The study by Canadian scientists found that the impact of cannabis use on skills, memory, and behavior is worse than alcohol.
The research also suggests cannabis works as a “gateway drug” by reducing the mental capacity to control the impulses associated with addictive behavior.
3,800 adolescents were tracked from the age of around 13 over a period of four years.
The researchers, from the University of Montreal, are urging teenagers to delay their use of cannabis, saying it affects “important cognitive functions.”
A statistical link was already established between drug use and impaired cognition in teenagers. However, previous studies had not been able to establish that cannabis use was causing the drop in cognition.
Dr. Patricia Conrod, from the Department of Psychiatry at Université de Montréal, said, “Our study is unique in that it followed a large sample of high school students from 7th to 10th 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.”
The researchers looked at year-to-year changes in substance use and cognitive development in recall memory, perceptual reasoning, inhibition, and working memory.
The study, published in the American Journal of Psychiatry, found that cannabis and alcohol use in adolescence was associated with generally lower performance on all cognitive domains.
“However, 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,” said Conrod
“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 behaviours, and might explain why early onset cannabis use is a risk factor for other addictions.”