Doubt cast on cannabis, schizophrenia link
A British study has cast doubt on the supposed link between cannabis use and schizophrenia, but at least one Australian researcher says the study needs more evidence.
Previous research has suggested cannabis use increases the risk of being diagnosed with either psychosis or schizophrenia.
This latest study, led by Dr. Martin Frisher of Keele University, examined the records of 600,000 patients aged between 16 and 44, but failed to find a similar link.
"An important limitation of many studies is that they have failed to distinguish the direction of association between cannabis use and psychosis," the authors write in the September edition of the journal Schizophrenia Research.
They point out that "although using cannabis is associated with a greater risk of developing psychosis, there is also evidence of increased cannabis use following psychosis onset."
Not as predicted
Frisher and colleagues compared the trends of cannabis use with general practitioner records of schizophrenia and psychosis.
They argue that if cannabis use does cause schizophrenia, an increase in cannabis use should be followed by an increase in the incidence of schizophrenia.
According to the study, cannabis use in the United Kingdom between 1972 and 2002 has increased four-fold in the general population, and 18-fold among under-18s.
Based on the literature supporting the link, the authors argue that this should be followed by an increase in schizophrenia incidence of 29 per cent between 1990 and 2010.
But the researchers found no increase in the rates of schizophrenia and psychosis diagnosis during that period. In fact, some of the data suggested the incidence of these conditions had decreased.
"This study does not therefore support the specific causal link between cannabis use and the incidence of psychotic disorders," the authors say. "This concurs with other reports indicating that increases in population cannabis use have not been followed by increases in psychotic incidence."
Study 'just a start'
Prof. Joseph Rey of the Faculty of Medicine at the University of Sydney, whose previous research has identified a link between cannabis and schizophrenia, is skeptical of the study's results.
"Not showing that there is a link does not mean there is no link," he says. "There might be other factors at play that may reduce the incidence of diagnosed schizophrenia."
According to Rey, "this study is just a start and the evidence suggesting that cannabis use does increase the risk of schizophrenia is quite strong. We need more evidence to counteract what we already know."
The authors of the study say that while they cannot completely dismiss all alternative explanations of their data, such explanations "do not appear to be plausible."