Hospital and emergency room patients diagnosed with cannabis use disorder — defined as an inability to stop using cannabis even when the drug is causing harm — died at almost three times the rate of individuals without the disorder over the next five years, according to a study published on Thursday, the largest on the subject.
Patients with cannabis use disorder were 10 times as likely to die by suicide as those in the general population. They were also more likely to die from trauma, drug poisonings and lung cancer. Those numbers suggest that cannabis use disorder is about half as dangerous as opioid addiction and slightly less dangerous than alcohol use disorder, the researchers said.
A second report, published on Tuesday, found that more cases of schizophrenia and psychosis in Canada have been linked to cannabis use disorder since the drug was legalized.
“Many people think, ‘Oh, cannabis is not harmful — it’s organic, it’s natural; how great,’” said Dr. Laura Bierut, a psychiatrist at Washington University School of Medicine in St. Louis who is an author of an editorial accompanying the study of death risk. But the marijuana sold today is far more potent, and more harmful, than what baby boomers smoked in the 1960s and 1970s, she said.
“It is a public health threat just like alcohol,” Dr. Bierut said.
Recent research suggests that three in 10 cannabis users will develop cannabis use disorder, defined as being unable to stop using cannabis even though it’s causing serious health and social problems. As with alcohol, many people use marijuana recreationally without adverse effects or addiction.
The researchers took advantage of records in Ontario that capture millions of residents’ encounters with the government health system, which covers 97 percent of the population there.
From the records, the scientists in Thursday’s study identified 106,994 people who were diagnosed with cannabis use disorder during an emergency department visit or hospitalization between 2006 and 2021.
The researchers linked the records with vital statistics records and found that 3.5 percent of them died within five years of treatment for the disorder. In a matched comparison group of people of the same age and sex, the death rate was 0.6 percent.
The authors then made adjustments to account for other risk factors that may have contributed to their deaths, including mental health disorders, other substance use and conditions like heart disease and cancer.
Even when taking deaths by those other causes out of the equation, the researchers concluded that patients with cannabis use disorder were at a 2.8-fold increased risk of death compared with the general population. The risk was greatest in young adults ages 25 to 44.
Dr. Daniel Myran, an assistant professor of family medicine at University of Ottawa and the study’s first author, noted that these are most likely underestimates of cannabis’s toll.
“Our estimate is that for every person treated for C.U.D. there are another three who didn’t seek care,” he said. “So this is not just C.U.D., but bad enough that they sought care for it.”
He also noted that the study could not conclusively determine whether the cannabis itself increased death risk, or whether it was other lifestyle or health factors that happen to be more common among heavy cannabis users.
“Either way, this group is really, really high risk, and could benefit from intervention and monitoring and prevention,” he said.
On Tuesday, the same research group published a study looking at whether Canada’s recent legalization of marijuana affected rates of psychosis and schizophrenia.
Cannabis use has been associated with the development of schizophrenia, a serious mental illness, as well as transient psychotic episodes characterized by a loss of contact with reality. Some research has found no association between cannabis legalization and an increase in these disorders, but many studies are too small to detect changes in the prevalence of psychotic disorders, which are rare.
The new study analyzed cannabis-linked psychosis in Canada during three periods: before the country made cannabis legal (2006 to 2015), amid widening use of medical and nonmedical cannabis (2015-2018) and after the recreational use of cannabis was made legal (2018 to 2022).
The rates of schizophrenia were stable over time. But the percentage of cases attributable to cannabis use disorder increased to 10.3 percent during the legalization period, up from 3.7 percent before legalization, the authors found. The rate of psychosis (without a diagnosis of schizophrenia) nearly doubled after legalization.
Young adults ages 19 to 24 were most vulnerable, said Jodi Gilman, an associate professor of psychiatry at Harvard Medical School who wrote a commentary about the study.
“This is a period of the life span when the brain is still developing and still vulnerable to the effects of cannabis,” Dr. Gilman said. Psychosis and schizophrenia are also known to develop in young adulthood, she added, “so you have a double hit.”
