New MRI Study Reveals How Cannabis Alters Brain Activity and Weakens Memory

A massive new study sheds light on how cannabis affects the brain, particularly during cognitive tasks.

Researchers analyzed over 1,000 young adults and found that both heavy lifetime use and recent cannabis consumption significantly reduced brain activity during working memory tasks. This impairment was linked to worse performance on tasks requiring focus, problem-solving, and instruction-following. Brain imaging revealed that key regions responsible for decision-making and attention were affected.

Cannabis Use and Brain Function: A Groundbreaking Study

A new study published in JAMA Network Open examines how both recent and long-term cannabis use affects brain function during cognitive tasks.

As the largest study of its kind, researchers analyzed over 1,000 young adults aged 22 to 36 using brain imaging technology. They found that 63% of heavy lifetime cannabis users showed reduced brain activity during a working memory task, with a similar impact seen in 68% of recent users.

Reduced Brain Activity and Working Memory Impairment

This decrease in brain activity was linked to poorer performance on working memory — a crucial function that helps people retain and use information for tasks. For instance, working memory enables someone to follow instructions, mentally visualize concepts, or solve problems like math equations.

“As cannabis use continues to grow globally, studying its effects on human health has become increasingly important. By doing so, we can provide a well-rounded understanding of both the benefits and risks of cannabis use, empowering people to make informed decisions and fully comprehend the potential consequences,” said the study’s first author Joshua Gowin, PhD, assistant professor of radiology at the University of Colorado School of Medicine on the University of Colorado Anschutz Medical Campus.

In the study, heavy users are considered young adults who’ve used cannabis more than 1000 times over their lifetime. Whereas, using 10 to 999 times was considered a moderate user, and less than 10 times was considered a nonuser.

MRI Scans Reveal Cannabis’ Impact on Brain Function

The researchers studied the neural response of participants during a magnetic resonance imaging (MRI) session and gave them seven cognitive tasks to complete. The tasks tested working memory, reward, emotion, language, motor skills – such as tapping a finger to map brain control, relational assessment and theory of mind.

The researchers found that cannabis had a statistically significant effect on brain function during working memory tasks, meaning the observed impact is very unlikely to be due to random chance. This effect was seen in both recent and lifetime cannabis users. The impact was less significant for the other tasks.

“We applied the highest standards to our research, setting rigorous thresholds for statistical significance across all seven cognitive function tests. To minimize the risk of false positives, we employed false discovery rate (FDR) correction. While some of the other tasks indicated potential cognitive impairment, only the working memory task showed a statistically significant impact,” adds Gowin.

Key Brain Regions Affected by Heavy Cannabis Use

During working memory tasks, the researchers found heavy cannabis use appeared to reduce brain activity in certain areas of the brain (dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula). These regions of the brain are involved in important cognitive functions such as decision-making, memory, attention, and emotional processing.

However, Gowin mentions their research also suggests that abstaining from using cannabis before doing a cognitive task could help to improve performance. “People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well. For example, heavy users may need to be more cautious,” Gowin says.

Unanswered Questions: The Future of Cannabis Research

He adds, “There are a lot of questions we still need answers to regarding how cannabis impacts the brain. Large, long-term studies are needed next to understand whether cannabis use directly changes brain function, how long these effects last, and the impact on different age groups.”

Reference: “Brain Function Outcomes of Recent and Lifetime Cannabis Use” 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, 28 January 2025, JAMA Network Open.
DOI: 10.1001/jamanetworkopen.2024.57069