Long-Term Cannabis Users Lost 5.5 IQ Points and Had Smaller Hippocampi by Midlife | Research

The takeaway

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.

Long-term cannabis users, neuroscience researchers, policymakers

-5.5 IQ points from childhood to age 45

What the researchers 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.

Why it 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 numbers in context

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 the study worked

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.

What this study cannot tell us

Observational design cannot definitively prove causation. Cannabis potency has changed over the study period. Dunedin cohort may not represent all populations.

How to read the evidence

Landmark longitudinal cohort with childhood baselines, 94% retention, repeated measures, brain imaging, and extensive confounder control.

When this study was published

Published in 2022

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.

Questions still open

Will long-term cannabis users show elevated dementia rates in later life? Are the cognitive deficits reversible with sustained abstinence, or are they permanent?

Common 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.

Read the original research

Long-Term Cannabis Use and Cognitive Reserves and Hippocampal Volume in Midlife.

The American journal of psychiatry, 179(5), 362-374

View on PubMed ↗Publisher’s paper ↗

Citation

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