Researchers from the UC San Diego School of Medicine reported on Apr. 23 that teenagers who begin using cannabis experience slower gains in thinking and memory skills as they grow. The findings come from a study published in Neuropsychopharmacology, which analyzed data from more than 11,000 participants in the Adolescent Brain Cognitive Development Study.
The research is significant because adolescence is a key period for brain development, and changes during this time can affect learning and daily functioning. Natasha Wade, PhD, assistant professor of psychiatry at UC San Diego School of Medicine and lead author of the study, said: “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. These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”
The team followed over 11,000 children starting at ages 9 to 10 through ages 16 and 17. They tracked both cognitive performance and substance use by combining self-reports with biological testing methods such as hair, urine, and saliva samples. Across various skills—including memory, attention, language, and processing speed—teens who used cannabis showed restricted growth compared to those who did not use it.
In some cases younger users performed similarly or slightly better than non-users initially; however as they began using cannabis their progress leveled off while their peers continued to improve. The study also found that teens with evidence of tetrahydrocannabinol (THC) exposure showed worse memory over time than those without such exposure. Teens exposed only to cannabidiol (CBD) did not show this pattern.
“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.” She added: “Delaying cannabis use supports healthy brain development. As cannabis becomes more widely available, it’s important for families and teens to understand how it may affect the developing brain.”
The researchers note that while their findings do not prove causation between cannabis use and slowed cognitive growth—since other factors like environment or personality could play a role—they accounted for many influences including family background and prior cognitive performance.
The team plans further tracking into young adulthood to better understand long-term effects related to timing or frequency of use.
