A recent study published in JAMA Psychiatry offers new insights into how long-term, frequent cannabis use may affect the brain in ways that raise the risk of psychosis.
Researchers from London, Ontario—including teams at the London Health Sciences Centre Research Institute and Western University’s Schulich School of Medicine & Dentistry—used advanced brain imaging to show how cannabis use disorder is linked to high levels of dopamine in the brain, a chemical strongly tied to psychosis.
Cannabis use disorder refers to a pattern of cannabis use where individuals continue using the drug despite it causing problems in their daily life. This kind of ongoing, heavy use is different from occasional recreational use, and it has been increasing since the legalization of cannabis in many parts of the world, including Canada.
Emergency departments across the country have reported more patients coming in with symptoms of psychosis after using cannabis, particularly since legalization.
The brain chemical at the center of this new study is dopamine. Dopamine is important for mood, motivation, movement, and learning. But too much of it can disrupt normal brain function. In particular, unusually high dopamine levels have long been linked to psychotic episodes and conditions such as schizophrenia.
This study is the first to clearly show a biological connection—a “straight line,” as senior author Dr. Lena Palaniyappan describes—between cannabis use, increased dopamine activity, and psychosis.
Using a brain scan called neuromelanin-sensitive MRI, researchers could measure dopamine activity by looking at a pigment called neuromelanin. Neuromelanin builds up over time when the brain produces too much dopamine, and it appears as black spots on the scans.
What they found was striking: in young adults with cannabis use disorder, the black spots in key areas of the brain were darker than expected for their age. In some cases, their brain scans looked like they belonged to someone a decade older. These darkened areas were mostly found in two parts of the midbrain—the substantia nigra and the ventral tegmental area—both known to be involved in psychosis.
The study involved 61 participants between the ages of 18 and 35. Some had cannabis use disorder, and some didn’t. A few in each group were also experiencing their first episode of schizophrenia. Regardless of whether they had schizophrenia or not, those with cannabis use disorder had higher levels of neuromelanin, showing signs of excess dopamine.
Researchers say these findings are especially important for adolescents. Since legalization, doctors have noticed more teens showing early, mild psychotic symptoms after using cannabis.
In some cases, these early episodes are followed by more serious, long-term psychotic conditions. The team hopes that by showing brain images of the areas affected, they can help young people understand the risks before more damage is done.
Dr. Betsy Schaefer, one of the study’s authors, explains that this research helps clarify the biological reasons behind the mental health risks tied to cannabis. While not everyone who uses cannabis will develop psychosis, those with certain risk factors—including genetics or previous mental health conditions—may be especially vulnerable.
The research team hopes these findings will lead to greater public awareness and smarter public health policies. As cannabis use becomes more common, especially among young people, it’s important to understand that it can have real effects on the brain—effects that may not be visible at first but can build up over time.
First author Jessica Ahrens, a Ph.D. candidate at McGill University, says she hopes the study will encourage doctors and patients to talk more openly about cannabis use. If someone is struggling with stress, anxiety, or sleep issues, health professionals should offer safer treatment options and resources instead of defaulting to cannabis.
In summary, this study offers strong evidence that long-term, heavy cannabis use can lead to changes in the brain’s dopamine system—changes that are known to increase the risk of psychosis.
The findings don’t suggest cannabis should be avoided by everyone, but they do highlight the importance of using it responsibly, especially for young people and those with other mental health concerns.
If you care about cannabis, please read studies that what you need to know about cannabis and heart attack, and CBD from cannabis may help inhibit COVID-19 infection.
For more information about cannabis, please see recent studies that medical cannabis could help reduce depression, and results showing this stuff in cannabis may protect aging brain, treat Alzheimer’s.
The research findings can be found in JAMA Psychiatry.
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