Several studies have shown that chronic cannabis use is linked to a higher incidence of schizophrenia among men in their early 20s, the age when the disease is usually diagnosed. The first paper on the topic, a Swedish study published in 1997, found that heavy cannabis use was associated with a sixfold increase in schizophrenia risk. In the decades since, social scientists have unearthed a strong link between heavy cannabis use and other severe psychological illnesses, including clinical depression and bipolar disorder.
Now a new longitudinal study has examined the medical records of all citizens of Denmark over the age of 16, some 6.5 million people in all, for patterns of diagnosis, hospitalization and treatment for substance use between 1995 and 2021. In the paper, published in the journal JAMA Psychiatry in May, Dr. Oskar Hougaard Jefsen of Aarhus University and colleagues showed that people who had previously been diagnosed with cannabis use disorder were almost twice as likely to be diagnosed later with clinical depression. According to the Centers for Disease Control and Prevention, cannabis use disorder is characterized by craving marijuana, using it more often than intended, spending a lot of time using it, and having it interfere with friends, family and work.
Even more dramatically, the paper also found that people with cannabis use disorder were up to four times as likely to be diagnosed later with bipolar disorder with psychotic symptoms.
Reefer madness, literally: A new study involving the entire adult population of Denmark found a clear link between cannabis abuse and mental illness, writes Susan Pinker. https://t.co/AdnvFvGN8Y via @WSJ
— Gary Rosen (@garyrosenWSJ) July 6, 2023