Cannabis and schizophrenia

Many people with schizophrenia use cannabis and its effects on the illness are unclear.

To evaluate the effects of cannabis use on people with schizophrenia and schizophrenia-like illnesses.

We searched the Cochrane Schizophrenia Group Trials Register (April 2007) which is based on regular searches of BIOSIS, CENTRAL, CINAHL, EMBASE, MEDLINE and PsycINFO.

We included all randomised trials involving cannabinoids and people with schizophrenia or schizophrenia-like illnesses.

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.

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).

At present, there is insufficient evidence to support or refute the use of cannabis/cannabinoid compounds for people suffering with schizophrenia. This review highlights the need for well designed, conducted and reported clinical trials to address the potential effects of cannabis based compounds for people with schizophrenia.