As cannabis legalization spreads globally for both recreational and medical purposes, questions about its safety, particularly for young adults, remain a topic of intense debate. Adolescence and early adulthood are critical periods for brain development, which continues until the mid-to-late 20s.
While smaller studies have explored the effects of cannabis on young brains, a groundbreaking study led by researchers at the University of Colorado is the largest of its kind, offering new insights into the potential risks.
“With cannabis use on the rise worldwide, understanding its impact on human health is more important than ever,” said Joshua Gowin, PhD, the study’s lead author and assistant professor of radiology at the University of Colorado School of Medicine.
“Our goal is to provide a balanced view of both the benefits and risks of cannabis use, helping people make informed decisions and fully grasp the potential consequences.”
The research team analyzed data from 1,003 adults aged 22 to 36, including magnetic resonance imaging (MRI) scans, urine toxicology reports, and detailed information about cannabis use. The participants had an average age of 28.7 years, with 53.1% being female. The group was predominantly White (76%) but also included Asian (6.3%) and Black (13.7%) individuals.

To determine recent cannabis use, participants provided urine samples on the day of their MRI scans, which were tested for THC metabolites. Those with positive results were classified as recent users.
Lifetime cannabis use was also categorized: heavy users had consumed cannabis 1,000 or more times, moderate users between 11 and 1,000 times, and non-users fewer than 10 times.
During the MRI scans, participants performed seven tasks designed to assess various cognitive functions, including emotion processing, reward response, motor skills, working memory, language, logical reasoning, and social information processing.
The study revealed that both recent and heavy lifetime cannabis use were associated with a statistically significant reduction in brain activity—but only during the working memory task.
“We applied rigorous standards to our research, using strict thresholds for statistical significance and employing false discovery rate (FDR) correction to minimize false positives,” Gowin explained. “While some tasks hinted at potential cognitive impairment, only the working memory task showed a clear and significant impact.”
Working memory, which involves temporarily holding and manipulating information to guide decision-making and behavior, is essential for tasks like following directions, remembering conversations, or executing multi-step processes like cooking.
The researchers observed that reduced brain activity in heavy cannabis users during the memory task was most evident in specific brain regions: the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula. These areas play critical roles in emotion regulation, cognition, and social behavior, often working together to manage complex tasks.
The findings suggest that abstaining from cannabis before engaging in cognitively demanding tasks may improve performance, particularly for recent users. However, heavy users face additional challenges.
“For recent users, abstaining before a task that requires working memory could enhance performance,” the researchers noted. “However, the exact duration of abstinence needed is unclear. Studies suggest residual cognitive effects may persist for 2 to 4 weeks after stopping use.
Additionally, heavy users may experience withdrawal symptoms, which can last a week or more and further impair performance.”
The study has several limitations. As an uncontrolled, cross-sectional study, it cannot establish a causal relationship between cannabis use and reduced brain function. The findings are also specific to young adults and cannot be generalized to other age groups.
Furthermore, the researchers lacked data on THC dosage, potency, the presence of other cannabinoids like CBD, and the methods of cannabis consumption.
“There are still many unanswered questions about how cannabis affects the brain,” Gowin emphasized. “Large, long-term studies are needed to determine whether cannabis use directly alters brain function, how long these effects last, and how they impact different age groups.”
As cannabis use becomes more widespread, this study underscores the importance of understanding its potential risks, particularly for young adults whose brains are still developing. By shedding light on the cognitive effects of cannabis, researchers hope to empower individuals to make informed choices about their health and well-being.
The study was published in the journal JAMA Network Open.
Source: University of Colorado Anschutz Medical Campus