Long-Term Cannabis Users Lost 5.5 IQ Points and Had Smaller Hippocampi by Midlife
In a landmark 45-year cohort study, long-term cannabis users showed a 5.5-point IQ decline from childhood, poorer learning and processing speed, and smaller hippocampal volume, with effects specific to cannabis rather than other substances.
Quick Facts
What This Study Found
Long-term cannabis users showed a mean 5.5-point IQ decline from childhood to age 45, poorer learning and processing speed relative to childhood IQ, and informant-reported memory and attention problems. These deficits were specific to long-term cannabis users and were not present or were smaller among tobacco users, alcohol users, recreational users, or quitters. Smaller hippocampal volume was observed but did not statistically mediate cognitive deficits.
Key Numbers
1,037 participants; 94% retention to age 45; -5.5 IQ point decline; assessed at ages 18, 21, 26, 32, 38, 45; smaller hippocampal volume on MRI
How They Did This
Prospective cohort of 1,037 individuals born in Dunedin, New Zealand (1972-1973) followed to age 45 with 94% retention. Cannabis use assessed at ages 18, 21, 26, 32, 38, and 45. IQ tested at ages 7, 9, 11, and 45. Neuropsychological testing and brain MRI at age 45.
Why This Research Matters
This is one of the most methodologically rigorous longitudinal studies of cannabis and cognition, with childhood baseline IQ, repeated assessments, and controls for numerous confounders. The findings are difficult to dismiss.
The Bigger Picture
Midlife cognitive deficits and smaller hippocampal volume are risk factors for dementia. If long-term cannabis use contributes to both, the aging cannabis-using population may face elevated dementia risk.
What This Study Doesn't Tell Us
Observational design cannot definitively prove causation. Cannabis potency has changed over the study period. Dunedin cohort may not represent all populations.
Questions This Raises
- ?Will long-term cannabis users show elevated dementia rates in later life?
- ?Are the cognitive deficits reversible with sustained abstinence, or are they permanent?
Trust & Context
- Key Stat:
- -5.5 IQ points from childhood to age 45
- Evidence Grade:
- Landmark longitudinal cohort with childhood baselines, 94% retention, repeated measures, brain imaging, and extensive confounder control.
- Study Age:
- Published in 2022
- Original Title:
- Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife.
- Published In:
- The American journal of psychiatry, 179(5), 362-374 (2022)
- Authors:
- Meier, Madeline H(7), Caspi, Avshalom(3), R Knodt, Annchen, Hall, Wayne, Ambler, Antony, Harrington, HonaLee, Hogan, Sean, M Houts, Renate, Poulton, Richie, Ramrakha, Sandhya, Hariri, Ahmad R, Moffitt, Terrie E
- Database ID:
- RTHC-04060
Evidence Hierarchy
Follows a group of people over time to track how outcomes develop.
What do these levels mean? →Frequently Asked Questions
Does long-term cannabis use lower IQ?
This 45-year study found a 5.5-point IQ decline in long-term cannabis users from childhood to midlife. The decline was specific to cannabis and not explained by tobacco, alcohol, socioeconomic status, or family history.
Could this lead to dementia?
The authors noted that midlife cognitive deficits and smaller hippocampal volume are known dementia risk factors. They called for research tracking whether long-term cannabis users develop dementia at higher rates in later life.
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Cite This Study
https://rethinkthc.com/research/RTHC-04060APA
Meier, Madeline H; Caspi, Avshalom; R Knodt, Annchen; Hall, Wayne; Ambler, Antony; Harrington, HonaLee; Hogan, Sean; M Houts, Renate; Poulton, Richie; Ramrakha, Sandhya; Hariri, Ahmad R; Moffitt, Terrie E. (2022). Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife.. The American journal of psychiatry, 179(5), 362-374. https://doi.org/10.1176/appi.ajp.2021.21060664
MLA
Meier, Madeline H, et al. "Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife.." The American journal of psychiatry, 2022. https://doi.org/10.1176/appi.ajp.2021.21060664
RethinkTHC
RethinkTHC Research Database. "Long-Term Cannabis Use and Cognitive Reserves and Hippocampa..." RTHC-04060. Retrieved from https://rethinkthc.com/research/meier-2022-longterm-cannabis-use-and
Access the Original Study
Study data sourced from PubMed, a service of the U.S. National Library of Medicine, National Institutes of Health.
This study breakdown was produced by the RethinkTHC research team. We analyze and report published research findings without making health recommendations. All interpretations are based solely on the published abstract and study data.