Studies have long suggested a link between marijuana use and psychosis. New research sheds further light on this association, after finding that teenagers who increase their use of the drug are more likely to experience psychotic-like episodes.
The study also reveals that the link between frequent marijuana use in adolescence and the risk of psychotic symptoms may be largely mediated by symptoms of depression.
Lead study author Josiane Bourque, of the Department of Psychiatry at the University of Montreal in Canada, and colleagues published their findings today in the Journal of Child Psychology and Psychiatry.
According to the 2015 National Survey on Drug Use and Health, marijuana remains the "most commonly used illicit drug" in the United States, with more than 22.2 million past-month users.
In recent years, legalization of marijuana for recreational purposes, medicinal purposes, or both, has risen in the U.S. Studies have indicated that this rise in legalization has reduced perceptions of marijuana as a harmful drug, leading to an increase in its use, particularly among teenagers.
In 2016, around 5.4 percent of 8th graders, 14 percent of 10th graders, and 22.5 percent of 12th graders reported having used marijuana in the past month.
Risk of psychotic-like episodes increased
For this latest study, Bourque and team sought to determine how marijuana use in adolescence might influence the risk of psychotic-like experiences (PLEs).
"To clearly understand the impact of these results, it is essential to first define what psychotic-like experiences are: namely, experiences of perceptual aberration, ideas with unusual content and feelings of persecution," notes Bourque.
The research included the data of 2,566 teenagers from Canada, all of whom were aged between 13 and 16 years. Every year for a total of 4 years, the teenagers completed questionnaires that assessed their drug use and the occurrence of any psychiatric symptoms.
Additionally, the adolescents underwent a series of cognitive tests that assessed their IQ, long-term memory, working memory, and inhibitory control skills.
Compared with teenagers in the general population, the researchers found that teenagers who increased their frequency of marijuana use from occasionally to weekly or daily were at 159 percent greater risk of having recurrent PLEs.
"Although they may be infrequent and thus not problematic for the adolescent, when these experiences are reported continuously, year after year, then there's an increased risk of a first psychotic episode or another psychiatric condition," says Bourque.
"Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms."
What explains the findings?
On looking at the cognitive effects of marijuana use among adolescents, the researchers found that increasing use of the drug was only associated with poorer inhibitory control skills.
However, the researchers found that a lack of inhibitory control could not explain the increased likelihood of PLEs among adolescents with increasing marijuana use.
Instead, the team found that an increase in symptoms of depression among adolescents appeared to mediate the link between rising marijuana use and the greater risk of PLEs.
"Our results show that while marijuana use is associated with a number of cognitive and mental health symptoms, only an increase in symptoms of depression - such as negative thoughts and low mood - could explain the relationship between marijuana use and increasing psychotic-like experiences in youth," explains Bourque.
Overall, the researchers believe that their findings highlight the importance of drug intervention programs for adolescents who are at high risk of PLEs.
"While preventing adolescent marijuana use should be the aim of all drug strategies, targeted prevention approaches are particularly needed to delay and prevent marijuana use in young people at risk of psychosis," says senior study author Patricia Conrod, also of the Department of Psychiatry at the University of Montreal.
Conrod discusses the success of a school-based drug prevention program, called Preventure, that she created, which has been shown to lower marijuana use among teenagers by 33 percent.
She would now like to see whether this program might be effective for reducing marijuana use among teenagers with psychosis.
"While the approach seems promising, we have yet to demonstrate that drug prevention can prevent some cases of psychosis," she cautions.