"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 Natasha Wade, PhD, assistant professor in the Department of Psychiatry at UC San Diego School of Medicine and lead author of the study. "These differences may seem small at first, but they can add up in ways that affect learning, memory and everyday functioning."

Tracking Cannabis Use and Cognitive Growth

The research team followed 11,036 children from ages 9 to 10 through ages 16 and 17. During that period, the scientists monitored changes in cognitive performance as well as the participants' use of cannabis and other substances.

To measure cannabis exposure more accurately, the researchers used both participant reports and biological tests. Hair, urine and saliva samples helped identify drug exposure ranging from recent use to use that had occurred several months earlier.

Teenagers who used cannabis showed slower improvement across several areas of cognition, including memory, attention, language and processing speed. Some of these participants initially performed as well as, or slightly better than, other children at younger ages. After they grew older and began using cannabis, however, their cognitive gains tended to level off while teens who did not use cannabis continued to improve.

THC Linked to Worsening Memory

The researchers also examined whether specific cannabis compounds were associated with different outcomes. Within a smaller group of participants, teenagers with evidence of tetrahydrocannabinol (THC) exposure, the primary intoxicating compound in cannabis, experienced greater declines in memory performance over time than teens who did not use cannabis.

The same pattern was not observed among teenagers with evidence of cannabidiol (CBD) exposure, although the number of participants in that group was limited.

"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."

Small Differences May Have Real Consequences

Although the cognitive differences identified in the study were relatively modest, the researchers said they may still have meaningful effects. The brain develops rapidly during adolescence, so even minor changes in memory, attention or thinking speed could influence classroom performance, learning and everyday activities.

The findings do not establish that cannabis directly caused the slower cognitive growth. Environmental conditions, personality traits and other factors could also contribute to the differences.

Still, the researchers adjusted their analysis for many possible influences. These included family background, mental health, the use of other substances and each participant's cognitive abilities before cannabis use began.

Researchers Will Follow Participants Into Adulthood

The research team plans to continue monitoring the participants as they enter young adulthood. Future analysis may offer a clearer picture of the long-term effects of cannabis and reveal whether the age at which use begins, as well as how often cannabis is used, influences 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."

Additional co-authors on the study include: Ryan M. Sullivan, Alexander L. Wallace, Veronica Szpak, Joanna Jacobus and Susan F. Tapert from UC San Diego School of Medicine; Rachel Visontay, Louise Mewton and Hollie Byrne from the University of Sydney; Krista M. Lisdahl from University of Wisconsin-Milwaukee; Marilyn A. Huestis from Huestis & Smith Toxicology; and Priscila Dib Gonçalves from Columbia University.

The study was funded, in part, by the National Institute on Drug Abuse (DA050779, PI: Wade), (DA064409 PI: Sullivan), (DA062011 PI: Wallace), (T32 AA013525 PI: Riley/Spadoni to Szpak), (R01DA062432 MPI Lisdahl, Hillard), (2U01DA041025 MPI Lisdahl, Larson), (K01DA057389 PI: Gonçalves), and (NARSAD/Brain Behavior Research Foundation Gonçalves).