As a part of recent study into the psychological consequences of marijuana, a team of researchers in the United Kingdom looked at the details of first-episode psychotic incidents treated at hospitals in South London. They found that roughly 24 percent of the 410 patients treated for a previously undiagnosed psychotic disorder smoked high-potency weed on a daily basis. The results were weighed against a control group of 370 randomly selected South London residents.
"Compared with those who had never tried cannabis, users of high potency 'skunk-like' cannabis had a threefold increase in risk of psychosis," explained lead study author Dr. Marta Di Forti, a professor at the Institute of Psychiatry, Psychology and Neuroscience at King's College London. "The risk to those who use every day was even higher; a fivefold increase compared to people who never use."
Researchers say its not only further evidence of the drug's long-term psychological effects, but highlights the need to pay closer to marijuana potency (the concentration of THC) as a variable in medical research.
But critics of the research -- or at least the way the research is presented -- say that just because patients being treated for psychosis are more likely to have smoked marijuana on a regular basis, doesn't mean the drug encouraged or cause their mental disorder. The doctors and researchers responsible for the latest paper, published in the journal Lancelot, acknowledge that they can't show the correlation to be causal.
"Ecological studies such as this are pretty weak evidence for causation -- if you just look at population level information like this, then you can't be sure that the people using cannabis are the same people developing psychosis," Suzi Gage, a University of Bristol researcher exploring the links between substance abuse and mental health outcomes, told The Washington Post. "There could be distinct underlying sub-populations that get masked."
But Gage says the fact that neither hash nor low-potency pot were linked with higher rates of psychosis points to a potentially interesting area of future research.
"This new research is an important step on the road to understanding the nature of the association between cannabis and psychosis, but once again media exaggeration or misrepresentation of the findings could risk the message being ignored by the people most likely to benefit from it," Gage wrote in an editorial for The Guardian. "Just in the same way that a pint of beer of an evening is likely to have a different health impact to a pint of vodka, the same could be true for skunk compared to hash. I look forward to more research investigating this."
Gage points out that most research focuses on the effects of THC. Previous studies have suggested that cannabidiol, another cannabinoid, could offer anti-psychotic effects. Skunk has high levels of THC but only trace amounts of cannabidiol. High-potency weed is the most widely available version of the drug on the illicit market.
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