High-potency cannabis, especially when used daily, is “strongly linked” to the risk of developing psychotic disorders such as schizophrenia and paranoia, scientists said yesterday.
In Amsterdam and London — where marijuana with high levels of tetrahydrocannabinol (THC) has long been the rule rather than the exception — 50 and 30 percent of new psychosis cases respectively were associated with potent forms of the drug.
The findings, reported The Lancet, bolster a growing body of research connecting pot to a range of mental health disorders.
With piecemeal legalization and decriminalization, consumption in North America and Europe has increased markedly over the past two decades, even as levels of the drug’s mind-bending molecule, THC, have risen four or five-fold.
“The use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms,” said lead author Marta di Forti, a professor at the Institute of Psychiatry, Psychology and Neuroscience at King’s College London. “Our findings also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level.”
Across 11 European cities or regions and one in Brazil, one in five new cases of psychosis were associated to daily cannabis use, and one in 10 to high-potency pot, the study found.
If high-THC cannabis — including strains such as “trainwreck,” “gorilla glue” and “hindu kush” — were no longer available, “12 percent of cases of first-episode psychosis could be prevented across Europe,” the researchers said.
In Amsterdam, the incidence of first-time psychosis would likely drop from 38 to 19 per 100,000 people per year, while in London, the number of cases would decline from 46 to 32 per 100,000 people.
“As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital public health importance to consider the potential adverse effects that are associated with daily cannabis use, especially high-potency varieties,” Di Forti said.
The researchers established a baseline by identifying all people known to have experienced a first episode of psychosis from 2010 to 2015. They found 901 cases.
These were then compared with 1,237 healthy adults to tease out the risk factors associated with psychosis.
All participants provided information about how frequently they had used marijuana and other drugs over a period of years.
The scientists also classified the potency of the cannabis consumed as either “high” — more than 10 percent THC — or “low” — less than 10 percent THC.
In some locations, THC levels are 20 percent or higher, the researchers said.
Nearly 30 percent of patients who had experienced a first episode of psychosis were daily cannabis users, compared with less than 7 percent of those in the control group.
For high-potency pot, the corresponding figures were 37 and 19 percent respectively.
Adjusting for social factors and other drug use, the authors found that daily cannabis users were three times more likely to experience psychosis for the first time than people who had never used it.
For daily use of high-potency cannabis, it increased to five times.
While the correlation between marijuana use and mental disorders was strong, a cause-and-effect relationship can only be inferred, the authors said.
Royal College of Psychiatrists registrar Adrian James said that the findings were to be “taken seriously” and that his institution “is reviewing the mental health effect of cannabis use.”
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