A landmark investigation into cannabis effects on brain function has uncovered concerning findings about working memory in young adults. Published in JAMA Network Open in January 2025, this research represents the most comprehensive examination of its kind to date.

The study analysed brain imaging data from more than 1,000 participants aged between 22 and 36, making it the largest investigation ever conducted into how marijuana impact on cognition manifests in everyday mental tasks. Researchers discovered that both regular and recent users demonstrated measurable changes in brain activity when performing memory-related challenges.

Working Memory Takes the Biggest Hit

Working memory, the cognitive system that allows us to hold and manipulate information temporarily, emerged as particularly vulnerable. This mental function enables people to follow spoken instructions, mentally calculate figures, or remember a phone number long enough to dial it.

The findings were striking: 63% of heavy lifetime users and 68% of recent users exhibited reduced brain activity during working memory tasks. These individuals showed decreased function in critical brain regions including the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula. These are areas responsible for decision-making, attention, and emotional processing.

Dr Joshua Gowin, assistant professor of radiology at the University of Colorado School of Medicine and the study’s lead author, emphasised the importance of understanding these cannabis effects on brain health. “As cannabis use continues to grow globally, studying its effects on human health has become increasingly important,” he explained.

How the Research Was Conducted

Participants underwent magnetic resonance imaging (MRI) scans whilst completing seven distinct cognitive assessments. These tasks examined working memory, reward processing, emotional responses, language abilities, motor skills, relational thinking, and theory of mind.

The research team applied rigorous statistical standards, employing false discovery rate correction to minimise the risk of false positives. Whilst some cognitive domains showed potential impairment, only working memory demonstrated statistically significant marijuana impact on cognition. This means the observed effects were extremely unlikely to result from chance.

For the purposes of the study, heavy users were defined as young adults who had consumed cannabis more than 1,000 times throughout their lives. Moderate users ranged from 10 to 999 lifetime uses, whilst non-users had consumed cannabis fewer than 10 times.

The Abstinence Factor

Interestingly, the research suggests that abstaining from cannabis before performing cognitive tasks might improve outcomes. However, Dr Gowin cautioned that the relationship isn’t straightforward: “People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well. For example, heavy users may need to be more cautious.”

This nuance highlights the complexity of cannabis effects on brain function and suggests that withdrawal itself may temporarily affect cognitive performance in regular users.

What Comes Next?

Despite the scale of this investigation, Dr Gowin acknowledged that many questions remain unanswered. “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 next to understand whether cannabis use directly changes brain function, how long these effects last, and the impact on different age groups.”

The research team included scientists from the University of Colorado Anschutz Medical Campus, the National Institutes of Health, and the Centre for Addiction and Mental Health in Toronto. Their collaborative effort represents a significant step forward in understanding the marijuana impact on cognition, particularly amongst young adults whose brains are still developing.

As cannabis legalisation expands across numerous jurisdictions worldwide, these findings provide crucial evidence for individuals making decisions about use. The documented effects on working memory (a fundamental cognitive ability required for countless daily activities) underscore the importance of understanding potential risks alongside any perceived benefits.

The study’s publication in a prestigious peer-reviewed journal adds weight to the evidence base, offering healthcare professionals and policymakers robust data to inform guidance and support services for cannabis users.

Source: The Star, University of Colorado Anschutz