Researchers at the University of California San Diego studied over 11,000 American teenagers from ages 9–10 through 16–17 to understand how cannabis use affects brain development. The team published their findings in Neuropsychopharmacology, tracking changes in memory, attention, language and processing speed alongside participants' cannabis and other substance use.
This is one of the largest long-term investigations of adolescent brain development in the United States, making it a significant source of evidence about cannabis and teen cognition.
Cannabis users showed slower gains in memory and thinking skills
Teenagers who used cannabis did not improve in cognitive abilities at the same rate as their peers who did not use it. Some cannabis users performed as well as, or even slightly better than, non-users in early childhood. However, once they began using cannabis, their cognitive growth slowed while non-users continued to improve steadily across memory, attention, language and processing speed.
Researchers used biological tests—hair, urine and saliva samples—alongside self-reported use to identify cannabis exposure accurately, including both recent and past use dating back several months. This careful measurement helps ensure the findings reflect real patterns rather than reporting errors.
THC appears to drive the slowdown; CBD showed a different pattern
When the team examined specific cannabis compounds, teenagers with evidence of THC (tetrahydrocannabinol), the intoxicating ingredient in cannabis, experienced greater declines in memory over time than non-users. This suggests THC may be the primary driver of the cognitive changes observed.
Teenagers with evidence of CBD (cannabidiol) exposure did not show the same pattern, though the number of participants in that group was small. Importantly, some products marketed as CBD-only still contain THC, highlighting how difficult it can be to know what is actually in cannabis products.
Small differences during adolescence may matter more than they appear
Although the cognitive differences identified were modest, researchers emphasize these changes could have real consequences. The teenage brain develops rapidly, so even small declines in memory, attention or processing speed may affect classroom performance and everyday activities. A teen who falls slightly behind in thinking speed, for example, may struggle to keep pace with schoolwork or absorb new information as quickly as peers.
The study shows correlation, not direct causation—meaning cannabis use was associated with slower cognitive growth, but the research does not prove cannabis directly caused this outcome. Also, all participants came from the United States, so findings may differ in other countries or populations.