While cannabis has been a controversial topic of debate and discussion throughout the world, its use has increased globally driven by legalization and growing societal acceptance. Many people once saw cannabis as harmless, but new research now reveals its damaging effects on the brain.
This research highlights the risk of psychosis from cannabis use, especially in young adults. A groundbreaking McGill University study, published in JAMA Psychiatry, shows that cannabis reduces brain connectivity in people already at higher risk of psychosis. It also finds that cannabis use increases their chances of developing psychosis.
How Cannabis Affects Brain Connectivity
The McGill University study, published last year in November and led by Dr. Romina Mizrahi and PhD student Belen Blasco, used advanced brain scanning technology to examine 49 participants aged 16 to 30. The analysis includes those with recent psychotic symptoms and those at high risk of psychosis. The study discovered a marked decrease in synaptic density in individuals at risk of psychosis compared to a healthy control group.
Synaptic density refers to the number of synapse units in the brain per area. Synapses are important connections between neurons, imperative for brain connectivity and communication. This reduction was further aggravated by cannabis use, suggesting the drug interrupts the brain’s natural process of synaptic pruning, which is important for healthy brain development. Dr. Mizrahi explained, “Cannabis appears to disrupt the brain’s natural process of refining and pruning synapses, which is essential for healthy brain development. Not every cannabis user will develop psychosis, but for some, the risks are high. Our research helps clarify why”.
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The Link Between Cannabis and Psychosis
Cannabis has long been associated with increasing the risk of developing psychosis, which can progress to schizophrenia. The McGill study is the first to measure the structural changes in the brains of a high-risk population in real time. The research identified reduced synaptic density in brain regions such as the prefrontal cortex, anterior cingulate cortex, and striatum – areas imperative for cognitive and emotional ability. These reductions were observed not only in individuals diagnosed with psychosis but also in those at high risk for psychosis, even before full symptoms emerged.
The study also revealed that cannabis use amplifies these dispositions, particularly during periods of brain maturation. The striatum, a region associated with motivation and reward processing, showed the greatest impact. Blasco noted, “Current medications largely target hallucinations, but they don’t address symptoms that make it difficult to manage social relationships, work, or school. By focusing on synaptic density, we may eventually develop therapies that enhance social function and quality of life for those affected”.
Broader Implications for Cognitive Function
The effects of cannabis on the brain also affects cognitive ability. A separate and recent study published in JAMA Network Open provides new clarity on the effects of cannabis on cognitive ability and particularly on working memory. The study, involving 1,003 adults aged 22–36, found that frequent, heavy cannabis users showed reduced brain activity within regions that pertain to working memory, decision-making, and emotional processing. This decline was most noted in the dorsolateral prefrontal cortex, dorsomedial prefrontal cortex, and anterior insula.
The research found that 63% of long-term cannabis users showed reduced brain activity during working memory tasks, with a similar number seen in 68% of recent users. The study revealed cannabis damages working memory—the mental skill needed for following instructions or doing calculations. Long-term/heavy users of cannabis were defined as those who had used cannabis more than 1,000 times in their lifetime, while moderate users had used it 10 to 999 times, and nonusers fewer than 10 times.
Joshua Gowin, PhD, the study’s first author, stated, “As cannabis use continues to grow globally, studying its effects on human health has become increasingly important. By doing so, we can provide a well-rounded understanding of both the benefits and risks of cannabis use, empowering people to make informed decisions and fully comprehend the potential consequences”.
Acute Effects of Cannabis on Brain Networks
The researchers explored the acute effects of cannabis on the brain in young adults and adolescents. Using functional magnetic resonance imaging (MRI), researchers studied the impact of cannabis on brain networks in resting state. Cannabis negatively affects how key brain networks communicate, including those involved in daydreaming, decision-making, focus, memory, and emotions. Contrary to popular belief, cannabidiol (CBD) did not act against THC’s effects and, in some cases, increased their effects.
Future Directions in Research and Treatment
In the next phase of their research, the McGill University study’s team will check if these brain changes can help spot psychosis risk earlier, allowing for quicker treatment. By focusing on synaptic density, researchers hope to develop therapies that address negative symptoms like social withdrawal and lack of motivation, which are extremely difficult to treat with existing medications.
Blasco explained, “By focusing on synaptic density, we may eventually develop therapies that enhance social function and quality of life for those affected”. This approach could significantly change the treatment of psychosis, offering hope for individuals struggling with symptoms that current medications fail to address.
Conclusion
While cannabis legalisation gains global traction and societal attitudes towards cannabis change, these studies help highlight our understanding of cannabis’s potential risks to brain activity and cognitive function. From disrupting synaptic pruning to reducing brain connectivity and amplifying psychosis risk, cannabis poses significant challenges to brain health. As research progresses, it is important to balance the benefits of cannabis with a clear understanding of its neurological impact.
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