In a world where cannabis is increasingly legalized and normalized, questions about its effects on the brain have never been more pressing. A recent study, the largest and most comprehensive of its kind, offers some of the clearest insights yet into how cannabis use affects brain function during cognitive tasks.

Researchers uncovered a striking pattern: heavy cannabis users show reduced brain activity during working memory tasks, the cognitive function that helps us retain and use information in the moment. The findings suggest that the drug’s effects on the brain may be more persistent than previously thought.

The Brain on Cannabis: A Closer Look

The study was led by Dr. Joshua Gowin, an assistant professor of radiology at the University of Colorado School of Medicine. It examined over 1,000 young adults aged 22 to 36. Using advanced brain imaging technology, the researchers discovered that 63% of heavy lifetime cannabis users and 68% of recent users exhibited diminished brain activity during working memory tasks. This decline was associated with poorer performance on tasks that require holding and manipulating information, such as following instructions or solving a simple math problem.

“As cannabis use continues to grow globally, studying its effects on human health has become increasingly important,” Gowin said. “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 study categorized participants based on their cannabis use. They defined heavy users as those who had used cannabis more than 1,000 times over their lifetime. Moderate users had used it between 10 and 999 times, while nonusers had used it fewer than 10 times. This classification allowed the researchers to explore how varying levels of use might correlate with brain function. They also tested for recent cannabis use by analyzing urine samples.

Cannabis and Working Memory

Participants underwent magnetic resonance imaging (MRI) while completing seven cognitive tasks. These tasks tested working memory, reward processing, emotion regulation, language skills, motor skills, relational reasoning, and theory of mind (the ability to understand others’ mental states). Among these, working memory tasks stood out.

The researchers found a statistically significant reduction in brain activity during these tasks. And both recent and lifetime cannabis users showed this trend. This raises questions about how long cannabis’s acute effects linger and whether they might compound over time with heavy use. Other tasks showed less pronounced effects.

“We applied the highest standards to our research, setting rigorous thresholds for statistical significance across all seven cognitive function tests,” Gowin explained. “While some of the other tasks indicated potential cognitive impairment, only the working memory task showed a statistically significant impact.”

The study pinpointed specific areas of the brain showing reduced activity in heavy cannabis users during working memory tasks. These regions — the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula — are linked with higher-order cognitive functions, including decision-making, memory, attention, and emotional processing.

The Bigger Picture

In the United States alone, nearly half of all states have legalized recreational cannabis, and its use is on the rise. Yet, despite its growing acceptance, the long-term effects of cannabis on the brain remain poorly understood.

The study’s findings align with earlier research showing that cannabis use can alter brain structure and function. For example, studies have found that heavy cannabis use is associated with reduced thickness in the prefrontal cortex, a region involved in decision-making and self-control. However, these weren’t causal links. One could argue that people with these innate cognitive characteristics are more prone to cannabis use in the first place.

Other research has shown that cannabis can disrupt the brain’s endocannabinoid system, which plays a key role in regulating mood, memory, and appetite.

But the new study goes further by examining how these changes manifest during specific cognitive tasks. The reduced brain activation seen in heavy users could help explain why some people who use cannabis heavily struggle with memory and attention in their daily lives. It also raises concerns about the potential long-term consequences of cannabis use, especially for young people whose brains are still developing.

Interpreting the Findings

The researchers caution that their study is cross-sectional, meaning it provides a snapshot in time rather than tracking changes over years. This makes it difficult to establish causality — whether cannabis use directly causes these brain changes or whether people with certain brain characteristics are more likely to use cannabis heavily. Longitudinal studies, which follow participants over time, are more suited for answering these types of questions.

In any event, Gowin noted that abstaining from cannabis well before engaging in cognitive tasks could improve performance. This may be something worth considering if you need to pass an upcoming important exam, for instance. “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.”

For now, the message is clear: while cannabis may be increasingly accepted, it is not without risks. In the meantime, Gowin’s advice is simple: “Be mindful of how cannabis affects you. Understanding its impact on your brain is the first step toward making informed decisions.”

The findings appeared in the journal JAMA Network Open.