It has been well established that the use of alcohol during adolescence can have a significant impact on cognitive functions into adulthood. However, due to drug classification and stringent regulations by governmental agencies, information about the long-term effects of marijuana use, especially by early teens, has been limited. Yet now, a new study by investigators at CHU Sainte-Justine and the University of Montreal shows that beyond the role of cognition in vulnerability to substance use, the concurrent and lasting effects of adolescent cannabis use can be observed on important cognitive functions and appear to be more pronounced than those observed for alcohol.        

Findings from the new study were published in the American Journal of Psychiatry through an article titled “A Population-Based Analysis of the Relationship Between Substance Use and Adolescent Cognitive Development.”

“While many studies have reported group differences in cognitive performance between young users and non-users, what had yet to be established was the causal and lasting effects of teen substance use on cognitive development,” explains lead study investigator Jean-François Morin, a doctoral candidate at the University of Montreal.

“Very few studies are designed to look at this question from a developmental perspective,” adds senior study investigator Patricia Conrod, Ph.D., professor in the department of psychiatry at the University of Montreal. “Our study is unique in that it followed a large sample of high school students from seventh to tenth grade using cognitive and substance-use measures. Using this big-data approach, we were able to model the complex nature of the relationship between these sets of variables.”

To understand the relationship between alcohol and cannabis use on cognitive development among adolescents at all levels of consumption (abstinent, occasional consumer, or high consumer), the research team followed a sample of 3,826 Canadian adolescents over a period of four years.

Using a developmentally sensitive design, the authors investigated relationships between year-to-year changes in substance use and cognitive development across some cognitive domains, such as recall memory, perceptual reasoning, inhibition, and working memory. Multi-level regression models were used to simultaneously test vulnerability and concurrent and lasting effects on each cognitive domain. The study found that vulnerability to cannabis and alcohol use in adolescence was associated with generally lower performance on all cognitive domains.

“Further increases in cannabis use, but not alcohol consumption, showed additional concurrent and lagged effects on cognitive functions, such as perceptual reasoning, memory recall, working memory, and inhibitory control,” Dr. Conrod notes. “Of particular concern was the finding that cannabis use was associated with lasting effects on a measure of inhibitory control, which is a risk factor for other addictive behaviors, and might explain why early-onset cannabis use is a risk factor for other addictions.”

“Some of these effects are even more pronounced when consumption begins earlier in adolescence,” Morin adds.

In a context where policies and attitudes regarding substance use are being reconsidered, this research highlights the importance of protecting youth from the adverse effects of consumption through greater investment in drug-prevention programs.

“It will be important to conduct similar analyses with this cohort or similar cohorts as they transition to young adulthood when alcohol and cannabis use become more severe,” Dr. Conrod remarks. “This might be particularly relevant for alcohol effects: while this study did not detect effects of teen alcohol consumption on cognitive development, the neurotoxic effects may be observable in specific subgroups differentiated based on the level of consumption, gender, or age.”

“We also want to identify if these effects on brain development are related to other difficulties such as poor academic performance, neuroanatomical damage, and the risk of future addiction or mental health disorders,” Morin concludes.

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