older adolescent or young adult who might have a more developed brain and might not be as sensitive to the negative effects of nicotine," he added.
Developing brains are more vulnerable, experts say
Experts have previously warned that nicotine changes the brain, interfering with development up until about 26 years old.
Adam Leventhal, director of the Health, Emotion, and Addiction Laboratory at the University of Southern California, told CNN earlier this year that "there's concern that the adolescent brain may be more vulnerable to the addictive effects of nicotine."
He added that "the circuits underlying pleasure and the pursuit of novel, enjoyable experiences develop much faster than the circuits that promote decision-making, impulse control and rational thinking."
The nation's top doctor has also weighed in. Former Surgeon General Dr. Vivek H. Murthy, in a 2016 report, said that "compared with older adults, the brain of youth and young adults is more vulnerable to the negative consequences of nicotine exposure."
The link between nicotine and marijuana is not new. A study published last year, for example, found that teens who vaped and used hookah were up to four times more likely to use marijuana later.
Some teens put cannabis into e-cigarettes, with almost 1 in 11 middle and high school students saying they have vaped marijuana, according to a report published last year.
While more than two dozen states have legalized marijuana in some way -- either for medical or recreational use -- Chadi warned that the drug is not risk-free, especially in young people. It can affect memory, learning and higher-level functioning, he said, and "those are risks that don't go away."