A new study of over 1000 young adults found heavy lifetime cannabis use was associated with decreased brain activation during working memory tasks, with effects persisting even after accounting for recent use.
Investigators analyzed functional MRI data from 1003 participants aged 22 to 36 who completed 7 cognitive tasks as part of the Human Connectome Project. The sample included 88 heavy cannabis users (>1000 lifetime uses), 179 moderate users (10 to 999 uses), and 736 nonusers (<10 uses).
Heavy lifetime cannabis use was linked to significantly lower brain activation during working memory exercises compared to nonusers. The effect remained after controlling for recent use, demographic variables, and other substance use.
"These findings identify negative outcomes associated with heavy lifetime cannabis use and working memory in healthy young adults that may be long-lasting," wrote Joshua L. Gowin, PhD, of the University of Colorado Anschutz Medical Campus, and colleagues in JAMA Network Open.
The reduced activation was most pronounced in brain regions with high densities of cannabinoid receptors, including the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula.
"Because THC can reduce CB1 density, this could provide a mechanism to explain findings that cannabis use is associated with lower cortical thickness in the dorsomedial PFC and dorsolateral PFC," the researchers noted.
Recent cannabis use, determined by positive THC urine screens in 106 participants, was associated with poorer performance on working memory, episodic verbal memory, and theory of mind tasks. However, these associations did not remain significant after adjusting for race and education.
The study found no significant relationship between cannabis dependence diagnosis and brain function, suggesting that "factors associated with diagnosis, such as social and legal consequences, may be unrelated to brain function outcomes attributable to cannabis use."
Study limitations included its cross-sectional design, young adult sample, and lack of data on THC dose, potency, and route of administration. The researchers called for large longitudinal studies to assess causality and determine how long these effects persist.
The findings come amid increasing cannabis legalization and rising potency of cannabis products. The researchers emphasized the need to educate users about potential cognitive consequences, particularly regarding working memory tasks.
"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," the authors wrote, though they noted the optimal abstinence duration remains unclear.
The study was funded by grants from the National Institute of Mental Health and National Institute on Drug Abuse. The Human Connectome Project provided the data through the WU-Minn Consortium.
Full disclosures can be found in the study.