Heavy users of marijuana might face an increased risk of bipolar disorder and depression, a new study suggests.
The analysis of more than 6.6 million Danish individuals found that having cannabis use disorder doubled or even tripled the odds for most forms of depression or bipolar disorder. When it came to bipolar disorder in particular, "cannabis use disorder was associated with higher risk for psychotic than non-psychotic subtypes of bipolar disorder," the researchers noted.
These results, however, cannot prove that marijuana use was the cause of these conditions, the investigators said.
"Our results are compatible with the idea that cannabis may increase the risk of developing depression or bipolar disorder, but we cannot prove this for certain, since we are working with observational data," explained lead researcher Dr. Oskar Jefsen, from the psychosis research unit at Aarhus University in Denmark. "Unequivocal evidence would require a randomized controlled trial, but this would be unethical."
Still, one expert noted that while the study can't prove that cannabis was the cause of bipolar disorder or depression, it adds to growing evidence that the drug is not harmless.
“This is a powerful and well-conducted study that adds to the emerging evidence of the added risk of cannabis regular use to the development of serious mental health disorders," said Beatriz Carlini, director of the Cannabis Research and Education program and an acting associate professor of psychiatry and behavioral sciences at the University of Washington's School of Medicine.
As the legalization of marijuana spreads in the United States, people must recognize that cannabis use is not benign, she said.
"While we should celebrate that, as a society, we moved away from the stigmatization of cannabis users, and wide exaggeration of the risks of using cannabis, we should be concerned about our current cultural moment, when cannabis has been portrayed as a harmless substance," Carlini said.
This study begs the question of whether bipolar disorder and depression make abusing cannabis more likely rather than causing these problems, said Paul Armentano, deputy director of NORML, which advocates for the legalization of marijuana.
"It is well-established that people either predisposed to or who suffer from bipolar disorder and other similar conditions tend to consume all controlled substances, not just cannabis, at rates far higher than the general population," he said. "This is especially true for tobacco use."
Yet, there is little support for the idea that smoking cigarettes either causes psychiatric illness or that it should be criminalized for adults to benefit mental health, he said
"Studies frequently show that the relationship between the use of cannabis and many psychiatric conditions is bidirectional, meaning that in many instances, a diagnosis often predicts later cannabis use," Armentano said. "In short, these relationships are most likely correlative, and they are often complex, multi-directional, and they are far from unique to cannabis."
Some populations, however, are more vulnerable to cannabis' effects, particularly when the substance is used frequently or at earlier ages, Armentano noted.
"Those suffering from bipolar disorder, schizophrenia and certain other mental health issues, or who are predisposed to these events, likely fall into this category and arguably ought to approach cannabis and other substances with a greater degree of caution," he said.
The report was published online May 24 in the journal JAMA Psychiatry.
For more on the effects of marijuana, see the U.S. Centers for Disease Control and Prevention.
SOURCES: Oskar Jefsen, MD, psychosis research unit, Aarhus University Hospital-Psychiatry, Aarhus University, Denmark; Paul Armentano, deputy director, NORML; Beatriz Carlini, PhD, MPH, director, Cannabis Education and Research, and acting associate professor, psychiatry & behavioral sciences, University of Washington School of Medicine, Seattle; JAMA Psychiatry, May 24, 2023, online