Summary: A new, large brain imaging study highlights a potential downside of heavy cannabis use in young adults. Examining more than 1,000 participants aged 22 to 36, researchers report that heavy lifetime cannabis use (defined here as more than 1,000 lifetime uses) is associated with reduced neural activity in brain regions essential for attention, decision-making, and executive control. The decline was most apparent during working memory tasks, suggesting frequent use in early adulthood may weaken the mental workspace used to hold and manipulate information for everyday problem-solving.
In this sample, 63% of heavy lifetime users and 68% of recent users showed diminished neural response during working memory challenges. While prior studies in older adults have suggested possible neuroprotective or different effects with age-related brain changes, this study emphasizes that the impact of cannabis can vary across the lifespan and that frequent use during young adulthood may impair cognitive systems needed for tasks that require short-term retention and manipulation of information.
Key Facts
- Study scale and method: The study used functional MRI (fMRI) to assess brain activation during cognitive tasks in the largest sample to date for this type of analysis.
- Primary cognitive finding: Out of seven cognitive domains tested, working memory was the only domain to show a statistically significant decline associated with heavy cannabis use.
- Regions affected: Heavy users showed reduced activity in the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula—areas central to executive control, attention, memory, and emotional processing.
- Usage threshold: The most pronounced effects were observed among those reporting more than 1,000 lifetime uses.
- Potential for recovery: Preliminary results indicate that abstaining before cognitive tasks may improve performance, suggesting some effects could be functional and partially reversible with behavioral change.
- Publication and institution: The study appears in JAMA Network Open and includes contributions from researchers at the University of Colorado.
This cross-sectional analysis used data from the Human Connectome Project and included cognitive testing paired with fMRI scans. Participants completed seven cognitive tasks during scanning that probed working memory, reward processing, emotion recognition, language, motor control, relational reasoning, and theory of mind. Urine toxicology was used to assess recent cannabis exposure, and lifetime use was classified via a structured assessment.
Make informed decisions
“As cannabis use continues to increase globally, studying its effects on human health is increasingly important,” said Joshua Gowin, PhD, the study’s first author and an assistant professor of radiology. The researchers emphasize understanding both potential benefits and risks so individuals can make informed decisions about use and its consequences.
For the purposes of this study, participants were grouped as nonusers (fewer than 10 lifetime uses), moderate users (10 to 999 uses), and heavy users (1,000 or more lifetime uses). The research team applied rigorous statistical controls, including false discovery rate correction, to reduce the likelihood of false-positive findings across multiple cognitive comparisons.
Statistically significant effect on working memory
The researchers found that heavy lifetime cannabis use was significantly associated with lower brain activation during the working memory task after correcting for multiple comparisons. Although some other tasks suggested potential impairments, only working memory showed a robust, statistically significant association. Recent cannabis use was linked to poorer performance and lower activation in some tasks, but those associations did not remain significant after correction.
Working memory supports everyday activities such as following multi-step instructions, holding a phone number briefly while searching for a pen, or mentally manipulating numbers to solve a problem. Reduced activation in regions like the dorsolateral prefrontal cortex and anterior insula may reflect diminished capacity in networks that support these functions.
Gowin noted that abstaining from cannabis prior to testing appeared to improve performance in some participants, indicating that at least part of the observed effect may be functional rather than structural. Still, the authors call for large, long-term studies to determine whether heavy cannabis use directly alters brain function, how durable those effects might be, and how age and other factors influence outcomes.
Questions and answers
A: Working memory is the brain’s short-term workspace: the ability to hold and manipulate information for brief periods while completing a task, like remembering part of a sentence while finishing it or keeping a number in mind long enough to write it down.
Q: How does this study differ from research on older adults?
A: This study focused on young adults, a period when the brain retains considerable plasticity. Frequent cannabis use during this stage may blunt the activity of executive networks. In older adults, cannabis effects could interact differently with age-related brain changes.
Q: Are the effects permanent?
A: The study suggests some impairment is functional because abstaining before tasks improved performance in some cases. However, long-term recovery timelines and the permanence of effects remain unclear and require extended longitudinal research.
Editorial notes
- This article was edited by a Neuroscience News editor.
- The journal paper was reviewed in full by the editorial team.
- Additional context was provided by staff to clarify methods and findings.
About this research summary
Author: Julia Milzer
Source: University of Colorado
Contact: Julia Milzer, University of Colorado
Image credit: Neuroscience News
Abstract — Brain Function Outcomes of Recent and Lifetime Cannabis Use
Importance
Cannabis use has increased globally, yet the full effects on brain function are not fully defined. Better understanding recent and long-term activation patterns associated with cannabis is essential for public health.
Objective
To examine the association between lifetime history of heavy cannabis use and recent cannabis exposure with brain activation across multiple cognitive domains in a large sample of young adults in the United States.
Design, setting, and participants
This cross-sectional study used data from the Human Connectome Project collected between 2012 and 2015. It included young adults aged 22–36 with MRI scans, urine toxicology results, and detailed cannabis use histories. Data were analyzed in 2024.
Exposures
Lifetime heavy cannabis use was assessed with a semi-structured clinical interview. Participants were categorized as heavy users (>1,000 lifetime uses), moderate users (10–999 uses), or nonusers (<10 uses). Urine testing assessed recent use, and diagnostic criteria for cannabis dependence were also considered.
Main outcomes and measures
Brain activation during seven fMRI tasks (working memory, reward, emotion, language, motor, relational reasoning, and theory of mind) served as the primary outcomes. Mean activation from task-relevant regions was analyzed using linear mixed-effects models to evaluate associations with lifetime and recent cannabis use.
Results
The final sample included 1,003 adults (mean age 28.7 years). Of these, 88 were classified as heavy lifetime users, 179 as moderate users, and 736 as nonusers. Heavy lifetime use was associated with lower activation during the working memory task in regions including the anterior insula and prefrontal cortex. Recent use showed associations with lower activation and performance in working memory and motor tasks, though those did not remain significant after correction for multiple comparisons. No other task domains showed robust associations with lifetime use, recent use, or dependence diagnosis.
Conclusions and relevance
In this sample of healthy young adults, a history of heavy cannabis use was associated with reduced brain activation during working memory challenges. These findings point to negative outcomes related to heavy lifetime cannabis use and working memory that may persist, underscoring the need for further long-term research.