A new meta-analysis has found that the risk of developing psychosis is increased by 40 per cent among people who have used cannabis in their lifetimes. If a causal link is assumed, then given estimates of cannabis use in the UK, and extrapolating from incidence rates of psychosis found in a Danish study, this increased risk equates to 800 extra yearly cases of schizophrenia in the UK among 15- to 34-year-olds.
The new meta-analysis by Theresa Moore and colleagues - reportedly the most extensive ever - was published in The Lancet in July (see tinyurl.com/2ordy3), and included 11 longitudinal studies relevant to psychosis, as well as several studies looking at affective disorders. Among heavy users, the increased risk for psychosis was between 50 and 200 per cent. The authors concluded: ‘We believe that there is now enough evidence to inform people that using cannabis could increase their risk of developing a psychotic illness later in life.’
However, there are serious problems with interpreting observational data of the kind included in the meta-analysis. For instance, it is possible that people start taking cannabis to alleviate psychotic symptoms, and also that any psychotic symptoms observed among users are the temporary consequence of intoxication.
Although many of the analysed studies took measures to overcome these difficulties, for example by excluding participants who had psychotic symptoms at baseline, and discounting psychotic symptoms that were clearly a result of intoxication, Moore and colleagues cautioned: ‘The possibility that this association [between cannabis use and psychosis] results from confounding factors or bias cannot be ruled out, and these uncertainties are unlikely to be resolved in the near future.’
Responding to the new analysis, Dr Jon Cole (University of Liverpool) said: ‘The most sensible explanation for these results is that individuals who are pre-disposed to develop schizophrenia are more likely to develop the disorder if they use a large amount of cannabis.’
Cole added that a crucial question that remains to be addressed is why young people at risk of psychosis are using so much cannabis in the first place. ‘A credible explanation,’ Cole said, ‘is that as they start to develop the symptoms of schizophrenia they are drawn to the daily use of cannabis - i.e. self-medication. As with all self-medication, the use of controlled drugs will probably worsen rather than improve symptoms, and this will set up a vicious cycle of cannabis use and symptom aggravation. Whether these people would have developed schizophrenia if they had not used cannabis is an unanswerable question, but as people who don’t smoke cannabis develop schizophrenia it is safe to assume that some probably would.’
The government said the results of the new meta-analysis would be taken into account during the current review of the classification of cannabis. The drug was downgraded from Class B to C in 2004. CJ
|
The Psychologist
