Long-term cannabis and the midlife brain: what a 45-year cohort found, and what it cannot tell you

**Research review** ## What is known, what is not, and the risks Cannabis is psychoactive, so the risks come first. - **What is known:** in one New Zealand birth cohort, people who had used cannabis over many years had lower IQ by midlife, learned more slowly, processed information more slowly, and had a smaller hippocampus. These are observational findings [S-001]. - **What is not known:** whether these changes lead to dementia in later life. The study did not measure dementia, and the authors say that this question needs more research [S-001]. - **Risks:** the paper gives no safe dose, frequency or form of use, and it should not be read as if it did. Do not start, continue or adjust your use because of this article. If memory, attention or mood worries you or someone close to you, talk to a doctor first. ## The hook More people in midlife and later life are using cannabis. The question is no longer only what it does to a young brain. It is what it may do to the same brain decades later. The study appeared in the American Journal of Psychiatry in 2022. ## What was measured - **The group.** 1,037 people born in 1972–1973 in Dunedin, New Zealand, followed up to age 45, with 94% retention [S-001]. - **Timing.** Cannabis use and dependence were assessed at ages 18, 21, 26, 32, 38 and 45. IQ was measured at 7, 9, 11 and 45. At 45, participants had neuropsychological tests and hippocampal MRI [S-001]. - **Comparisons.** Long-term cannabis users were compared with people who had never used it, midlife recreational users, long-term tobacco smokers, long-term drinkers, and people who had quit cannabis [S-001]. ## What was found - By midlife, long-term cannabis users had lost about 5.5 IQ points on average since childhood. Their learning and processing speed were worse than their childhood IQ would predict. Informants reported more memory and attention problems [S-001]. - These deficits could not be explained by persistent tobacco, alcohol or other drug use, childhood socioeconomic status, low childhood self-control, or family history of substance dependence [S-001]. - Long-term tobacco and alcohol users, midlife recreational cannabis users and people who had quit cannabis showed no such deficits, or smaller ones [S-001]. - Hippocampal volume was smaller in long-term users. According to the paper, though, that smaller volume did not account for the cognitive deficits [S-001]. ## What this does not show - **Cause.** This is an observational study. Even with childhood IQ and other factors taken into account, it cannot prove that cannabis produced these changes. - **Dementia.** Late-life dementia was not measured. The authors treat it as an open question [S-001]. - **Generalisation.** This is one cohort from one region. The study does not show whether the results apply to today's products or to other patterns of use. ## What to do next If this question concerns you or someone close to you, start with a doctor. The article does not offer a "safe level", and you should not go looking for one in it. ## What was verified and what was meant | Claim | Status | Source | |---|---|---| | Meier et al., American Journal of Psychiatry, 2022, 179(5): 362–374 | verified against the abstract (Europe PMC, 8 Nov 2026); full text not opened | S-001 | | 1,037 participants, Dunedin, followed to age 45, 94% retention | verified against the abstract | S-001 | | Long-term users lost about 5.5 IQ points from childhood to age 45, on average | verified against the abstract | S-001 | | Poorer learning and processing speed; informant reports of memory and attention problems | verified against the abstract | S-001 | | Deficits were not explained by tobacco, alcohol, other drugs, childhood socioeconomic status, childhood self-control or family history | verified against the abstract | S-001 | | Tobacco and alcohol users, midlife recreational users and quitters showed no such deficits, or smaller ones | verified against the abstract | S-001 | | Hippocampal volume was smaller in long-term users; it did not explain the deficits | verified against the abstract | S-001 | | Use is rising among midlife and older adults | verified against the abstract | S-001 | | Link to late-life dementia | not verified: the study did not measure it; the authors call for more research | S-001 | | Cannabis causes the deficits | not shown: observational design | S-001 | | "Hippocampus is rich in cannabinoid receptors" as an explanation | background claim from the old post, not in the abstract; dropped | — | | A frequency threshold ("more than once a year, less than weekly" without deficits) | not in the abstract; dropped, because it would read as a safe level | — | | "The authors admit a conflict with earlier data"; the role of the social environment | not in the abstract; dropped | — | Author: Serge Luboff. Reviewer: not assigned. Medical review: not done. This is not medical advice and does not replace a consultation with a doctor.