The truth about cannabis: Researchers from largest study on its effects on the brain have a warning for young adults

The myth that cannabis is harmless because it’s natural has been further debunked—by scientists in a pot-friendly state. New research from the University of Colorado Anschutz Medical Campus shows that heavy lifetime cannabis use may harm working memory in young adults.

In what the university calls the largest study of its kind, researchers used brain imaging technology to explore the effects of recent and lifetime cannabis use on brain function among more than 1,000 people ages 22 to 36. During a task assessing working memory—the short-term storage of information that can be applied to activities such as reasoning and problem-solving—the majority of recent (68%) and heavy lifetime (63%) cannabis users showed reduced brain activity. The findings were published Jan. 28 in the journal JAMA Network Open.

“As cannabis use continues to grow globally, studying its effects on human health has become increasingly important,” study coauthor Joshua Gowin, PhD, an assistant professor of radiology at the University of Colorado School of Medicine, said in a news release about the research. “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.”

Participants were assigned seven types of cognitive tasks designed to test language, reward, emotion, motor function, theory of mind, working memory, and relational or logical reasoning. Gowin and his colleagues used functional magnetic resonance imaging (fMRI) to analyze their neural responses. While urine samples were used to determine participants’ recent cannabis use, lifetime cannabis use was defined as:

• Heavy: 1,000+ times or diagnosis of cannabis dependence

• Moderate: 10–999 times

• Nonuser: <10 times

In the working memory task, heavy lifetime cannabis use was associated with lower brain activation, a link that remained even when discounting participants who had recently used cannabis. Variables including alcohol use, nicotine use, age at first cannabis use, and demographics didn’t explain the connection, the authors noted.

“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,” Gowin said. “While some of the other tasks indicated potential cognitive impairment, only the working memory task showed a statistically significant impact.”

What’s more, Gowin and his team found in the working memory task that heavy cannabis use correlated to reduced activity in specific areas of the brain—the anterior insula, dorsolateral prefrontal cortex, and dorsomedial prefrontal cortex—involved in cognitive functions such as attention, memory, decision-making, and emotional processing.

Recent cannabis use was linked to poorer performance and lower brain activation in the motor and working memory tasks, but its associations with brain activation didn’t withstand FDR correction. The team said its findings support previous research indicating that heavy cannabis use, even absent recent use, alters brain activation. In addition, the study suggests abstaining from cannabis ahead of cognitively demanding situations may improve performance.

“People need to be aware of their relationship with cannabis since abstaining cold turkey could disrupt their cognition as well,” Gowin said. “For example, heavy users may need to be more cautious.”

One study limitation is that it was difficult to pinpoint when recent cannabis users had last consumed the drug, not to mention the dose, potency, and route of administration. And despite the large sample size, subgroups such as women with a positive urine sample were relatively small, the authors said.

“There are a lot of questions we still need answers to regarding how cannabis impacts the brain,” Gowin said. “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.”

If you or a loved one are struggling with cannabis use, help is available 24/7. Visit FindTreatment.gov, text your zip code to HELP4U (435748), or call the SAMHSA National Helpline at 800-662-HELP (4357) or TTY: 800-487-4889.

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This story was originally featured on Fortune.com

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