A recent study suggests that long-term cannabis use could be linked to changes in brain structure and function, especially in later years.
However, this study—published in the open-access journal BMJ Mental Health—also points out that the connections found between cannabis use and brain changes may not directly indicate that one causes the other.
In other words, while some differences are seen in the brains of people who have used cannabis over their lifetime, it’s unclear if cannabis is actually the reason for these changes.
With cannabis now more widely legalized for both medical and recreational use, the number of people using it has grown significantly. Yet, researchers argue that this increase has happened without a complete understanding of the potential long-term effects on the brain.
Prior studies have shown possible links between cannabis use and negative effects on thinking abilities, brain structure, and brain function. However, they haven’t proven that cannabis is the direct cause of these effects, nor have many of these studies focused on older adults.
To explore this further, scientists examined data from nearly 16,000 cannabis users in the UK Biobank study. This large, long-term study provided both genetic data and brain scans.
The researchers used a special approach called Mendelian randomization, which leverages genetic information as an indirect indicator of a person’s likelihood to use or depend on cannabis.
This method helps explore whether cannabis use might cause specific outcomes, such as changes in brain structure.
Participants reported how many times they had used cannabis throughout their lives. Responses ranged from “once or twice” to “more than 100 times.”
This data allowed researchers to categorize participants based on lifetime cannabis use, dividing them into groups of low-frequency users (up to 10 uses) and high-frequency users (11 times or more).
They also considered other factors that might influence brain health, such as smoking, alcohol use, mental health, and lifestyle.
When the team looked at the brain scans, they found several structural and functional differences between people who had used cannabis and those who hadn’t.
For example, cannabis users generally showed reduced integrity in the brain’s white matter, which is essential for quick communication between different parts of the brain.
This reduction was most noticeable in the corpus callosum, a thick bundle of nerves that links the left and right sides of the brain.
Additionally, those who used cannabis appeared to have lower connectivity in brain regions associated with the “default mode network”—the part of the brain that is active when we’re daydreaming or letting our minds wander.
Notably, this part of the brain is rich in cannabinoid receptors, which interact with the active compounds in cannabis.
Interestingly, these changes in brain structure and function were not closely linked to how often or how recently people had used cannabis.
There were also some differences between men and women: in men, the changes were mainly seen in functional brain connections, while in women, the changes were more about white matter structure.
To better understand these findings, the team applied Mendelian randomization to test whether cannabis use or dependence was likely causing these brain differences.
Here, the results were inconclusive—this genetic analysis did not find strong evidence to confirm that cannabis use directly led to the observed brain changes. This raises questions about whether other factors, unrelated to cannabis, might explain the results.
For instance, the researchers suggest that some other aspect, such as family health history, diet, or use of other medications, could have influenced the findings in the observational part of the study.
They also note that Mendelian randomization may not detect subtle effects and might miss variations that show up only at certain life stages, while observational studies capture changes at a specific moment.
The study has a few limitations. It relied on participants’ memory of their cannabis use, which may not be entirely accurate. The researchers also used data from the UK Biobank, a group largely made up of healthy, white individuals, limiting how well the results might apply to other populations.
Additionally, the study didn’t include enough people with cannabis use disorders to look specifically at the effects of heavy, ongoing cannabis use.
Given these findings, the researchers advise caution in interpreting their results. They emphasize that more research is necessary to fully understand how cannabis use may impact brain health, particularly in terms of potency and long-term effects.
If you care about Alzheimer’s, please read studies about Vitamin D deficiency linked to Alzheimer’s, vascular dementia, and Oral cannabis extract may help reduce Alzheimer’s symptoms.
For more information about brain health, please see recent studies about Vitamin B9 deficiency linked to higher dementia risk, and results showing flavonoid-rich foods could improve survival in Parkinson’s disease.
The research findings can be found in BMJ Mental Health.
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