Bharti Jayshankar
February 1, 2025: A study published in JAMA Network Open reveals that heavy cannabis use may impair brain function during critical cognitive tasks, particularly those involving working memory.
This is the largest neuroimaging study of its kind, involving over 1,000 young adults. It found that 63% of lifetime heavy users and 68% of recent users exhibited reduced brain activity in regions governing decision-making and memory. The findings underscore growing concerns about cannabis’s cognitive impacts as legalization expands globally.
Research insights
Researchers at the University of Colorado Anschutz Medical Campus analyzed 1,000+ participants aged 22–36 using functional MRI scans during seven cognitive tasks. Heavy lifetime use was defined as consuming cannabis over 1,000 times (equivalent to daily use for nearly three years), while moderate use ranged from 10–999 instances. Non-users had fewer than 10 lifetime uses.
During working memory tasks—essential for following instructions or solving problems—heavy users showed diminished activity in the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula. These areas govern memory, attention, and emotional processing. Notably, the study applied rigorous statistical thresholds (false discovery rate correction) to minimize false positives, with working memory being the only task showing significant impairment.
“As cannabis accessibility rises, understanding its health implications is vital,” said lead author Dr. Joshua Gowin. “Our work highlights a clear association between use and reduced cognitive efficiency, but it also suggests abstaining before tasks might mitigate effects.” He cautioned heavy users against abrupt cessation, which could disrupt cognition due to withdrawal.
Cannabis use
Cannabis use among young adults has surged alongside legalization. According to the 2021 National Institute on Drug Abuse survey, 43.5% of U.S. adults aged 19–30 reported past-year use, up from 34% in 2016. While 24 states have legalized recreational cannabis, health experts emphasize balancing benefits (e.g., pain relief) with risks like dependency and cognitive decline.
Worldwide, about 147 million people, 2.5% of the world population, consume cannabis (annual prevalence) compared with 0.2% consuming cocaine and 0.2% consuming opiates, according to the World Health Organisation.
Dr. Nora Volkow, NIDA director (unaffiliated with the study), noted, “This study adds robust evidence that heavy cannabis use affects brain function. However, longitudinal research is needed to confirm causation.” Others, like Harvard neuroscientist Dr. Staci Gruber, cautioned that individual factors (e.g., THC potency, age of first use) may influence outcomes.
The study’s cross-sectional design prevents conclusions about long-term effects or causality. Self-reported usage data may also introduce bias. Dr. Gowin emphasized the need for decade-spanning studies to track how brain changes evolve and affect different age groups.
For individuals using cannabis heavily, the findings suggest moderation and strategic abstinence before tasks requiring sharp memory. Public health campaigns may need to address cognitive risks alongside addiction education.
As global cannabis policies shift, this study serves as a critical reminder of the substance’s complex trade-offs. While not universally detrimental, heavy use appears to carry measurable cognitive costs—a reality users and policymakers must navigate with care.