Heavy Cannabis Use Really Could Affect Your Memory, Largest Ever Study Suggests

Between 2010 and 2015, the team of researchers recruited a total of 1,003 young people aged 22 to 36 to take part in a series of tests at Washington University in Saint Louis, Missouri, while having their brains scanned using functional magnetic resonance imaging (fMRI). Participants who claimed to have used cannabis more than 1,000 times during their lifetime were classed as heavy users, those who had consumed the drug between 11 and 1,000 times were labeled moderate users, and those who had used it 10 times or fewer were deemed non-users.

All participants also provided a urine sample on the testing day so that the researchers could gauge how recently they had last used cannabis.

The volunteers were then presented with a series of challenges designed to assess seven key facets of cognition: emotion, reward, language, motor skills, relational assessment, theory of mind, and working memory. The latter of these refers to the ability to hold information in one’s mind while working through a problem.

Overall, 63 percent of heavy stoners and 68 percent of recent users showed reduced brain activation during working memory tasks. The brain regions most affected were the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula, all of which play a key role in vital cognitive functions including memory, decision-making, and attention.

The study authors also note that all of these brain regions have high concentrations of cannabinoid 1 (CB1) receptors, which is the main binding target for THC, the psychoactive component of cannabis. Previous research in both humans and animals has indicated that frequent exposure to THC reduces the density and sensitivity of CB1 receptors in these regions, which may explain the link between heavy cannabis use and reduced cognitive functioning.

Importantly, the urine test results showed that this association remained detectable in heavy users who hadn’t recently consumed cannabis. 

Noting that this is the largest study of its kind ever conducted, the researchers mention that none of the other six areas of cognition appear to be impacted by cannabis use, and suggest that abstaining from weed for an extended period of time may help to undo the deficits brought about by frequent consumption. 

“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 write. “Further studies are required to determine guidance on the length of abstinence that may be necessary to improve cognitive performance.”

In a statement, study author Joshua Gowin added that those considering giving up cannabis may also experience a period of poorer cognitive function before things get better. “People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well,” he said. “For example, heavy users may need to be more cautious.”

Speaking more generally about the study’s findings, Gowin states that “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.”

The study has been published in the journal JAMA Network Open.