Study: Marijuana Use Not Linked to Cognitive Decline or Dementia Risk in Older Adults
- The study by Yale University and Oxford researchers found no evidence that marijuana use is linked to faster cognitive decline or increased dementia risk in older adults.
- Data from the UK Biobank and U.S. Million Veteran Program revealed no significant longitudinal cognitive differences or dementia incidence related to cannabis use, despite slightly better baseline cognitive scores among some marijuana users.
- Researchers attributed better baseline cognitive performance to confounding factors like education and socioeconomic status, rather than marijuana use itself, and found no dose-response relationship supporting cognitive improvement from cannabis.
- The study does not prove marijuana is harmless, especially at high doses or prolonged use, and calls for further research with more detailed exposure data and longer follow-up to fully understand marijuana’s impact on cognitive aging.
A new study published by BMJ Mental Health and conducted by researchers from Yale University School of Medicine and the University of Oxford found no evidence that marijuana use is associated with faster cognitive decline or a higher risk of dementia in older adults. Researchers examined data from two large cohorts, the UK Biobank and the U.S. Million Veteran Program, to explore how marijuana use may relate to cognitive performance and dementia risk later in life. The study looked at multiple areas of cognition, including memory, reasoning and processing speed, while also using Mendelian randomization to test whether any apparent associations might be causal.
In the UK Biobank portion of the study, marijuana users showed slightly better performance at baseline on tests of numeric memory and fluid intelligence. But those differences did not hold up when researchers looked at how cognition changed over time. According to the study, “no significant differences were observed in longitudinal cognitive change.” In the Million Veteran Program, researchers also found that cannabis use disorder was not significantly associated with incident all-cause dementia.
The study’s conclusion was direct, stating that “cannabis use was not linked to longitudinal cognitive decline or dementia risk.” Researchers added that, “within the limits of these cohorts, we found no evidence that cannabis use contributes substantially to cognitive ageing or dementia risk in older adults.”
The findings are notable given the growing use of marijuana among older adults for both medical and recreational purposes. Researchers said previous studies have often been limited by small sample sizes or cross-sectional designs, making it harder to determine whether marijuana itself plays a meaningful role in long-term cognitive outcomes.
Importantly, the authors said the modestly better baseline scores seen among some users were likely not caused by marijuana itself. Instead, they pointed to factors such as education, socioeconomic status and other confounding variables as more likely explanations. The lack of a dose-response pattern, along with the absence of meaningful findings in the longitudinal and genetic analyses, further weakened the idea that marijuana use was improving cognition.
The researchers were also careful not to frame the findings as proof that marijuana is harmless. They noted that the study did not establish safety at higher doses or with prolonged use, and it did not capture detailed information on potency or long-term exposure patterns. The authors said “occasional or prior use may not substantially affect cognitive ageing,” but stressed that more research with better exposure data and longer follow-up is still needed.
Overall, the study adds to growing evidence that marijuana use, at least as measured in these large cohorts, does not appear to be a major driver of cognitive decline or dementia in older adults.