Cannabis and risk of psychosis
As a part of recent study into the psychological consequences of cannabis, a team of researchers in the United Kingdom looked at the details of first-episode psychotic incidents treated at hospitals in South London. “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.”
But critics of the study say that just because patients being treated for psychosis are more likely to have smoked cannabis on a regular basis, doesn’t mean the drug caused their mental disorder. Di Forti and her colleagues 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 on the association between drugs and psychosis told The Washington Post.
Di Forti M, Marconi A, Carra E, Fraietta S, Trotta A, Bonomo M, Bianconi F, Gardner-Sood P, O’Connor J, Russo M, Stilo SA, Marques TR, Mondelli V, Dazzan P, Pariante C, David AS, Gaughran F, Atakan Z, Iyegbe C, Powell J, Morgan C, Lynskey M, Murray RM. Proportion of patients in south London with first-episode psychosis attributable to use of high potency cannabis: a case-control study. Lancet Psychiatr. 2015 Feb 18. [in press]