BACKGROUND: The cognitive effects of cannabis use in older adults remain unclear, despite increasing use for medical and recreational purposes in this age group.

OBJECTIVE: To investigate associations between cannabis use, cognitive performance and dementia risk in older individuals, using large population cohorts and Mendelian randomisation (MR) to explore potential causal relationships.

METHODS: Observational analyses were conducted using the UK Biobank (UKB) and the US Million Veteran Program (MVP). In UKB, cross-sectional and longitudinal cognitive performance across five domains was compared between lifetime cannabis users (up to 18 975 participants) and non-users (up to 60 598 participants). In MVP, cannabis use disorder (CanUD; n=12 222) was examined in relation to incident all-cause dementia using Cox proportional hazards models. MR analyses assessed potential bidirectional causal relationships between cannabis use, cognitive function and dementia.

FINDINGS: At baseline, cannabis users performed modestly better on tests of numeric memory (beta=0.07, 95% CI 0.06 to 0.09, p<0.001) and fluid intelligence (beta=0.12, 95% CI 0.10 to 0.13, p<0.001), but no significant differences were observed in longitudinal cognitive change. In MVP, CanUD was not significantly associated with dementia risk (HR=1.11, 95% CI 0.97 to 1.26, p=0.12). MR analyses provided no evidence of a causal relationship between cannabis use and either cognitive performance or dementia risk.

CONCLUSIONS: Cannabis use was not linked to longitudinal cognitive decline or dementia risk. Within the limits of these cohorts, we found no evidence that cannabis use contributes substantially to cognitive ageing or dementia risk in older adults. Further research with detailed exposure assessment and longer follow-up is warranted to confirm these findings.

CLINICAL IMPLICATIONS: Cannabis use in older adults does not appear linked to faster cognitive decline or higher dementia risk. Occasional or prior use may not substantially affect cognitive ageing, though safety at higher doses or prolonged use remains uncertain. Clinicians should inquire about cannabis history and consider cognitive screening in routine care.