Patients who used marijuana more likely to stop treatment and need stronger medication, study says

People with bipolar disorder who use cannabis are more likely to develop the illness earlier and experience more intense manic and psychotic symptoms, according to a new study from Spain.

The research, led by scientists at Hospital Clínic of Barcelona and published in the Journal of Affective Disorders (July 2025), followed nearly 300 adults after they were hospitalized for manic or mixed episodes. The goal was to better understand how substance use—including cannabis—affects the long-term course of bipolar disorder.

Researchers tracked 279 patients for three years after discharge. They monitored hospital readmissions, emergency room visits, and substance use. 

About 28% of participants reported using marijuana. Compared to those who didn’t, marijuana users developed bipolar disorder roughly five years earlier, at an average age of 25 instead of 31.

The study also found that cannabis users were twice as likely to show signs of psychosis during manic episodes and more likely to require long-acting injectable antipsychotic medication. They were also nearly four times more likely to stop their treatment before being hospitalized again.

Despite those findings, cannabis use was not linked to a higher number of hospital readmissions or emergency psychiatric visits over the three-year follow-up. In contrast, tobacco use was strongly tied to more hospitalizations and emergency care.

While the cannabis findings don’t show a direct link to relapse, the researchers say cannabis still signals a potentially more aggressive form of bipolar disorder. 

One related study found that in some patients, manic symptoms appeared shortly after quitting cannabis, suggesting that withdrawal may trigger episodes in vulnerable individuals.

The team calls for more research into how cannabis affects brain chemistry in people with mood disorders, especially during withdrawal. They also recommend that doctors consider substance use histories when treating newly diagnosed bipolar patients.

The study was published in the July 2025 issue of the Journal of Affective Disorders.

Comment: There is recent research linking early cannabis consumption to increased mental health issues in America. In addition to mania, it can also trigger psychosis. https://www.psychiatrist.com/news/cannabis-use-triggers-brain-changes-linked-to-psychosis/

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