Study of more than 11,000 teens finds cannabis use tied to slower gains in memory, focus and thinking speed as well as worse memory over time during key years of brain developmen
Across a range of skills — including memory, attention, language and processing speed — teens who used cannabis showed restricted growth over time compared to those who did not.
Researchers from University of California San Diego School of Medicine have found that teenagers who begin using cannabis show slower gains in thinking and memory skills as they grow. The study, published on April 20, 2026 in Neuropsychopharmacology, analyzed data from more than 11,000 participants in the Adolescent Brain Cognitive Development (ABCD) Study, the largest long-term study of brain development in U.S. youth.
“Adolescence is a critical time for brain development, and what we’re seeing is that teens who start using cannabis aren’t improving at the same rate as their peers,” said Natasha Wade, PhD, assistant professor of psychiatry at UC San Diego School of Medicine and lead author of the study. “These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”
The researchers followed 11,036 children starting at ages 9 to 10 through ages 16 and 17, tracking both their cognitive performance and substance use. To get a clearer picture of cannabis use, the team combined self-reports with biological testing — such as hair, urine and saliva samples — which can detect recent to several months of drug exposure.
Across a range of skills — including memory, attention, language and processing speed — teens who used cannabis showed restricted growth over time compared to those who
Natasha Wade, PhD did not. In some cases, these teens performed just as well as — or even slightly better than — others when they were younger. But as they got older and started using cannabis, their progress leveled off, while their peers continued to improve.
The study also looked more closely at different components of cannabis. In a smaller group of participants, teens with evidence of tetrahydrocannabinol (THC) exposure — the main intoxicating ingredient in cannabis — showed worse memory over time than those who did not use cannabis. Teens with evidence of cannabidiol (CBD) did not show the same pattern, although that group was small.
“These results point to THC as a likely driver of the changes we’re seeing,” Wade said. “It also highlights how complicated cannabis products can be, especially since some products labeled as CBD may still contain THC.”
While the differences seen in the study were relatively modest, researchers say they could still matter. During adolescence, the brain is rapidly developing, and even small changes in memory, attention or thinking speed can affect school performance and daily life.
The researchers note that the study does not prove cannabis use directly causes these changes. Other factors — such as environment or personality — may play a role. However, the team accounted for many of these influences, including family background, mental health and use of other substances, as well as for each participant’s prior cognitive performance.
The team will continue tracking participants into young adulthood to better understand the long-term effects of cannabis use, including how timing and frequency of use may shape brain development.
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