Heavy cannabis use reduces brain activity during working memory tasks

Key takeaways:

• Overall, 63% of heavy lifetime cannabis users showed reduced brain activity during a working memory task.
• Not using cannabis before cognitively demanding situations may help with performance, researchers said.

Heavy use of cannabis recently or at any point in a person’s lifetime was tied to decreases in brain function during a working memory task, a new cross-sectional analysis showed.

The data indicate “that abstaining from cannabis prior to cognitively demanding situations will likely help with performance,” the researchers explained in JAMA Network Open.

Joshua Gowin, PhD, an assistant professor of radiology at the University of Colorado School of Medicine, noted that primary care providers “are often the first contact point that a person with substance use problems will have with potential treatment providers.”

“I encourage PCPs to ask about alcohol and cannabis use, and if warranted, they should provide resources on where the patient can seek treatment,” he told Healio.

In the study, Gowin and colleagues assessed possible correlations between recent and lifetime cannabis use and brain function using MRI scans, urine toxicology and cannabis use data from 1,003 adults aged 22 to 36 years.

The study’s primary outcome focused on the differences in brain activity as participants undertook several tasks that were administered during functional MRI sessions — including those that assessed reward, emotion, language, working memory, motor skills, relational assessment and theory of mind.

“We applied the highest standards to our research, setting rigorous thresholds for statistical significance across all seven cognitive function tests,” Gowin said in a press release.

He added that the researchers adjusted their analysis to minimize the risk for false-positive results.

Of the cohort, 8.8% were identified as having heavy lifetime cannabis use, defined as having over 1,000 uses.

The researchers found that heavy recent and lifetime cannabis use was associated with lower brain activity during a working memory task (Cohen’s d = 0.28; 95% CI, 0.5 to 0.06), with 63% and 68% of heavy and recent lifetime cannabis users having reduced brain activity during one of these tasks, respectively, according to the release.

The regions where decreased brain activity was linked to a history of heavy cannabis use included the medial prefrontal cortex, anterior insula and dorsolateral prefrontal cortex (all P < .02). The release noted that these areas are important for decision-making, attention, emotional processing and memory.

The researchers said brain activity levels during the working memory tasks were associated with intelligence, verbal episodic memory performance and education, “suggesting that they are meaningful indicators of cognitive function.”

Gowin told Healio that he “was surprised” by some aspects of the results.

“I expected to see larger effects for recent use rather than heavy lifetime use, but we saw that lifetime use had larger effects on working memory in this study,” he explained. “I also expected to see larger effects in language-related processing, but the effects were more specific to working memory. In hindsight, the findings make sense with what has been shown before — memory is one of the mental tasks most affected by THC.”

The researchers could not establish causality between cannabis use and decreased brain activity because of the study’s design. Additionally, the size of some of the study’s subgroups was small, hurting its statistical power, Gowin and colleagues acknowledged.

For future research, “we need to explore the degree to which these effects on memory can recover with abstinence or a reduction in use,” Gowin said. “We also are interested in exploring whether other mental health conditions, like ADHD, modify the effects of cannabis on the brain.”

References:

• Gowin J, et al. JAMA Netw Open. 2025;doi:10.1001/jamanetworkopen.2024.57069.
• Largest study ever done on cannabis and brain function finds impact on working memory. Available at: https://www.eurekalert.org/news-releases/1071666. Published Jan. 28, 2025. Accessed Jan. 30, 2025.