Cannabis and the Hippocampus: What a 30-Year MRI Study Found

The hippocampus contributes to learning and the formation and retrieval of memory. Earlier research has reported mixed findings about cannabis use, memory and brain structure. The 2026 CARDIA analysis asked whether cumulative exposure measured repeatedly over decades was associated with hippocampal volume, white-matter integrity or blood flow in middle age.

The study included 648 participants with an average age of 55. Cannabis use had been assessed during follow-up visits beginning in 1985. Researchers summarized exposure in cannabis-years, where one cannabis-year represented 365 days of reported use, and adjusted the analyses for factors including tobacco, alcohol, education and other health variables.

The researchers found no statistically significant association between cumulative cannabis exposure and hippocampal volume, fractional anisotropy or cerebral blood flow. Results were also not significantly different when examined by sex or tobacco-use history. The finding was consistent across several sensitivity analyses.

The result applies to three MRI measures in one brain region within this cohort. It does not prove that cannabis has no acute cognitive effects, produces no changes elsewhere in the brain or is risk-free. It also does not show that cannabis improves memory or protects against age-related cognitive decline.

Heavy users were probably underrepresented. The study did not have detailed information about THC or CBD content, product potency, route of use, dependence or the exact timing of recent consumption before MRI. Exposure was self-reported at visits spaced two to five years apart, and MRI was performed at one point in middle age.

The authors suggested examining other neural mechanisms, including task-based imaging and more detailed hippocampal subregions. Future work also needs better product characterization, clearer dose information, stronger representation of heavy use and attention to social and health factors that may influence both cannabis exposure and cognition.

The responsible headline is precise: this study did not detect an association with the hippocampal outcomes it measured. That is different from saying cannabis has no effect on the brain. Strong research communication states exactly what was measured, what was found and what remains unknown.