• Posted January 30, 2025

Heavy Weed Use Affects Young Adults' Short-Term Memory

Heavy weed use appears to dull the brains of young adults, particularly affecting their short-term working memory, a new study suggests.

MRI scans showed less brain activation in heavy tokers performing tasks that tested their ability to temporarily hold a limited amount of information at the ready for immediate mental use, according to results published Jan. 28 in JAMA Network Open.

“Lifetime heavy cannabis use history was associated with lower brain activation related to working memory, with a small to medium effect size,” concluded the research team led by Joshua Gowin, an assistant professor of radiology with the University of Colorado Anschutz Medical Campus.

Previous research has linked marijuana use to reduced memory, planning, decision making and social ability, but these studies have typically contained few participants who were regular or heavy weed users, the team noted.

For the study, researchers analyzed MRI scans and cognitive tests performed on more than 1,000 young adults as part of a large-scale project to comprehensively map the human brain.

Participants underwent brain MRI scanning as they performed seven different tasks associated with brain function, researchers said.

Researchers classified about 9% of the participants as heavy weed users because they said in a questionnaire they’d used cannabis 1,000 or more times. 

Another 18% were considered moderate users because they reported using weed 10 to 999 times. The rest were considered non-users.

Results showed lower activation of certain brain regions during the working memory task among heavy weed users, researchers said.

Those regions included the anterior insula, a region associated with emotional processing, and the medial prefrontal cortex and dorsolateral prefrontal cortex, regions associated with higher-level functions like planning, decision making, cognitive flexibility and working memory.

“These are regions that have a relatively high density of (cannabinoid) receptors,” researchers wrote, and thus could be impacted by heavy weed use.

The study also found that recent weed use was associated with poorer performance and less brain activation during working memory tests, but the associations were not statistically significant.

“The association we observed between recent use and working memory task activation and performance suggests that abstaining from cannabis prior to cognitively demanding situations will likely help with performance,” researchers stated.

“The exact duration of this period of abstinence is unclear, but studies suggest that residual cognitive effects of cannabis may remain for 2 to 4 weeks after abstinence,” they continued.

Larger studies are needed to track the effect of weed use on brain function, researchers said.

In the meantime, “our findings highlight the need to educate cannabis users about the consequences of recent and heavy lifetime cannabis use on cognitively demanding working memory tasks,” they concluded.

More information

The U.S. Centers for Disease Control and Prevention has more on cannabis and brain health.

SOURCE: JAMA Network Open, Jan. 28, 2025