Heavy cannabis use linked to reduced brain activity during memory tasks, study finds

A new study published in JAMA Network Open has found that individuals with a history of heavy cannabis use showed lower brain activation during a working memory task. This association held even after excluding participants who had recently used cannabis. The results suggest that frequent cannabis use may have lasting associations with how the brain supports memory-related processes, even in the absence of recent consumption.

As the legal landscape surrounding cannabis continues to shift, both in the United States and internationally, its use has become more widespread—and with it, concerns about its effects on the brain. The increasing accessibility of cannabis has been accompanied by rising rates of cannabis use disorder, higher product potency, and a growing public perception that the drug poses little harm. In response to these trends, researchers are working to clarify whether and how cannabis use might affect brain function over time, especially in young adults whose brains are still developing.

Previous studies have shown that cannabis can impair cognitive performance in the short term, particularly in areas like memory and attention. However, it is less clear whether these effects persist after someone stops using the drug. Brain regions rich in cannabinoid receptors—such as the prefrontal cortex—appear to be especially sensitive to tetrahydrocannabinol (THC), the main psychoactive compound in cannabis. Some evidence has shown structural differences in the brains of frequent cannabis users, but large-scale, well-controlled studies linking these changes to actual brain function have been limited.

“I had been talking with my colleague about the effects of cannabis on mental processing and what we know and what we don’t know,” explained lead author Joshua L. Gowin, an assistant professor of radiology at the University of Colorado Anschutz Medical Campus. “We realized that although neuropsychological effects had been clearly established, such as verbal learning and memory, no similarly clear effects had been established for brain function. We wanted to see if we could establish a similar breakdown of which aspects of brain function might be most associated with cannabis use.”

For their study, the researchers used data from the Human Connectome Project, a large study that included brain scans and behavioral data from over 1,000 adults between the ages of 22 and 37. The sample included people with varying histories of cannabis use, ranging from nonusers to those who had used the drug more than 1,000 times in their lifetime. Participants underwent functional magnetic resonance imaging (fMRI) while completing seven different tasks designed to activate brain areas responsible for memory, reasoning, emotion, motor skills, language, reward processing, and social understanding.

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To distinguish between short-term and long-term effects, the researchers took two key measures. First, they assessed recent use using a urine screen that detects cannabis metabolites typically present for up to 10 days after use. Second, they categorized lifetime use using structured interviews. Participants were grouped as nonusers (10 or fewer lifetime uses), moderate users (11 to 999 uses), or heavy users (1,000 or more uses). The researchers also considered a range of other factors that could influence brain activity, such as age, sex, income, education, alcohol and nicotine use, and family background.

The main finding was that individuals with a history of heavy cannabis use showed lower brain activation during a task involving working memory, which requires people to hold and manipulate information over short periods. This association remained even after controlling for whether participants had used cannabis recently. The reduction in brain activity was most pronounced in brain regions involved in executive control, such as the dorsolateral prefrontal cortex, medial prefrontal cortex, and anterior insula—all of which are known to contain high levels of cannabinoid receptors.

“We think there is some suggestion that cannabis use is associated with changes in brain function, and that might be important information for someone to consider if they are a regular cannabis user or are considering using it,” Gowin told PsyPost.

Interestingly, the researchers found no strong link between cannabis dependence diagnosis and brain activity. This suggests that formal diagnoses may not fully capture the extent of cannabis-related brain function changes. Rather, it was the frequency of use over a lifetime that appeared most relevant. The researchers also found that individuals who had recently used cannabis showed decreased brain activity during working memory and social cognition tasks, but these results did not remain statistically significant after adjusting for other factors such as race and education.

In addition to measuring brain activation, the researchers looked at how participants performed on several tasks. Recent cannabis users performed worse on working memory and verbal memory tasks, even when they were not intoxicated at the time of testing. However, lifetime heavy use was not clearly linked to performance deficits, which suggests that the brain activation differences might not always translate to observable behavior—or that effects may be subtle or emerge under specific conditions.

The working memory findings were especially notable because of their link to cognitive abilities that are important in daily life, such as decision-making, focus, and learning. Brain activation in the relevant areas also correlated with participants’ scores on intelligence tests and verbal memory tasks. This suggests that changes in brain activation are not random but may reflect real differences in how efficiently the brain is operating during mental tasks.

The study also explored social cognition, often referred to as “theory of mind”—the ability to infer what others are thinking or feeling. Recent cannabis use was linked to lower performance on these tasks, although the association with brain activity was weaker and did not hold up to more rigorous analysis. The authors note that reduced social cognitive processing has been linked to mental health conditions like schizophrenia, and that this line of research may warrant further exploration.

“I was expecting to see more differences in language processing, but we didn’t find anything there,” Gowin said. “There was some indication that social processing might be associated with cannabis use, and that intrigued us and we hope to learn more about that in the future.”

Another unexpected finding was a sex difference in the motor task: men who had recently used cannabis showed reduced brain activity, while women did not. Although this result was limited to one task, it raises the possibility that cannabis may affect the brain differently in men and women, something that previous studies have suggested but not conclusively demonstrated.

Despite its strengths, the study has some limitations. “The most important caveat is that this is a cross-sectional study, so we can’t conclusively say that cannabis caused the changes in brain function,” Gowin noted. “We also didn’t examine ADHD, and numerous people have suggested that ADHD could be an important confound.”

Still, the findings contribute to a growing body of evidence that cannabis use is associated with changes in how the brain responds during cognitive tasks, especially those involving memory. The researchers emphasize the need for more long-term studies to determine whether these changes are reversible, how long they last, and whether they have meaningful effects on behavior, mental health, or life outcomes.

“My goal for this is to try to better understand how substance use affects the brain, and to provide truthful messaging about the relationship and the brain to the public, neither overplaying or underplaying potential effects, but simply knowing the truth,” Gowin said.

The study, “Brain Function Outcomes of Recent and Lifetime Cannabis Use,” was authored by Joshua L. Gowin, Jarrod M. Ellingson, Hollis C. Karoly, Peter Manza, J. Megan Ross, Matthew E. Sloan, Jody L. Tanabe, and Nora D. Volkow.