| Doubt cast on cannabis, schizophrenia link|
|A new study in the UK 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 schizophrenia or other psychotic disorders.
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 latest 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.
They argue if cannabis use does cause schizophrenia, then an increase in cannabis use should be followed by an increase in the incidence of schizophrenia.
According to the study, cannabis use in the UK 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% between 1990 and 2010.
But the researchers found no increase in the diagnosis of schizophrenia or other psychotic disorders 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."
Professor Joseph Rey of the Faculty of Medicine at the University of Sydney, whose previous research has identified a link between cannabis and schizophrenia, is sceptical of the study's results.
"Not showing that there is a link does not mean there is no link," he says.
Rey says that there may be other factors at play that have reduced the overall incidence of diagnosed schizophrenia.
"The evidence suggesting that cannabis use does increase the risk of schizophrenia is quite strong."
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".