According to a study published in *BMJ Mental Health*, older adults who have used cannabis do not appear to have faster cognitive decline or increased risk of dementia. The study also found no genetic evidence that cannabis use directly causes poor brain health later in life.
As cannabis becomes available for both medical and recreational use, its popularity among older adults is increasing. This has led to concerns that cannabis may accelerate memory loss or increase the likelihood of developing dementia. Previous studies have yielded conflicting results, with some suggesting long-term cognitive impairment and others finding little evidence of lasting harm. However, many of those studies were relatively small or could not determine whether cannabis itself was responsible for the observed effects.
To better answer this question, researchers led by Saba Ishrat at the University of Oxford analyzed data from two large health databases. The UK Biobank analysis included up to 18,975 people who had ever used cannabis and 60,598 people who had never used cannabis, with the average age of cannabis users being around 58 years and the average age of non-users being around 62 years.
The team also examined electronic health records from the Million Veterans Program, which includes 12,222 people with cannabis use disorders and more than 210,000 comparison participants, to examine dementia diagnoses. Cognitive tests assessed included working memory, reasoning, processing speed, executive function, and visual memory. Finally, the researchers conducted a Mendelian randomization analysis using genetic data to test whether cannabis use itself was more likely to cause cognitive decline or dementia.
The results were generally reassuring. At the start of the study, people who had actually used cannabis performed slightly better on tests of numerical memory, reasoning, and problem-solving compared to non-users. However, these differences were small and did not lead to improved long-term outcomes. The researchers hypothesized that higher test scores among cannabis users may be confounded by educational background and socio-economic status, as cannabis users in the UK Biobank dataset tended to have higher levels of both.
Over time, cannabis users did not experience a rapid decline in thinking ability, and people with cannabis use disorder were not significantly more likely to develop dementia than those without the disorder.
Genetic analysis strengthened these findings. Because genetic variation is assigned prenatally and is less influenced by lifestyle and environmental factors, Mendelian randomization can provide stronger evidence for causality than observational studies alone. The analysis found no evidence that cannabis use causes cognitive decline or dementia, and no evidence that cognitive decline increases the likelihood of cannabis use.
The results were largely consistent even after looking at men and women, different frequencies of cannabis use, different age groups, and different ethnicities.
“Clinicians can consider that occasional or previous cannabis use may not be the main cause of cognitive aging in this population,” Ishrat et al. concluded. “However, these results do not prove the safety of cannabis, especially at high doses or long-term use, and should not be taken as a recommendation for its use.”
Please note some limitations. For example, researchers rely on self-reported data on cannabis use in the UK Biobank dataset, which introduces risks of recall and reporting bias. This study could not determine whether different concentrations, doses, or methods of use of cannabis products, cannabidiol (CBD) and tetrahydrocannabinol (THC), have different effects.
The study, “Cannabis Use, Cognitive Function, and Dementia Risk in Older Adults: Observational and Genetic Analysis,” was authored by Saba Ishrat, Daniel F. Levey, Joel Gelernter, Klaus P. Ebmeier, and Anya Topiwala.
