A large observational study by researchers at Oxford Population Health and the Department of Psychiatry has found no evidence that cannabis use contributes to cognitive decline in later life, or that it increases the risk of dementia. The findings are published in BMJ Mental Health.
Cannabis is the most widely used illicit substance globally, with use increasing significantly among older adults in recent years, largely driven by its legalisation for medical and recreational use in many countries. Some studies have found associations between cannabis use and impairments to memory, recall and brain processing speed, but the findings are inconsistent and little is known about how the drug’s psychoactive compounds influence gradual declines in brain function associated with ageing, or neurogenerative diseases such as dementia.
The study set out to address this gap using data from two large cohorts followed over a number of years – the UK Biobank (UKB) and the US Million Veteran Program (MVP). Nearly 19,000 UKB participants who self-reported as having ever used cannabis were divided into low frequency (1 – 10 uses of cannabis) or high frequency users (11 – 100+ times). MVP participants were identified as cannabis users through a history of ‘cannabis use disorder’ recorded in their electronic health records.
Cannabis users’ performance in five different cognitive tests, including numeric memory, pair matching and problem solving, was then compared with a control group of non-cannabis users.
The cannabis users performed significantly better on some of the tests than non-users, although the authors note that this was less likely due to any benefit from the cannabis than the fact that cannabis users in the UKB cohort tended to come from a higher socioeconomic background and have higher levels of education than non-users. Genetic analyses also supported this conclusion.
Saba Ishrat, DPhil candidate in the Department of Psychiatry, and lead author of the paper, said ‘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.’
Dr Anya Topiwala, Senior Clinical Researcher at Oxford Population Health and Honorary Consultant Psychiatrist at Oxford Health NHS Foundation Trust, said ‘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.’
The researchers acknowledged the limitations of using data from the UK Biobank due to the participants being predominantly healthier than the general population and less likely to be heavy cannabis users. Conversely, relying on diagnoses of cannabis use disorder in the MVP group likely created a bias towards more severe cases. They were also unable to measure the potency of cannabis used.