The largest study yet into the effects of cannabis on the brains of 18- to 36-year-olds has found that the drug can reduce brain function during cognitive tests. The findings will help people to make an informed decision about the pros and cons of using cannabis.
As more and more countries and states legalize the production and sale of cannabis for recreational and medical use, there’s an ongoing debate about how harmless it is, especially for young adults. Adolescence is a critical period for brain development, which continues until the mid-to-late 20s.
There have been some small studies that looked at the impact of cannabis use on young people’s brain function, but a study led by researchers from the University of Colorado is the largest yet.
“As cannabis use continues to grow globally, studying its effects on human health has become increasingly important,” said the study’s first author, Joshua Gowin PhD, assistant professor of radiology at the University of Colorado School of Medicine on the University’s Anschutz Medical Campus. “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.”
The researchers obtained data from 1,003 adults aged 22 to 36 that included magnetic resonance imaging (MRI) scans, urine toxicology, and information about cannabis use. The participants’ mean age was 28.7 years, and 53.1% were female. The sample was predominantly White (76%) but included Asians (6.3%) and Black (13.7%) participants.
To assess whether they’d used cannabis recently, participants provided a urine sample on the day they had their MRI scan, which was tested for the presence of a THC metabolite. If a participant’s urine test was positive, they were categorized as a recent user. Participants’ lifetime cannabis use was also assessed and categorized: heavy users had used 1,000 or more times during their lifetime, moderate users had used between 11 and 1,000 times, and non-users had used fewer than 10 times. MRIs measured brain activity while the participants undertook seven tasks designed to test emotion, reward, motor function, working memory, language, relational or logical reasoning, and theory of mind or social information processing.
The researchers found that both recent and heavy lifetime cannabis use was associated with a statistically significant reduction in brain activity only during the working memory task.
“We applied the highest standards to our research, setting rigorous thresholds for statistical significance across all seven cognitive function tests,” Gowin said. “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.”
Working memory is the temporary retention of a limited amount of information that can be accessed to guide decision-making and behavior. Following directions, remembering a conversation while it’s going on, and following the steps of a recipe are all examples of working memory.
They observed that the lower brain activity seen in heavy cannabis users during the memory task was most pronounced in specific areas of the brain: the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula. Each region contributes to essential brain functions related to emotion, cognition, and social behavior. They often work together to process complex tasks and decisions.
The researchers said that their findings suggest that, for those who used cannabis recently, abstaining before a cognition-heavy task may help with performance. However, for heavy users, abstaining brings with it other important considerations.
“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,” said the researchers. “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. Furthermore, in heavy users, abstaining from cannabis may also lead to withdrawal symptoms, which may last for a week or more following cessation and could also affect performance.”
The study has limitations. It was an uncontrolled, cross-sectional study, so the association seen between cannabis and brain function can’t be considered causal. And because the participants were young adults, the results can’t be generalized to other age groups. The researchers also lacked data on typical THC dose and potency, additional components such as cannabidiol (CBD), and how the cannabis was administered.
“There are a lot of questions we still need answers to regarding how cannabis impacts the brain,” said Gowin. “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 study was published in the journal JAMA Network Open.
