Heavy Cannabis Use Linked to Memory Deficits and Brain Function Changes

Emerging research continues to uncover potential risks associated with heavy cannabis use, including stroke and heart attack. Now, a new study published in JAMA Network Open suggests that frequent cannabis use may significantly impair the brain’s working memory, raising concerns about its impact on cognitive performance.  

The study analyzed data from 1,003 adults aged 22 to 36. Among them, 88 participants were identified as heavy lifetime cannabis users, defined as individuals who used cannabis 1,000 times or more. Sixty-three percent of heavy users displayed reduced brain activity during a working memory task, compared to 68% of recent cannabis users—a pattern that persisted even after excluding those who had used cannabis recently.  

Joshua Gowin, PhD, an assistant professor of radiology at the University of Colorado School of Medicine and the study’s lead author, explained the importance of working memory. Examples include following instructions given by a supervisor or recalling a blind spot while driving. These types of short-term memory tasks involve the brain’s capacity to hold and process information for immediate use.  

Using magnetic resonance imaging (MRI), researchers assessed participants' brain activation during tasks involving working memory, motor skills, reward processing, emotion, and language. They found that heavy cannabis use was strongly linked to reduced brain activation during working memory tasks—but less so during other cognitive functions.  

Residual Effects of Cannabis and Cognitive Performance  

Participants who had recently used cannabis were identified through urine samples, which measured THC concentrations. A THC level above 50 ng/mL typically indicates cannabis use within the past 10 days, though frequent users may show elevated levels for weeks or even months after stopping.  

The study authors advised that abstaining from cannabis before cognitively demanding activities may improve performance. However, existing evidence also shows that residual cognitive effects can last up to two weeks after cannabis use. For heavy users, abruptly quitting could result in withdrawal symptoms such as difficulty sleeping or mood changes, which may further impact cognition during the recovery period.  

“People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well,” Gowin said. More research is needed to determine how cannabis may directly change brain function and how long-lasting these effects are across different age groups.  

Cannabis and Developing Brains  

Itai Danovitch, MD, chair of the Department of Psychiatry and Behavioral Neurosciences at Cedars Sinai in Los Angeles, stressed the persistent effects of cannabis on brain functioning, particularly in young people whose brains are still developing.  

“Young people are especially vulnerable because their brains haven’t finished developing,” Danovitch explained. “This study reinforces concerns for this population, in the context of other research that’s been done.”  

The increasing availability of cannabis and cannabis-related products has raised alarm among public health experts about substance use in younger adults. A recent observational study, also published in JAMA Network Open, found that cases of schizophrenia associated with cannabis use disorder nearly tripled in Ontario, Canada, as cannabis became more widely accessible since 2006.  

Danovitch emphasized that cannabis impacts brain capacity in ways that carry tangible risks, such as impaired motor skills and reaction times. “That’s why you shouldn’t drive while using cannabis—it affects motor performance and safety,” he said.  

Balancing Awareness About Cannabis Risks  

Danovitch noted that cannabis is often portrayed with extreme views: some consider it harmless, while others emphasize its dangers. The reality lies in between. “Cannabis is neither as harmful as it’s often made out to be nor as harmless. It’s important people understand there are risks—and these risks are most significant for individuals whose brains are still developing.”  

As legalization continues to expand, promoting balanced education about cannabis remains vital, especially for younger populations at greater risk for long-term effects.