Is cannabis truly harmless? A new study from McGill University challenges that assumption, revealing a striking biological link between heavy cannabis use and psychosis. The culprit: an overstimulated dopamine system that may hold the key to understanding how marijuana use affects mental health.
Key Points at a Glance
- McGill researchers identified elevated dopamine activity in cannabis users with psychotic symptoms.
- The study used neuromelanin-sensitive MRI to measure dopamine levels in the brain’s striatum.
- Increased dopamine correlated with both cannabis use severity and psychosis risk.
- Non-users and participants without psychosis showed no such dopamine elevation.
- Findings raise public health concerns, particularly for frequent cannabis users under 25.
With cannabis legalization expanding across the globe, questions about its long-term mental health effects have never been more urgent. Now, a groundbreaking study from McGill University sheds new light on a question long debated in both scientific circles and public discourse: does cannabis use contribute to psychosis? The answer, according to the researchers, may lie deep within the brain’s dopamine system.
Published in JAMA Psychiatry, the study examined how heavy cannabis use affects dopamine activity in the brain—specifically in the striatum, a region involved in motivation, reward, and cognitive function. Using neuromelanin-sensitive MRI scans, researchers observed that individuals with cannabis use disorder (CUD) had significantly higher dopamine levels in this area compared to non-users and people without psychosis. The more frequent and intense the cannabis use, the higher the dopamine signal.
Why does this matter? Because elevated dopamine in the striatum is a known hallmark of psychotic disorders such as schizophrenia. Hallucinations, delusions, and disordered thinking are thought to arise in part from this chemical imbalance. The McGill study strengthens the hypothesis that cannabis—especially when used heavily—can mimic or trigger these changes in vulnerable individuals.
The research team included 61 participants, encompassing people with and without cannabis use disorder, and individuals in the early stages of schizophrenia. Interestingly, participants without CUD or psychosis showed no abnormalities in dopamine levels, indicating a specific neurochemical footprint linked to cannabis-related mental health risk.
“This is some of the most direct evidence to date that frequent cannabis use can dysregulate dopamine function in a way that mirrors the biology of psychosis,” said the study’s lead authors. “It highlights a potential mechanism by which cannabis may increase the risk of developing mental illness.”
While the link between cannabis and psychosis has been observed for years in epidemiological data, this study is among the first to identify a clear biological pathway. Previous studies often relied on behavioral assessments or broad diagnostic categories. Here, brain imaging offers a more concrete, objective window into the neurochemical shifts taking place.
What’s particularly alarming is how common heavy cannabis use has become—especially among youth. In Canada, for example, one in five young people report using cannabis daily or near-daily. These findings suggest that frequent use during adolescence and early adulthood—critical periods for brain development—could carry lasting mental health consequences.
The researchers emphasize that not everyone who uses cannabis will develop psychosis. Genetics, environment, and pre-existing vulnerabilities all play roles. However, the evidence is growing that cannabis can be a risk factor for some—particularly when used heavily and over long periods.
There’s also hope. One question the study raises is whether the observed dopamine abnormalities are reversible. If cannabis use stops, can the brain recalibrate? Future research aims to answer that, and it could inform treatment approaches for both cannabis use disorder and early psychosis.
As cannabis becomes more accessible and socially accepted, these findings underscore the importance of informed use. Public health messaging, especially for younger users, may need to evolve to reflect the science. What’s clear is that cannabis is not without consequence—and understanding its effects on the brain is essential for responsible policy and personal decision-making.
Source: McGill University
