Exactly what does marijuana do to a young person’s brain?
A new study in the American Journal of Psychiatry suggests that cannabis may actually have a more negative impact on teens’ cognitive development than alcohol.
The study’s results put up a warning sign to teens that regular use of marijuana, for instance, could have long-lasting effects on their brains.
“The study was originally designed to evaluate the impact of alcohol on adolescent cognitive development,” Patricia Conrod, PhD, lead author and professor of psychiatry at the University of Montreal, told Healthline. “This was years back, it took a while to get funded and set up. Ten years ago, we understood a lot about alcohol but very little about cannabis. In the interim years, we have come to understand more about cannabis than we did.”
She said that she would have expected to see that alcohol had a negative impact on cognitive development, but it was a surprise to see just how impactful cannabis was on young people’s cognition.
Conrod said her team took a “big data” approach to the study. They looked at 3,826 teens starting from seventh grade from 31 Montreal-area schools over the course of four years. The students who participated sent back annual reports that documented their level of alcohol and marijuana use. The researchers also gave the teens cognitive tests to gauge the teens’ working memory, perceptual reasoning, recall memory, and inhibition.
To make sure they got the most honest responses from the students, these reports were confidential. Parents and teachers, not allowed.
The study authors reported that teens who used cannabis more often than others had cognitive function changes that appeared “to be more pronounced than those observed for alcohol.”
Conrod said the results should be a cautionary tale to teens as they contemplate marijuana use at a young age.
“Our findings suggest young people should do everything they can to delay the onset of their cannabis use, if not avoid it entirely,” she added. “I do not recommend it; clearly there are health risks associated with cannabis.”
It’s certainly not the first study to look at what cannabis could do to cognitive development. In June, JAMA Psychiatry published a review that looked at 69 past studies on cannabis use among young people. The authors found that some past studies might have actually overstated “the magnitude and persistence of cognitive deficits” tied to cannabis use. They found in their review that abstaining from cannabis for 72 hours or longer could also diminish some of the negative impact of cannabis on a young person’s brain.
This new research is larger than a lot of these older studies. Danielle Ramo, PhD, associate professor in residence and licensed psychologist in the department of psychiatry and the Helen Diller Family Comprehensive Cancer Center at University of California, San Francisco (UCSF), told Healthline that making a direct comparison between alcohol and cannabis use made this study unique. She said most studies of this kind usually just look at one substance.
“This study presents stronger evidence that alcohol and cannabis both affect the teen brain by impacting memory and executive functioning,” said Ramo. “However, it went further to suggest that if cannabis use persists throughout adolescence, the impacts on cognitive functioning are greater over time, and this effect was stronger among those who started using earlier.”
Ramos explained that teens who start to use these substances earlier in life may face greater consequences as they age.
“Earlier use that persists throughout adolescence is associated with greater burden on teens’ ability to process new information and to ‘stop and think’ in the face of complex stimuli,” Ramo said. “These effects may even be greater than the effects of alcohol on the teen brain.”
She said the study’s findings are particularly timely as marijuana enters more of the mainstream.
“In an era in which cannabis laws are becoming more permissive, the message to teens should still be that cannabis use is detrimental to the teen brain, and use should be avoided to ensure healthy brain development,” Ramo added.
It’s no secret that cannabis use is becoming ever more popular among teens. The National Institute on Drug Abuse (NIDA) for Teens reports that marijuana is the most commonly used illicit substance by both teens and adults. Its study that came out last year showed that about 45 percent of U.S. 12th graders have had marijuana in their lifetime. It found that 5.9 percent of these students had marijuana or hashish on a daily basis.
Ramo said that more still needs to be understood about how these substances impact the brain. She said that alcohol hits the brain primarily in the hippocampus, the part that forms new memories. Heavy alcohol users are less likely to form new memories, something that is stronger in teens than adults. She said cannabis has been shown to also affect this part of the brain more strongly in adolescents than adults.
“A key difference between the alcohol and cannabis literature to date is related to when the effects occur on the brain. Some of the findings around the cognitive effects of cannabis use have only been shown when use begins and is heavy in adolescence,” she said. “The effects of alcohol on the brain have tended to arise regardless of whether use starts early or starts in adulthood.”
For her part, Conrod said it would be fascinating — and important — to do this kind of study with adults, but acknowledged this would be hard to carry out.
A school model gives you a built-in community that you can follow and consistently track for a set period of time, and young people’s cognition is still being dramatically molded and shaped in the teen years. She said another interesting place to look into would be neonatal development. How does cannabis consumption of pregnant women impact their babies?
For Conrod, she said the next step for her research is to look into how we can predict who will be a likely cannabis user at this age and then develop intervention strategies to warn of possible health risks for these people.
“I would like to look into how we could help young people delay their onset of cannabis use and look at those who are most at risk for experiencing mental health and cognitive harm from cannabis use,” she added.