People who smoke high-potency marijuana daily are nearly five times more likely to develop a psychotic disorder than those who have never used the drug, according to a new study published in the Lancet Psychiatry journal.
The research, which was published online Tuesday, adds to previous studies that have shown causal links between the drug and the onset of severe mental health disorders.
"Our findings confirm previous evidence of the harmful effect on mental health of daily use of cannabis, especially of high-potency types," the study's authors wrote. "Importantly, they indicate for the first time how cannabis use affects the incidence of psychotic disorder."
Researchers at King's College in London and others analyzed data gathered from 11 areas in Europe and Brazil from May 2010 to April 2015. The researchers compared 901 patients who were diagnosed with their first episode of psychosis to a control group of 1,237 from the same sites.
Psychosis is a term that describes when a person loses touch with reality. Psychotic disorders, including schizophrenia and delusional disorder, are often triggered by genetic factors, trauma, and other environmental stresses.
What are the details?
Scientists gathered a detailed history of the participants' lifetime marijuana use, along with other recreational drugs, including whether or not they had ever used, current habits, and, the age of first use, among others.
It is the biggest study to look at the impact of marijuana on psychiatric disorders.
"This study is groundbreaking," Dr. Kevin Sabet, president of Smart Approaches to Marijuana, said in a statement. "It is the first to show how marijuana impacts population rates of psychosis — and its results are chilling. For years we have known that low potency marijuana was damaging to mental health. Now the scientific literature is catching up with the rapidly increasing THC potency we are seeing on the market today."
The study found that marijuana users who smoked daily were three times more likely to be diagnosed with a psychotic disorder than those who never used the drug.
The risk increased to nearly five times more likely in those who smoked high-potency marijuana on a daily basis.
"If we think there's something particular about (high-potency) cannabis, then making that harder to get a hold of, could be a useful harm-reduction measure," Suzanne Gage of the University of Liverpool, told The Associated Press. Gage did not contribute to the study.
People with a family history of psychotic disorders or other risk factors could be at an even higher risk if they smoke pot, according to Gage.
"That could be the thing that tips the scale for some people," she told the AP. "Cannabis for them could be an extra risk factor, but it definitely doesn't have to be involved. If you use cannabis, it doesn't mean you are definitely going to develop psychosis."
The study was funded by Britain's Medical Research Council, São Paulo Research Foundation, and the Wellcome Trust, among others.