Large U.S. brain study tracks 11,000 adolescents
Researchers at the University of California San Diego School of Medicine report that teenagers who begin using cannabis show slower gains in thinking and memory skills as they grow older. The findings, published April 20, 2026 in Neuropsychopharmacology, draw on data from more than 11,000 participants enrolled in the long-running Adolescent Brain Cognitive Development Study, the largest study of youth brain development in the United States.
The team followed 11,036 children beginning at ages 9 and 10 through ages 16 and 17. Researchers tracked cognitive performance alongside substance use, combining self-reported information with biological testing, including hair, urine and saliva samples, to detect cannabis exposure.
Slower improvement in memory and thinking skills
Across multiple cognitive domains such as memory, attention, language and processing speed, teens who used cannabis showed more limited growth over time compared with peers who did not use the substance.
Lead author Natasha Wade, PhD, assistant professor of psychiatry at UC San Diego School of Medicine, said the differences became clearer as participants aged.
“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,” Wade said. “These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”
In some cases, teens performed similarly to or slightly better than their peers before initiating cannabis use. However, once use began, their progress tended to level off while others continued to improve.
THC identified as likely contributing factor
The study also examined specific cannabis components. In a smaller subgroup, adolescents with evidence of exposure to tetrahydrocannabinol, the primary psychoactive compound in cannabis, showed worsening memory performance over time compared with non-users.
Participants with evidence of cannabidiol exposure did not show the same pattern, though researchers noted that the CBD sample was limited.
“These results point to THC as a likely driver of the changes we’re seeing,” Wade said, adding that some products marketed as CBD may still contain measurable levels of THC.
Modest differences with potential long-term impact
While the observed effects were described as modest, researchers emphasized that adolescence is a period of rapid neurological development. Even small disruptions in attention, memory or processing speed may influence academic performance and daily functioning.
The authors caution that the findings do not establish direct causation. Other variables such as environment, personality and family background could also play roles. However, the analysis accounted for numerous factors, including prior cognitive ability, mental health status and use of other substances.
The research team plans to continue following participants into young adulthood to assess longer-term effects and to better understand how timing and frequency of cannabis use may shape brain development.
“Delaying cannabis use supports healthy brain development,” Wade said. “As cannabis becomes more widely available, it’s important for families and teens to understand how it may affect the developing brain.”