Photo by Thought Catalog on Unsplash
Researchers track brain changes in unprecedented 1,000-person study
In a nutshell
- People who have used cannabis more than 1,000 times show reduced brain activity during memory tasks, even long after stopping use
- The effects were most noticeable in brain regions responsible for working memory
- While the study found impacts on memory function, other mental tasks like emotional processing, language, and logical thinking weren’t significantly affected by heavy cannabis use
AURORA, Colo. — Think cannabis effects wear off once you decide to quit for good? Think again. New research suggests that heavy marijuana use could impact your brain’s memory functions long after your last use. This finding comes from one of the largest studies ever conducted on how cannabis affects brain function.
Against a backdrop of increasing cannabis legalization and rising potency of marijuana products, researchers from the National Institute on Alcohol Abuse and Alcoholism, the University of Colorado School of Medicine, and the University of Toronto collaborated to conduct the largest study of its kind examining how both recent and long-term cannabis use affects brain function. Their findings, published in JAMA Network Open, suggest that heavy cannabis use may have lasting impacts on cognitive processing, even after someone stops using the drug.
“As cannabis use continues to grow globally, studying its effects on human health has become increasingly important. 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,” said the study’s first author Joshua Gowin, PhD, an assistant professor of radiology at the University of Colorado School of Medicine, in a statement.
The research team looked at brain scans from 1,003 young adults between ages 22 and 36. They found that people who had used cannabis heavily throughout their lives showed less brain activity during memory tasks compared to those who rarely or never used marijuana. This difference was most noticeable in brain regions that help us pay attention, make decisions, and temporarily store information.
Working memory is sort of like a mental sticky note – it’s what helps you remember a phone number just long enough to dial it. We use this type of memory countless times each day, from following a conversation to solving problems or remembering directions.
The researchers split participants into three groups based on their lifetime cannabis use: heavy users (more than 1,000 times), moderate users (10-999 times), and nonusers (fewer than 10 times). Out of everyone studied, about 9% were heavy users, 18% were moderate users, and 73% rarely or never used cannabis. Heavy users were more likely to be male, have lower income and education levels, and show recent cannabis use in drug tests.
“We applied the highest standards to our research, setting rigorous thresholds for statistical significance across all seven cognitive function tests. 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,” noted Gowin.
Beyond memory tasks, the study also looked at how participants’ brains responded during other mental challenges – things like processing rewards, recognizing emotions, using language, controlling movement, logical thinking, and understanding others’ perspectives. Interestingly, heavy cannabis use didn’t seem to significantly affect these other brain functions.
The team also found that people who had recently used cannabis (detected through drug testing) performed worse on memory and movement tasks. However, these short-term effects weren’t as statistically strong as the long-term impacts on memory.
The study revealed an interesting gender difference: men showed more pronounced effects than women when it came to movement-related brain activity while under the influence of cannabis. This adds to growing evidence that marijuana might affect men and women differently.
For anyone who uses cannabis regularly, the research suggests it might be helpful to take breaks before tackling mentally demanding tasks. However, Gowin warned that suddenly stopping cannabis use could also affect thinking abilities. “People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well,” he said. “Heavy users may need to be more cautious.”
Paper Summary
Methodology
Researchers leveraged data from the Human Connectome Project, which collected extensive brain imaging and behavioral data between 2012-2015. Participants underwent functional magnetic resonance imaging (fMRI) while performing seven different cognitive tasks. They also provided detailed information about their history of cannabis use through standardized interviews and underwent urine testing to detect recent use. The study controlled for potentially confounding factors like alcohol use, nicotine use, education, income, and demographic variables.
Results
Heavy lifetime cannabis users showed significantly lower brain activation during working memory tasks compared to non-users, with a moderate effect size (Cohen’s d = -0.28). This association remained even after excluding participants with recent cannabis use. The effect was most pronounced in brain regions with high densities of cannabinoid receptors. Recent cannabis use was associated with poorer performance on working memory and theory of mind tasks, though these effects didn’t survive the strictest statistical corrections.
Limitations
This research was cross-sectional rather than longitudinal, meaning it can’t definitively prove that cannabis use caused the observed differences in brain function. The study focused on young adults, so findings may not generalize to other age groups. Researchers lacked detailed information about when heavy cannabis use occurred in participants’ lives and couldn’t account for differences in marijuana potency or consumption methods.
Discussion & Takeaways
These findings suggest that heavy cannabis use may have lasting effects on brain function, particularly working memory, even after cessation. This has important implications for public health messaging around marijuana use. The research indicates that users may benefit from periods of abstinence before cognitively demanding tasks, though the optimal duration of such breaks remains unclear.
Funding & Disclosures
This research was supported by the National Institute of Mental Health, National Institute on Drug Abuse, and other NIH institutes through the Human Connectome Project. Several authors reported receiving various research grants, but no significant conflicts of interest were disclosed.
Publication Information
This study, titled “Brain Function Outcomes of Recent and Lifetime Cannabis Use,” was published in JAMA Network Open on January 28, 2025. The research was led by Joshua L. Gowin, PhD, and colleagues from multiple institutions including the University of Colorado, National Institute on Alcohol Abuse and Alcoholism, and National Institute on Drug Abuse.
