The study, published in Neuropsychopharmacology, analysed data from 11,036 children who were followed from ages 9 and 10 through ages 16 and 17.
Researchers monitored changes in their cognitive abilities while also tracking cannabis use and exposure to other substances.
The research used data from the Adolescent Brain Cognitive Development study, the largest long term investigation of brain development among young people in the United States.
Researchers found that teenagers who used cannabis showed slower improvement across several areas of cognitive performance, including memory, attention, language and processing speed.
Some participants who later began using cannabis initially performed as well as, or slightly better than, their peers.
However, as they grew older and began using cannabis, their cognitive development tended to level off, while teenagers who did not use cannabis continued to improve.
To measure cannabis exposure more accurately, researchers combined participants’ reports with biological testing. y
Hair, urine and saliva samples were used to identify exposure ranging from recent cannabis use to use that had occurred several months earlier.
The researchers also examined whether individual compounds in cannabis were associated with different cognitive outcomes.
In a smaller group of participants, teenagers with evidence of exposure to tetrahydrocannabinol, commonly known as THC, experienced greater declines in memory performance over time than teenagers without evidence of cannabis use.
Researchers did not observe the same pattern among teenagers with evidence of cannabidiol, or CBD, exposure, although the number of participants in that group was limited.
The findings point to THC as a likely contributor to the changes observed.
Although the differences in cognitive performance were relatively modest, researchers said they could still be meaningful because adolescence is a critical period for brain development.
Small changes in memory, attention or thinking speed could potentially affect learning, classroom performance and everyday activities.
However, the researchers stressed that the study does not prove that cannabis directly caused the slower cognitive development.
Environmental conditions, personality traits and other factors could also contribute to the differences.
The researchers adjusted their analysis for several possible influences, including family background, mental health, use of other substances and participants’ cognitive abilities before cannabis use began.
The research team plans to continue following the participants into young adulthood.
Future research could help determine whether the age at which cannabis use begins and how frequently it is used influence long term brain development.
The researchers say delaying cannabis use may support healthier brain development during adolescence.
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