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Long-Term Cannabis Use May Shrink the Brain’s Hippocampus, Study Suggests

By Alexander Beadle

Published: Mar 30, 2022   
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Long-term cannabis use may detrimentally affect IQ, learning and processing speed, and even shrink the size of the brain’s hippocampus over time, according to a new study based on over four decades’ worth of cohort research.

Study participants who reported long-term cannabis use were seen to perform worse in cognitive function tests compared to non-users and exhibited a significantly greater mean decline between childhood and adulthood IQ measurements compared to their non-using peers. MRI scans of adult long-term cannabis users’ brains also showed significantly smaller hippocampal gray matter volumes than non-users.

The researchers say that these findings could not be otherwise explained by long-term alcohol or tobacco use, or by occasional cannabis use, suggesting that long-term cannabis use may have some unique detrimental effect on cognitive performance.

In light of this, they say that more research into the health of middle-aged and older people with a history of long-term cannabis use is needed, particularly to determine whether this user group might be at a greater risk of developing dementia or other cognition disorders.

Long-term users perform worse than peers in cognition tasks

Originally intended to be a study into early childhood development, the Dunedin Study has followed the lives of around one thousand children born in Dunedin, New Zealand, from their births in 1972 all the way through until the present day. This cohort has voluntarily undergone regular physical and mental health assessments as they have aged, generating a wealth of data that has been used in more than 1,300 research studies to date.

In this latest study, published in the American Journal of Psychiatry, researchers looked at self-reported cannabis use and dependence data from the Dunedin cohort measured at ages 18, 21, 26, 32, 38, and 45. Their IQ test scores, taken at ages 7, 9, and 11, and then again at age 45, were also included in the analysis to serve as a measure of cognitive function.

They found that individuals who were long-term cannabis users scored on average 5.5 points lower on their adult IQ test relative to those taken when they were a child. Notably, this childhood-to-adulthood IQ was significantly larger than the averages in long-term tobacco users (-1.5 points) and long-term alcohol users (-0.5 points).

Looking at the IQ test results in more detail, it was found that long-term cannabis users performed worse than their non-substance-using peers in most tests, and worse than long-term tobacco and alcohol users in tests of learning and memory.

Interestingly, this did not hold for midlife recreational cannabis users, whose results were more similar to the rest of their non-cannabis-using peers. The researchers say that this highlights the importance of future studies to not conflate long-term middle-aged cannabis users and other recreational users of the same age in their analyses.

Long-term users have smaller hippocampal volumes than non-users

At the most recent round of assessments for the Dunedin cohort, MRI scans of the study members’ brains were taken for the first time.

Previous research has indicated that heavy cannabis use may adversely affect the development of the hippocampus, a brain region that contains a high density of cannabinoid receptors and is critical to memory function and cognition. To test this, the researchers behind the new cannabis study used the new Dunedin MRI data to examine whether the long-term cannabis users had any noticeable differences in their hippocampal gray matter volume compared to their peers.

They found that long-term cannabis users had smaller overall hippocampal volumes compared to non-users, and smaller volumes in five of twelve subfields of the hippocampus, including the hippocampal-amygdala transition area. The long-term users also had smaller overall hippocampal volumes than the other midlife recreational cannabis users.

However, the researchers concluded that these differences in hippocampal volume did not statistically mediate the association seen between persistent cannabis use and poor cognition.

“Smaller hippocampal volume has been suggested as a possible mediator of cannabis-related cognitive deficits, because the hippocampus is rich in type 1 cannabinoid (CB1) receptors and is involved in learning and memory,” the study authors wrote.

“However, smaller hippocampal volume may be a reductionistic explanation for cannabis-related cognitive deficits. For example, in addition to the hippocampus, other CB1-rich brain regions, including those involved in reward and motivation, may play a role.”

Scientists urge more study into cannabis’ effects on the brain

The scientists behind this latest study note several limitations with the work. Firstly, the dataset relied on self-reported cannabis use data. Secondly, since the nature of the Dunedin study was to recruit participants from birth and follow them through their lives, there was no way to recruit certain numbers of substance users and so some of the subgroup sizes were small.

Additionally, this means that the long-term users often began using cannabis in the 1980s and 1990s, and the concentration of THC normally present in recreational cannabis has grown significantly since then. The researchers say that this could mean the effects of long-term cannabis use are actually underestimated in this study.

Further research into cannabis’ effects on the brain following long-term use are urgently needed, the researchers say, and this call is echoed by other scientists working in the field. Recent studies on teens have shown that cannabis use during adolescence can lead to thinning of the left and right prefrontal lobes, which are brain regions heavily involved in motor function, problem solving, memory, and other cognitive tasks. These detrimental effects “underscore the importance of further longitudinal studies of adolescent cannabis use, particularly given increasing trends in the legalization of recreational cannabis use,” the authors of that study said.

The American Heart Association also recently put out a statement in the journal Stroke detailing the currently known effects of cannabis on the brain, and appealing for more research to be funded and carried out to resolve some of the conflicting evidence surrounding cannabis and cognition.

For example, some studies concerning fractional anisotropy (FA) – a measurement used to describe the density of neurons in the brain – have claimed that people with cannabis use disorder (CUD) have lower FA readings than normal, whereas others have reportedly found no significant differences between people with CUD and controls.


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