Psychotic disorders, including schizophrenia and delusional disorder, are often triggered by genetic factors, trauma, and other environmental stresses.
The researchers established a baseline by identifying - in the cities and regions examined - all individuals known to have experienced a first episode of psychosis from 2010 to 2015.
"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 that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties", Marta Di Fori, lead author of the study, said in a statement. For those who used high-potency marijuana daily, the risk jumped to almost five times.
Skunk contains more THC, or delta-9-tetrahydrocannabinol, than regular cannabis, and THC can induce psychotic symptoms such as hallucinations, delusions and paranoia.
The findings were published in mental health journal The Lancet Psychiatry. "Fifteen years ago, nobody thought that cannabis thought increased the risk of psychosis", he said.
The study was funded by Britain's Medical Research Council, São Paulo Research Foundation, and the Wellcome Trust, among others.
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A third of psychosis cases in London are the result of smoking skunk, according to a study that underlines the danger of super-strength cannabis.
Use of high-potency cannabis was a strong predictor of psychotic illness in London and Amsterdam, where powerful versions of the drug were widely available, said the authors.
Still, the new study cannot rule out "reverse causation", meaning it could be that people with psychosis are more likely to use marijuana than people without the mental health condition, according to Suzanne Gage, of the University of Liverpool's Department of Psychological Sciences, who wrote a commentary accompanying the article. If you use cannabis, it doesn't mean you are definitely going to develop psychosis.
"That could be the thing that tips the scale for some people", she told the AP.
It's also possible that the people who experience psychotic episodes may be more likely to use marijuana, Gage points out in an accompanying editorial to the study.
And daily use of particularly potent marijuana ups the risk of developing psychosis even more.
But NORML Deputy Director Paul Armentano said it would be "premature at best, and sensational at worst" to conclude a causal relationship exists between marijuana use and psychiatric disorders.