Researchers at the UC San Diego School of Medicine found that teenage cannabis use is associated with slower cognitive development. The study, published on April 20, identified significant differences in verbal and visual memory, attention, and information processing between teens who use cannabis and those who do not.

Lead author Natasha Wade, assistant professor in the department of psychiatry, recruited more than 11,000 individual participants from the Adolescent Brain Cognitive Development Study. Funded by the National Institutes of Health, the ABCD Study is the largest long-term brain development study to analyze how experiences affect brain development, mental health, and substance use in youth in the U.S. For the past seven years, researchers have conducted annual imaging and surveys on participants beginning at ages 9 to 10 years old.

“The brain undergoes vast changes during adolescence, trimming away unnecessary connections and making new, faster connections,” Wade said. “Because the brain is so malleable during this time, what teens are exposed to may have a more significant impact than at other times in life.”

Through self-reports and biological testing of hair, urine, and saliva, researchers found that teenage cannabis use affects visual-spatial skills, attention, learning, and information processing. The researchers confirmed their hypothesis of differences in cognitive performance, finding that cannabis users performed 5% to 10% worse than nonusers on memory recall tests. They were surprised at how consistent these results were across every domain assessed. 

“Cannabis is theorized to alter a neuromodulatory system called the endocannabinoid system,” Wade said to The UCSD Guardian. “THC, the intoxicating component of cannabis, binds to receptors in this system, and regular THC decreases the activity of those receptors. These receptors are in key brain regions for cognition, like the prefrontal cortex, and so, the reduced activity may be reflected in cognitive performance.”

Wade explained that the study did not directly look at cannabinoid receptors and brain function, but at hair samples. Researchers found that teens whose hair samples contained THC performed worse on memory tasks than those without any cannabinoids in their hair. The study accounted for factors such as prenatal exposure to substances, mental health, family history of substance abuse, and sociodemographic background. Considering these controls, Wade still identified a relationship between cannabis and cognitive impairment over time. 

Igor Grant, professor in the department of psychiatry and director of the Center for Medicinal Cannabis Research, suggested that teenage marijuana use may cause a delay in social development.

“The main problems occur in sort of habits of use, which then distracts them from everything else,” Grant said to The Guardian. “People then may lose out on certain kind of social developments. So, if you just hang around with people who are stoners all the time, you may not be doing other things, like ordinary social interactions, or maybe sports, or whatever it may be that are part of normal development. So, people get left behind a little bit, who at a very young age, are involved with cannabis or other drugs.”

Grant said that, in his opinion, declining social development as a result of cannabis use is a more pressing problem than damage to a particular part of the brain.

“There’s been a recent study that was published in English; it’s called the English biobank in England,” Grant said. “What they found, for example, was that people who were actually using cannabis, and this is modest use, obviously, actually did better on cognitive tests later on and had healthier looking brains. Does that mean cannabis made them healthy? Well, no, probably not, but it doesn’t suggest that there was some huge impact from that.”

Wade said that the study did not offer conclusive results about whether the effects of cannabis are long term and if abstinence can recover lost cognitive abilities. In continued research with the ABCD Study, Wade hopes to understand how other substances may influence adolescent health and development.