A large-scale study by researchers at an Oxford University-based research facility has found no link between cannabis use and cognitive decline in older people, challenging concerns about the drug’s long-term effects on brain health.
The study, conducted by researchers from Oxford Population Health and the Department of Psychiatry, was published in the journal BMJ Mental Health and used data from two large datasets: the UK Biobank (UKB) and the US Million Veteran Program (MVP).
It was the largest observational analysis of cannabis and cognitive decline, and the first to use a Mendelian randomisation (MR) study approach to examine more thoroughly relationships between cannabis use and cognitive deficit.
Cannabis is used by an estimated 228 million people worldwide. In the past decade, there has been a surge in older people using the drug, driven by a decrease in stigma. Due to legalisation for medicinal use, more people are considering it as a potential treatment and not a recreational drug.
Some experts believe cannabis causes memory loss and other similar cognitive difficulties, despite evidence from research producing contradicting results. The study set out to address this gap using detailed data on cannabis use and patient health.
Researchers analysed data from nearly 19,000 people in the UKB between 2006 and 2010 and over 222,500 people in the MVP from 2011 to the present.
The UK data provided detailed cognitive test results over time from a large population sample, but had relatively few dementia cases among cannabis users. The US data, meanwhile, included many more cannabis users who developed dementia and had greater ethnic diversity. By combining the two datasets, researchers could compare findings across populations and levels of cannabis use, thereby enhancing the reliability of their conclusions.
“The use of both cohorts addresses complementary aspects of the research question,” the study said. “Together, these cohorts enhance external validity and allow triangulation of findings across different populations, exposure severities and outcome types.”
In the UKB cohort, cannabis users were divided into low frequency (one to 10 uses) or high frequency users (11 to 100+ times), whilst MVP participants were identified through a history of cannabis use disorder recorded in electronic health records.
Researchers compared sociodemographic, lifestyle, medical history and health behaviour data from a non-cannabis control group with data from patients in the cannabis cohort. The cohort was tested on five different cognitive tests, including numeric memory, pair matching and problem solving.
The results showed that cannabis users performed slightly better in some of the tests than non-users, although researchers said this could be attributed to those from the UKB cohort holding higher socioeconomic status and education, and shouldn’t be taken as evidence of cannabis use leading to improved outcomes in old age.
“Although cannabis users performed slightly better on some cognitive tests at baseline, this should not be interpreted as cannabis improving cognition. The differences we observed are more likely explained by underlying demographic, educational and socioeconomic factors that differ between users and non-users, rather than a protective effect of cannabis itself. Our findings underscore the importance of carefully distinguishing association from causation in research on brain health,” said Saba Ishrat, DPhil candidate in the Department of Psychiatry, and lead author of the paper.
The authors concluded that, although the cannabis-using group were found to have fewer instances of dementia and cognitive decline, cannabis should not be considered harmless.
“Across large UK and US cohorts, and using genetic approaches to explore potential causal relationships, we found no evidence that cannabis use was associated with accelerated cognitive decline or increased dementia risk in older adults. However, this should not be interpreted as cannabis being risk-free. Cannabis use is associated with other adverse health outcomes, and further research is needed to understand the effects of higher-dose, long-term use on brain health in later life,” said Dr Anya Topiwala, senior clinical researcher at Oxford Population Health and honorary consultant psychiatrist at Oxford Health NHS Foundation Trust.
