Teens who start using cannabis before age 15 are more likely to use the drug often later in their lives. They are also more likely to develop mental and physical health problems in young adulthood compared to their peers who did not use the drug in adolescence.
Those are the findings of a new study in JAMA Network Open.
"This further builds the case that cannabis use in adolescence adversely affects the [health] trajectories of those who use it," says psychiatrist Dr. Ryan Sultan at Columbia University, who wasn't involved in the new research.
The new study used data from the Québec Longitudinal Study of Child Development. Researchers in Montreal, Canada, have been following more than 1,500 kids since birth into young adulthood to understand the factors that influence their development and their health. Among the various aspects of the kids' lives and habits scientists have recorded is cannabis use between ages 12 and 17.
The researchers found that a majority of teens — 60% — didn't use cannabis in adolescence. Among the remaining 40%, half started using cannabis in their late teens and by the time they turned 17, they used the drug infrequently — less than once a month. The remaining 20% started using marijuana before they turned 15, and by age 17, they were using it at least once a month.
This last group was more likely to seek health care in young adulthood for both mental and physical health conditions compared to those who didn't use the drug as young teens.
"The risk is concentrated among those who start early and use frequently," says psychologist and the study's main author Massimiliano Orri at McGill University.
Early, frequent cannabis users had a 51% higher chance of seeking care for mental health problems in young adulthood compared to those who didn't use the drug. And that risk was calculated after controlling for a range of confounding factors that are known to influence health, like bullying or lack of parental involvement.
Similarly, early and frequent users of cannabis had an 86% higher chance of needing care for physical health needs.
"We have some indication that respiratory problems were most frequently reported and also accidents and unintentional injuries," says Orri.
These physical health problems may have to do with intoxication from cannabis, but could also be due to withdrawal symptoms, write Orri and his colleagues in the paper.
"That certainly makes sense," says psychologist Krista Lisdahl at the University of Wisconsin, Milwaukee, who also studies teenage brain development and health, but wasn't involved in the new research. "It's alarming and something that we really need to be tracking more closely here in the United States as well."
A major strength of the study is that Orri and his colleagues controlled for so many confounding factors, adds Lisdahl.
"There are a lot of factors that are co-linked with physical health as well as mental health," says Lisdahl. And the new study controlled for many of them, including "family, parental conflict and parenting style, as well as parental monitoring of the teens." They also looked at factors at the individual level like social skills, peer relationships and whether a kid was victimized by peers.
Prior studies have also found a link between teen cannabis use and a higher risk of developing mental health symptoms.
Sultan's recent research found a two to four times higher likelihood of developing psychiatric disorders for teens who used cannabis recreationally compared to adolescents who don't use the drug at all. Other studies have suggested a link between early cannabis use and psychosis in youth. He and his colleagues also found a higher risk of other impacts like poorer grades and truancy.
"The adolescent brain is continuing to develop in a very dynamic fashion during the adolescent period and all the way into young adulthood," says Lisdahl. "Using something like cannabis regularly during this period might disrupt that healthy neural development, especially in areas of the brain that are related to executive functioning, which is like problem-solving, planning, maybe controlling other kinds of behaviors and impulses, but also emotion regulation," she says.
Take for example, a teen who's prone to anxiety who turns to cannabis to feel less anxious, says Sultan. "If you start to do that on a regular basis, this is now your method for managing your anxiety," he says. "This becomes your coping skill and you become atrophied in any ability to manage it in another way."
The same thing can happen for someone who uses cannabis to manage their mood, he adds.
So, for someone who's already predisposed to some mental health symptoms, starting cannabis use in adolescence can make it more likely those symptoms will worsen over time into mental health disorders.
For all these reasons, Sultan — a child and adolescent psychiatrist — says he often talks to teens and parents about delaying using cannabis until they are 25, to minimize the risks of health and behavioral issues later on in life.
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