As cannabis laws evolve and cultural perceptions shift, one persistent question remains: does cannabis use, particularly when started young, have lasting effects on the brain? In one of the most rigorous long-term studies to date, researchers from Duke University and King’s College London provide compelling evidence that persistent cannabis use—especially when initiated during adolescence—is linked to lasting neuropsychological decline.
“Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline.”
Meier et al., 2012
Using data from the Dunedin Study, a longitudinal birth cohort followed from childhood to age 38, researchers measured cannabis use over two decades and tracked changes in IQ and cognitive functioning. Their findings suggest that chronic, adolescent-onset cannabis users experience measurable and lasting drops in cognitive performance, even after quitting. 
Inside the Study: A Unique Longitudinal Design
The strength of this study lies in its design. Unlike most cannabis studies, which rely on retrospective recall or cross-sectional data, this research followed 1,037 individuals from birth, testing their IQ at age 13—before any cannabis use—and again at 38. This approach allowed the researchers to observe within-person cognitive change and isolate the effects of persistent cannabis use.
Cannabis use and dependence were tracked through interviews at ages 18, 21, 26, 32, and 38. Participants were grouped based on their use history, ranging from never-used to those who had been dependent on cannabis at multiple timepoints.
The Findings: A Clear Pattern of Cognitive Decline
Persistent cannabis users—especially those who began in adolescence—experienced significant IQ decline from childhood to adulthood. The most heavily dependent users saw an average IQ drop of approximately 8 points.
Importantly, the decline was broad, spanning domains like memory, processing speed, verbal comprehension, and executive function. Even when accounting for potential confounding factors such as tobacco use, alcohol or other drug dependence, schizophrenia, and years of education, the association held strong.
The effect was dose-dependent: more frequent cannabis dependence was linked to greater cognitive decline.
Adolescent Onset Carries the Greatest Risk
The most striking aspect of the study was the difference in cognitive impact between those who started cannabis use in adolescence versus adulthood. Adolescent-onset users showed significantly greater declines in IQ, even when controlling for frequency of use and number of dependence episodes.
Adults who began using cannabis after the age of 18 did not demonstrate the same level of impairment, even if they were persistent users. This supports the idea that the adolescent brain is uniquely vulnerable to the neurotoxic effects of cannabis.
Quitting May Not Fully Reverse the Effects
Perhaps most concerning, the study found that adolescent-onset users who later quit cannabis did not recover their lost IQ. While quitting may prevent further decline, the data suggests that the damage to neuropsychological functioning may be long-lasting if cannabis use begins during critical periods of brain development.
Real-World Impacts: Cognitive Problems Noticeable to Others
This was not just a matter of test scores. Third-party informants—friends and family members—also reported more attention and memory problems in persistent users, reinforcing the everyday functional impact of the decline.
As the authors write, “Impairment was concentrated among adolescent-onset cannabis users, with more persistent use associated with greater decline.” The changes weren’t subtle or abstract; they affected daily life and were noticeable to the people who knew the participants best.
What This Means for Public Health and Policy
This study underscores the importance of delaying the onset of cannabis use until adulthood, when the brain is more fully developed. As cannabis becomes more accessible, fewer adolescents believe it poses serious health risks, and initiation is occurring at younger ages.
Public health campaigns and policy efforts should focus on communicating the risks of early use, much as they do with tobacco and alcohol. While cannabis may have therapeutic benefits, especially for adults with certain medical conditions, its use during adolescence appears to carry significant cognitive costs.
The findings from Meier et al. present a compelling case for cautious cannabis policy—one that acknowledges its medicinal potential but also respects the science of adolescent brain development. For individuals, families, educators, and policymakers alike, understanding the long-term risks of early cannabis use is essential in shaping informed, balanced approaches to legalization and youth prevention.
