Cannabis and schizophrenia
John Rathbone1, Hannele Variend2, and Hetal Mehta31HEDS, ScHARR, The University of Sheffield, Sheffield, UK 2CRHT, Becklin Centre, Leeds, UK 3Lancashire Care NHS Trust, Guild Lodge, Preston, UK
Background—Many people with schizophrenia use cannabis and its effects on the illness are unclear.
Objectives—To evaluate the effects of cannabis use on people with schizophrenia and schizophrenia-like illnesses.
Search methods—We searched the Cochrane Schizophrenia Group Trials Register (April 2007) which is based on regular searches of BIOSIS, CENTRAL, CINAHL, EMBASE,
MEDLINE and PsycINFO.
Selection criteria—We included all randomised trials involving cannabinoids and people with schizophrenia or schizophrenia-like illnesses.
Data collection and analysis—We extracted data independently. For dichotomous data we calculated relative risks (RR) and their 95% confidence intervals (CI) on an intention-to-treat basis, based on a fixed effects model. We calculated the numbers needed to treat/harm (NNT/ NNH). For continuous data, we calculated weighted mean differences (WMD) again based on a fixed effects model.
Main results—We identified one randomised trial. No significant differences were found between the Cannabis and Psychosis Therapy (CAP) intervention group and the Psychoeducaton (PE) intervention for use of cannabis at three months assessment (n=47, RR 1.04 CI 0.6 to 1.7). BPRS-extended scale scores at three months assessment (n=47, WMD −3.60 CI −12.8 to 5.6) and nine months assessment (n=47, WMD 0.80 CI −7.5 to 9.1) were non-significant between CAP and PE. We found no significant improvement in social functioning in the CAP group compared with PE (at 3 months, n=47, WMD −0.80 CI −10 to 8.4) and (at 9 months, n=47, WMD −4.70 CI −14.5 to 5.1).
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