UC San Diego study links teenage cannabis initiation to slower gains in memory and thinking skills

Researchers at the University of California San Diego School of Medicine have reported that teenagers who start using cannabis exhibit slower development of memory and thinking abilities than peers who do not use the substance. The analysis, published April 20, 2026, in Neuropsychopharmacology, drew on data from 11,036 participants in the Adolescent Brain Cognitive Development (ABCD) Study, the largest long-term investigation of brain development in U.S. youth (today.ucsd.edu).

Lead author Natasha Wade, PhD, assistant professor of psychiatry at UC San Diego, and colleagues followed children from ages 9–10 through ages 16–17. They tracked cognitive performance alongside substance use, combining self-reports with biological measures including hair, urine, and saliva samples to detect cannabis exposure over recent months to longer periods (today.ucsd.edu; nature.com).

Across domains that included immediate and delayed memory, processing speed, inhibitory control, visuospatial processing, language, and working memory, cannabis users showed altered trajectories with restricted growth relative to non-users (βs = −0.11 to −0.52). Some youths who later used cannabis performed as well as or slightly better than peers in late childhood, but their gains leveled off after initiation while non-users continued to improve (nature.com; sciencedaily.com).

In a secondary analysis of participants with repeated hair toxicology (n=645), evidence of delta-9-tetrahydrocannabinol (THC) exposure by age predicted worse episodic memory compared with controls (β = −0.60). No comparable difference appeared for those with cannabidiol (CBD) exposure versus controls, though the CBD group was small. Wade noted that the results point to THC as a likely driver and underscore the complexity of cannabis products, some of which labeled CBD may still contain THC (today.ucsd.edu; nature.com).

The observed differences were relatively modest. Researchers cautioned that the study does not establish direct causation; environmental factors, personality, or other variables could contribute. Analyses accounted for sociodemographics, family history of substance use disorder, prenatal exposures, early psychopathology, other substance use, and each participant’s prior cognitive performance, along with nesting for individual, site, and family effects (nature.com; sciencedaily.com).

“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.” She added that delaying cannabis use supports healthy brain development as the substance becomes more widely available (today.ucsd.edu).

The team plans to continue following the cohort into young adulthood to examine longer-term effects, including the roles of timing and frequency of use. The work was supported in part by the National Institute on Drug Abuse and involved collaborators from UC San Diego, the University of Sydney, University of Wisconsin-Milwaukee, Columbia University, and others (today.ucsd.edu; nature.com).