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SAN DIEGO — Researchers at UC San Diego have released findings from the largest long-term study of its kind in the U.S., revealing that adolescents who use marijuana show slower gains in critical cognitive skills, including memory, attention and thinking speed.
The UC San Diego School of Medicine published the study today after tracking more than 11,000 children starting at ages 9-10 and following them through ages 16-17 to measure both cognitive performance and substance use patterns.
This research distinguishes itself from previous studies through its use of toxicological testing—including hair and saliva samples—alongside traditional self-reporting methods, providing a more accurate assessment of cannabis use.
Teens using marijuana demonstrated somewhat slower rates of improvement on cognitive tests such as verbal recall and pattern comparisons compared to their non-using peers.
“After all of that, we see that there is still this relationship between patterns of cognitive performance over time and their cannabis use, that those who are using cannabis aren’t showing the same growth or gains as their peers who are not using cannabis,” said Natasha Wade, a clinical neuropsychologist at UC San Diego and lead author of the research.
Adolescence represents a particularly vulnerable period for marijuana’s effects on developing brains.
“What concerns me is that adolescence is a sensitive time,” said Wade.
Researchers controlled for confounding factors such as prenatal cannabis exposure and the teens’ mental health status, which strengthened the study’s conclusions about the direct link between marijuana use and cognitive development.
The testing revealed a specific connection between THC—marijuana’s primary intoxicating chemical—and poor memory performance. Teens who tested positive for THC through hair samples demonstrated the worst memory outcomes over time.
Even minor cognitive changes could carry significant practical implications, according to Wade.
“If there is even these minor small changes in their processing speed, in their ability to learn or recall information, when they have to do things for the first time like respond quickly to that brake light in front of them so they don’t get in a car accident, or when they need to do well on a test to get into the college they want, or just graduate, that any of these little changes might actually add up to have real-life, long-term outcome differences for them,” Wade told CBS 8.
Based on the research, Wade recommends that both parents and teenagers delay cannabis use.
“For parents and really for the teens themselves, I think it’s just: wait. It may be fine later on; we don’t fully have the data on that yet. But what we can say right now is that we know it’s riskier,” said Wade.
The research team plans to continue following the study participants to answer additional questions about cannabis use and cognition. Researchers are seeking additional funding from the National Institutes of Health with the goal of extending the study for another five to ten years, according to Wade.