More frequent marijuana use may damage an important memory skill

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More frequent use of marijuana damages the brain’s working memory, which could lead to issues with safety, communications and work success, a new study found.

“Working memory is the ability to retain information for a short period of time and use it,” said lead study author Joshua Gowin, an assistant professor of radiology at the University of Colorado’s Anschutz School of Medicine in Aurora, Colorado.

“An example is checking your blind spot when driving down the road,” Gowin said. “When you look back in front of you, you need to remember what you saw in the blind spot before you can make a good decision if you want to change lanes or not.

“When you’re in the middle of a conversation with your boss, you need to remember what they said long enough to respond to it,” Gowin said. “Losing working memory means that retaining that information might require more effort and be more challenging.”

Only a connection, not cause and effect

The new observational study, published Tuesday in the journal JAMA Network Open, cannot prove that cannabis harms the brain, said Carol Boyd, professor emerita and founding director of the Center for the Study of Drugs, Alcohol, Smoking and Health at the University of Michigan in Ann Arbor. She was not involved in the study.

However, the study does add scientific evidence to long-standing social beliefs about regular cannabis use, Boyd said in an email.

“For years, clinicians, family and friends who knew heavy cannabis users understood that their memories were ‘shot’ — there’s even a term for this, ‘stoner,’ as in ‘forgot the grocery list, couldn’t follow a recipe, couldn’t get it together,’” she said.

However, the study could not determine if working memory comes back if a heavy user quits and is a nonuser for an extended period of time, she added.

“Can the adolescent who used cannabis more than 1000 times between the ages of 15 and 20 years of age, but stopped using at 20 years, demonstrate improved working memory 10 years later?” Boyd said via email. “That is my 64-dollar question — where is the hope for the person with a chronic use disorder who wants their memory back?”

More frequent use mattered most

The study analyzed data from the Human Connectome Project, which gathers raw data from studies that focus on how age, development, disease and other factors impact the brain.

In what researchers are calling the largest dataset used to study cannabis and brain function, over 1,000 former or current cannabis users underwent brain scans while completing seven different types of cognitive tests between 2012 and 2015.

Those tests measured their working memory, how they used emotion and rewards in their thinking, and how the brain responded to language. In addition, researchers tested how the brain handled motor skills, as well as how each person’s brain responded to others in a social setting.

Study participants, who were between 22 and 36 years of age, provided urine samples on the day of testing to assess recent use. Tetrahydrocannabinol, or THC, which is what provides the euphoria associated with cannabis use, can be detected “for up to 2 weeks in the casual user and possibly longer in the chronic user,” according to the US Centers for Disease Control and Prevention.

Individuals were considered heavy cannabis users if they had used marijuana more than 1,000 times in their lives; moderate users if they had used weed between 10 and 999 times; and nonusers if their use was less than 10 times.

Researchers found cannabis use reduced brain activity in certain areas of the brain responsible for decision-making, memory, paying attention and emotional processing. However, the only test that reached statistical significance was working memory, such as remembering a shopping list or following verbal instructions.

Researchers found 63% of heavy lifetime cannabis users exhibited reduced brain activity during a working memory task, while 68% of people who tested positive for recent use of cannabis also demonstrated a similar impact.

“However, when we compared recent to chronic users of cannabis side by side, we found that chronic use seemed to be more important than recent use when it came to problems with working memory,” Gowin said.

The reduction in brain activation for heavy users relative to non-users was approximately 14%,” he said.

The study did not know the time frame for heavy cannabis use — were all the 1,000 uses in someone’s early 20s and they then had abstained for 10 years? Or had they become recent heavy users?

“Even if a chronic user had stopped using, they still showed cognitive decline in working memory,” he said. “So it did not seem to be contingent upon recent use, but it was related more to chronic use across their lifetime.”

Outstanding questions

Heavy cannabis users in the study tended to be male, with a lower educational and social economic status, and were more likely to be smokers and drink alcohol.

“However, we did include alcohol in our statistical models, and we found that alcohol did not explain the effects we saw, while cannabis use did,” Gowin said.

However, the study was unable to determine levels of THC in the marijuana that was used or control for existing psychological conditions such as ADHD, which is also known to impact working memory, Boyd said.

“It is possible that ADHD is a confounder, distorting the results between heavy cannabis use and impaired working memory, she added. “Young adults with ADHD are disproportionately represented among cannabis users.

“The amount of THC ingested, and the mode of ingestion were not provided,” Boyd said, so there is no way to know “how much THC is involved in chronic, heavy use.”

While it’s true that researchers were unable to determine if the cannabis was smoked, vaped or eaten, the study was conducted prior to 2015 when smoking marijuana was most common, Gowin said.

“And certainly smoking or inhaling cannabis means it crosses the blood-brain barrier more quickly than if you consume an edible,” he said.

So what’s the answer to Boyd’s $64 question: Will regular cannabis users get their working memory back? It’s just too soon to know, Gowin said.

“Some really interesting studies have paid people to stop using cannabis for a period of time, usually a month, and there’s some evidence that they recover some of their memory function over that time,” he said.

“And there is some evidence that a period of abstinence from alcohol can lead to recovery of brain tissue,” Gowin added. “But when it comes to chronic marijuana use and the brain, it’s an area in which investigation is ongoing. Right now we just don’t know.”

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