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5MinuteConsult Journal Club

Forget about Weed


Am J Psych 8 Mar 2022 (

Study Summary

Cohort of 1,037 individuals born in Dunedin, New Zealand, in 1972–1973 and followed through age 45, with 94% retention assessed marijuana use at ages 18, 21, 26, 32, 38, and 45 and IQ at ages 7, 9, 11, and 45. They found that long-term cannabis users had an IQ decline from childhood to midlife by on average 5.5 IQ points, poorer learning ability and processing speed, and increased memory and attention problems. It also correlates with smaller hippocampal volume.

These deficits could not be explained by other exposures like tobacco, alcohol, or other illicit drug use, childhood socioeconomic status, low childhood self-control, or family history of substance dependence.


Chronic use of marijuana correlates with a loss of IQ and memory that cannot be explained by other environmental factors.


Regular marijuana use, over time, can have strong clinical effects. Loss of 5 IQ points does not sound like much, but this is on top of the normal loss with aging. Memory loss, and less processing speed implies a diminished ability to function at work, and ultimately with self-care. A second, smaller study found those who used marijuana regularly for 7 or more years had reduced executive functioning scores and processing speeds (J Geriatr Psychiatry Neurol. 2021 Sep 28;8919887211049130.  doi: 10.1177/08919887211049130).

A two-fold response to consider: tell patients this information; regular use alters brain function and structure AND generates a differential diagnosis for why they are compelled to regular use. Could be they have an undiagnosed anxiety problem, or even hypomania.

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