Experts have warned that cannabis may not be as harmless as once thought, after recent research suggested that middle-aged – and older – adults who sought hospital or emergency room care because of cannabis use were almost twice as likely to develop dementia over the next five years, compared with similar people in the general population.
The study also found that when compared with adults who sought care for other reasons, the risk of developing dementia was still 23% higher among the cannabis users.
The New York Times reports that the study included the medical records of 6m people in Ontario from 2008 to 2021. The authors accounted for health and socio-demographic differences between comparison groups, some of which play a role in cognitive decline.
The data do not reveal how much cannabis the subjects had been using, and the study does not prove that regular or heavy cannabis use plays a causal role in dementia.
But the finding does raise serious concerns that require further exploration, said Dr Daniel Myran, the first author of the study, which was published in JAMA Neurology.
“Figuring out whether or not cannabis use or heavy regular chronic use causes dementia is a challenging and complicated question that you don’t answer in one study,” said Myran, an assistant professor of family medicine at the University of Ottawa. “This contributes to the literature and to a sign, or signal, of concern.”
Myran’s previous research has found that patients with cannabis use disorder died at almost three times the rate of individuals without the disorder over a five-year period. He has also reported that more cases of schizophrenia and psychosis in Canada have been linked to cannabis use disorder since the drug was legalised.
The latest study, focusing on dementia, adds to a growing body of literature on regular or heavy cannabis use and cognition. Researchers have reported impacts on verbal learning, memory and attention, while imaging studies have pointed to changes in the brain related to the use of cannabis and other substances.
Veterans with traumatic brain injuries in addition to cannabis use disorder may be at heightened risk for early-onset dementia, researchers reported last year. But many of the studies are relatively small.
The new report’s strength was its large sample and the ability to track patients over time who did not have a diagnosis of dementia when they entered the study, said Madeline Meier, an associate professor of psychology at Arizona State University, who was not involved in the study.
“They were able to rule out dementia at the time of the first cannabis visit, and were able to show the temporal order – the cannabis came first, and the dementia came second,” she said.
Her research, which has followed and periodically tested a cohort of more than 1 000 individuals over a period of many years, has linked cannabis use and neuropsychological decline.
“I think you want to combat this whole idea that cannabis is harmless and maybe even has some medical benefits,” Meier said. “This study is showing an association that I think people should take seriously and say, ‘Maybe this is putting me at risk’.”
She noted that the work by Myran and his colleagues also found that people seeking care for alcohol use were even more likely to receive a dementia diagnosis than were cannabis users.
“I’m worried about the substance abuse in that Baby Boomer age,” she said.
More and more people, including seniors, are using cannabis. Medical visits related to cannabis increased more than five-fold among adults 45 and older between 2008 and 2021, the new study found. Among adults 65 and over, visits increased almost 27-fold.
The study included more than 6m people aged 45 and over who did not have a diagnosis of dementia at the start of the research. Of them, 16 275 had an acute-care medical encounter because of cannabis.
The patients with cannabis-related visits were compared with the matched general population, and in a separate comparison, to 140 824 matched patients who needed medical care for all other reasons.
Within five years, 5% of those with acute care cannabis visits received a dementia diagnosis. The figure for individuals needing care for other reasons was 3.6%, and for similar individuals in the general population, 1.3%.
But people who are heavy cannabis users differ from those who are not, in a variety of ways, some of which may help explain the increased risk for dementia, Myran explained.
While some of the factors can be accounted for, he said, “you can’t control for all of them”.
Another unknown is self-medication, he said. Someone who has started experiencing symptoms of cognitive decline may be more likely to turn to cannabis. “If so, then it looks as if the cannabis is causing dementia, but it’s just on the pathway – they were already developing dementia.”
After making adjustments for age, sex, income and other factors, including other health conditions, he and his colleagues determined that patients who sought care for cannabis-related reasons were 1.23 times as likely as those who had got any kind of acute care to be diagnosed with dementia, and 1.72 times as likely as those in the healthier general population.
Study details
Risk of dementia in individuals with emergency department visits or hospitalisations due to cannabis
Daniel Myran, Michael Pugliese, Lyndsay Harrison, Nathan Stall, Colleen Webber.
Published in JAMA Neurology on 14 April 2025
Abstract
Importance
Cannabis use is associated with short-term memory impairment and long-term changes in brain structure; however, little is known about whether disordered cannabis use is associated with an increased risk of a dementia diagnosis.
Objective
To investigate the association between emergency department visits or hospitalisations (acute care encounters) due to cannabis and future dementia diagnosis.
Design, Setting, and Participants
Population-based, retrospective, matched cohort study using health administrative data from Ontario, Canada, between 2008 and 2021 (with follow-up until 2022) including all individuals aged 45 to 105 years living in Ontario who were eligible and did not have a diagnosis of dementia at cohort entry (2 620 083 individuals excluded).
Exposure
Individuals with incident acute care due to cannabis use, defined using International Classification of Diseases and Related Health Problems, Tenth Revision coding.
Main Outcomes and Measures
We used cause-specific adjusted hazard models to compare new diagnoses of dementia (from a validated algorithm) between individuals with acute care due to cannabis use with (1) individuals with all-cause acute care (excluding cannabis), (2) the general population, and (3) individuals with acute care due to alcohol use.
Results
The study included 6 086 794 individuals, of whom 16 275 (0.3%) had incident acute care due to cannabis use (mean age, 55.2 [SD, 8.3] years; 60.3% male). Annual rates of incident acute care due to cannabis use increased 5.0-fold in individuals aged 45 to 64 years (from 10.16 to 50.65 per 100 000) and 26.7-fold in individuals aged 65 years or older (from 0.65 to 16.99 per 100 000) between 2008 and 2021. Individuals with incident acute care due to cannabis use were at a 1.5-fold and 3.9-fold increased risk of dementia diagnosis within 5 years relative to individuals with all-cause acute care and the general population of the same age and sex, respectively (absolute rates of dementia diagnosis: 5.0% for cannabis-related acute care, 3.6% for all-cause acute care, and 1.3% in the general population). After adjustment for socio-demographics and chronic health conditions, individuals with acute care due to cannabis use remained at elevated risk relative to those with all-cause acute care (adjusted hazard ratio [aHR], 1.23; 95% CI, 1.09-1.39) and the general population (aHR, 1.72; 95% CI, 1.38-2.15). Individuals with acute care due to cannabis use were at lower risk than those with acute care due to alcohol use (aHR, 0.69; 95% CI, 0.62-0.76).
Conclusions and Relevance
Individuals with cannabis use severe enough to require hospital-based care were at increased risk of a new dementia diagnosis compared with those with all-cause hospital-based care or the general population. These findings have important implications considering increasing cannabis use among older adults.
See more from MedicalBrief archives:
Is marijuana as safe as we think?
Active ingredient in cannabis protects ageing brain cells, may help Alzheimer’s
Cannabis and brain health – American Heart Association scientific statement
Frequent cannabis use could affect crucial memory skill – US study