A major new study tracking thousands of American children over several years is raising concerns about how cannabis use may affect developing brains.

Researchers at the University of California San Diego followed more than 11,000 participants from late childhood into their teenage years, making it the largest long-term study of its kind in the U.S. The findings suggest that teens who begin using cannabis show slower progress in key cognitive areas compared to their peers.

According to the research, differences emerged across multiple skills, including memory, attention, language, and processing speed. While many of the teens started out on similar footing, those who reported cannabis use over time didn’t improve at the same pace as those who abstained.

“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 lead author Natasha Wade, an assistant professor of psychiatry at UC San Diego. “These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning.”

To build a clearer picture, the research combined self-reported behavior with biological testing, including hair, urine, and saliva samples, to confirm substance use over time. That approach helped strengthen the findings, which remained consistent across multiple areas of thinking ability.

The study also examined specific components of cannabis. In a smaller group, teens exposed to THC, the substance responsible for the drug’s psychoactive effects, showed more noticeable declines in memory over time. Those linked to CBD did not display the same pattern, though researchers cautioned that products labeled as CBD can still contain traces of THC.

While the cognitive differences observed were not dramatic, experts say the timing matters. Even modest changes during adolescence, a period when the brain is rapidly developing, can influence academic performance and long-term outcomes.

The research does not definitively prove that cannabis causes these changes, as factors like environment, personality, and mental health may also play a role. However, scientists accounted for many of these variables, including prior cognitive ability and family background, to better isolate the relationship.

The findings come as cannabis becomes more accessible across the country, and as some teens continue to use it despite declining overall rates. In places like New York, recent reports suggest roughly one in five teenagers still use marijuana, while cases involving high-dose edibles have also increased.

For researchers, the takeaway is less about alarm and more about awareness.