63% of heavy cannabis users show reduced working memory ability

Functional MRI reveals that 63% of heavy lifetime cannabis users show reduced brain activity during a working memory task -- and that 68% of recent users also show this impact, according to a study published January 28 in JAMA Network Open.

The findings identify negative outcomes associated with heavy lifetime cannabis use and working memory in healthy young adults that could be long-lasting, noted a team led by Joshua Gowin, PhD, of the University of Colorado School of Medicine in Aurora.

"As cannabis use continues to grow globally, studying its effects on human health has become increasingly important," Gowin said in a statement released by the university. "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."

Although the use of cannabis has increased, its effects on brain function are not fully known, Gowin and colleagues explained. Of particular concern is the effect of the drug on working memory, which allows a person to follow instructions or to mentally visualize and manipulate information.

The team conducted a study that assessed neural response during a functional MRI exam of 1,003 adults between the ages of 22 and 36. Each subject was given seven cognitive tasks to complete to test working memory, reward, emotion, language, motor skills, and relational assessment. Of the total study cohort, 6.3% were Asian, 13.7% were Black, and 76% were white.

The investigators evaluated participants' history of heavy cannabis use via the Semi-Structured Assessment for the Genetics of Alcoholism (SSAGA) and diagnosed cannabis dependence using Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition criteria. A heavy cannabis habit was defined as greater than 1,000 uses; a moderate habit as between 10 and 999 uses; and nonuse as fewer than 10 uses. Of the study cohort, 8.8% were classified as heavy users, 17.8% as moderate, and 73.4% as nonusers.

Gowin and colleagues found the following:

• Heavy lifetime cannabis use was associated with lower activation of working memory (Cohen d of -0.28, more than twice the standard deviation).
• Affected brain regions associated with a history of heavy cannabis use included the anterior insula, medial prefrontal cortex, and dorsolateral prefrontal cortex -- areas involved in cognitive functions such as decision-making, memory, attention, and emotional processing.

The findings underscore a need for more research, Gowin said.

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

The complete study can be found here.