Older adults who have used cannabis do not appear to experience faster cognitive decline or a higher risk of dementia, according to a study published in *BMJ Mental Health*. The research also found no genetic evidence that cannabis use directly causes poorer brain health later in life.
As cannabis becomes increasingly available for both medical and recreational use, its popularity has grown among older adults. This has raised concerns about whether cannabis might accelerate memory loss or increase the likelihood of developing dementia. Previous research has produced conflicting results, with some studies suggesting long-term cognitive impairment while others found little evidence of lasting harm. Many of those studies, however, were relatively small or unable to determine whether cannabis itself was responsible for the observed effects.
To better answer this question, researchers led by Saba Ishrat from the University of Oxford analyzed data from two large health databases. The UK Biobank analysis included up to 18,975 people who had used cannabis and 60,598 non-users, with cannabis users averaging about 58 years of age and non-users about 62 years.
The team also examined electronic health records from the Million Veteran Program, including 12,222 people with cannabis use disorder and more than 210,000 comparison participants, to investigate dementia diagnoses. Cognitive tests assessed included working memory, reasoning, processing speed, executive function, and visual memory. Finally, the researchers performed Mendelian randomization analyses using genetic data to test whether cannabis use itself is likely to cause cognitive decline or dementia.
The results were reassuring overall. At the beginning of the study, people who had used cannabis actually performed slightly better on tests of numeric memory, reasoning, and problem-solving ability, compared to non-users. However, these differences were small and did not translate into better long-term outcomes. The researchers hypothesize that the better test performance of cannabis users may be confounded by educational background and socioeconomic status, as cannabis users in the UK Biobank dataset tended to have higher levels of both.
Over time, cannabis users did not experience faster declines in thinking abilities, and people with cannabis use disorder were not significantly more likely to develop dementia than those without the disorder.
The genetic analyses strengthened these findings. Because genetic variants are assigned before birth and are less affected by lifestyle or environmental factors, Mendelian randomization can provide stronger evidence about cause and effect than observational studies alone. The analyses found no evidence that cannabis use causes cognitive decline or dementia, nor that poorer cognitive function increases the likelihood of cannabis use.
Results remained broadly consistent after examining males versus females, different frequencies of cannabis use, different age groups, and different ethnicities.
“Clinicians can consider that occasional or prior cannabis use may not be a major contributor to cognitive aging in this population,” Ishrat and colleagues concluded. “However, these results do not establish the safety of cannabis, particularly at higher doses or with prolonged use, and should not be taken as an endorsement of use.”
Some limitations are to be noted. For instance, the researchers relied on self-reported data for cannabis use in the UK Biobank dataset, which introduces the risk of recall and reporting bias. The study could not determine whether different cannabis products, cannabidiol (CBD) and tetrahydrocannabinol (THC) concentrations, doses, and methods of use might have different effects.
The study, “Cannabis use, cognitive function and dementia risk in older adults: observational and genetic analyses,” was authored by Saba Ishrat, Daniel F. Levey, Joel Gelernter, Klaus P. Ebmeier, and Anya Topiwala.
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