Scientists say more frequent use of marijuana could damage the brain’s working memory, which may lead to issues with safety, communications and work success, according to their recent research.
However, the findings could not prove that cannabis actually harms the brain, said one expert.
“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.
“An example is checking your blind spot when driving down the road. 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.
“Losing working memory means that retaining that information might require more effort and be more challenging.”
Only a connection, not cause and effect
The observational study, published in JAMA Network Open, cannot prove that cannabis harms the brain, said Carol Boyd, professor emerita and founding director of the Centre for the Study of Drugs, Alcohol, Smoking and Health at the University of Michigan, who was not involved in the study.
But it does add scientific evidence to long-standing social beliefs about regular cannabis use, she told CNN.
“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.
Yet the study could not determine if working memory comes back if a heavy user quits and is a non-user for an extended period of time, she added.
“Can the adolescent who used cannabis more than 1 000 times between 15 and 20 years old, but who stopped using at 20 years, demonstrate improved working memory 10 years later?” Boyd asked. “That is my 64m-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 analysed 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, more than 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 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 two weeks in the casual user and possibly longer in the chronic user”, according to the US Centres for Disease Control and Prevention.
Individuals were considered heavy users if they had ingested marijuana more than 1 000 times in their lives; moderate users if they had used weed between 10 and 999 times; and non users 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.
But 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.
“Yet 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 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.
“When we did include alcohol in our statistical models, we found that alcohol did not explain the effects we saw, while cannabis use did,” Gowin said.
The study was also unable to determine levels of THC in the marijuana that was used or control for existing psychological conditions like 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, 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 before 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 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. 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.”
Study details
Brain Function Outcomes of Recent and Lifetime Cannabis Use
Joshua Gowin, Jarrod Ellingson, Hollis Karoly, et al.
Published in JAMA Network on 28 January 2025
Abstract
Importance
Cannabis use has increased globally, but its effects on brain function are not fully known, highlighting the need to better determine recent and long-term brain activation outcomes of cannabis use.
Objective
To examine the association of lifetime history of heavy cannabis use and recent cannabis use with brain activation across a range of brain functions in a large sample of young adults in the US.
Design, Setting, and Participants
This cross-sectional study used data (2017 release) from the Human Connectome Project (collected between August 2012 and 2015). Young adults (aged 22-36 years) with magnetic resonance imaging (MRI), urine toxicology, and cannabis use data were included in the analysis. Data were analysed from January 31 to July 30, 2024.
Exposures
History of heavy cannabis use was assessed using the Semi-Structured Assessment for the Genetics of Alcoholism, with variables for lifetime history and diagnosis of cannabis dependence. Individuals were grouped as heavy lifetime cannabis users if they had greater than 1000 uses, as moderate users if they had 10 to 999 uses, and as nonusers if they had fewer than 10 uses. Participants provided urine samples on the day of scanning to assess recent use. Diagnosis of cannabis dependence (per Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition criteria) was also included.
Main Outcomes and Measures
Brain activation was assessed during each of the 7 tasks administered during the functional MRI session (working memory, reward, emotion, language, motor, relational assessment, and theory of mind). Mean activation from regions associated with the primary contrast for each task was used. The primary analysis was a linear mixed-effects regression model (one model per task) examining the association of lifetime cannabis and recent cannabis use on the mean brain activation value.
Results
The sample comprised 1003 adults (mean [SD] age, 28.7 [3.7] years; 470 men [46.9%] and 533 women [53.1%]). A total of 63 participants were Asian (6.3%), 137 were Black (13.7%), and 762 were White (76.0%). For lifetime history criteria, 88 participants (8.8%) were classified as heavy cannabis users, 179 (17.8%) as moderate users, and 736 (73.4%) as nonusers. Heavy lifetime use (Cohen d = −0.28 [95% CI, −0.50 to −0.06]; false discovery rate corrected P = .02) was associated with lower activation on the working memory task. Regions associated with a history of heavy use included the anterior insula, medial prefrontal cortex, and dorsolateral prefrontal cortex. Recent cannabis use was associated with poorer performance and lower brain activation in the working memory and motor tasks, but the associations between recent use and brain activation did not survive false discovery rate correction. No other tasks were associated with lifetime history of heavy use, recent use, or dependence diagnosis.
Conclusions and Relevance
In this study of young adults, lifetime history of heavy cannabis use was associated with lower brain activation during a working memory task. These findings identify negative outcomes associated with heavy lifetime cannabis use and working memory in healthy young adults that may be long lasting.
JAMA Network article – Brain Function Outcomes of Recent and Lifetime Cannabis Use (Open access)
CNN article – More frequent marijuana use may damage an important memory skill (Open access)
See more from MedicalBrief archives:
Cannabis and brain health – American Heart Association scientific statement
Does marijuana have any medical benefits?
Rise in number of young South Africans seeking help for cannabis abuse
Cannabis recreational consumption ‘not benign’ – Ontario cohort study