Henk S Temmingh, Taryn Williams, Nandi Siegfried, Dan J Stein

Up to 75% of people with serious mental illness (SMI) such as schizophrenia and bipolar disorder have co-occurring substance use disorders (dual diagnosis). Dual diagnosis can have an adverse effect on treatment and prognosis of SMI. To evaluate the effects of risperidone compared to treatment with other antipsychotics (first-generation and other second-generation antipsychotics) used in people with serious mental illness and co-occurring substance misuse. On 6 January 2016 and 9 October 2017, we searched the Cochrane Schizophrenia Group's Study-Based Register of Trials (including trial registers). We selected randomised trials of risperidone versus any other antipsychotic in people with SMI and substance abuse (dual diagnosis). We included trials meeting our inclusion criteria and reporting useable data. We excluded trials that either did not meet our inclusion criteria or met our inclusion criteria but did not report any useable data. We independently inspected citations and selected studies. For included studies, we independently extracted data and appraised study quality. For binary outcomes we calculated the risk ratios (RRs) and their 95% confidence intervals. For continuous outcomes we calculated the mean differences (MDs) and their 95% confidence intervals. We pooled data using random-effects meta-analyses and assessed the quality of evidence, creating a 'Summary of findings' table using the GRADE approach. We identified eight randomised trials containing a total of 1073 participants with SMI and co-occurring substance misuse. Seven of these contributed useable data to the review. There was heterogeneity in trial design and measurement. Risperidone was compared to clozapine, olanzapine, perphenazine, quetiapine and ziprasidone. Few trials compared risperidone with first-generation agents. Few trials examined participants with a dual diagnosis from the outset and most trials only contained separate analyses of subgroups with a dual diagnosis or were secondary data analyses of subgroups of people with a dual diagnosis from existing larger trials.For risperidone versus clozapine we found no clear differences between these two antipsychotics in the reduction of positive psychotic symptoms (1 randomised controlled trial (RCT), n = 36, mean difference (MD) 0.90, 95% CI -2.21 to 4.01, very low quality evidence), or reduction in cannabis use (1 RCT, n = 14, risk ratio (RR) 1.00, 95% CI 0.30 to 3.35, very low quality evidence), improvement in subjective well-being (1 RCT, n = 36, MD -6.00, 95% CI -14.82 to 2.82, very low quality evidence), numbers discontinuing medication (1 RCT, n = 36, RR 4.05, 95% CI 0.21 to 78.76, very low quality evidence), extrapyramidal side-effects (2 RCTs, n = 50, RR 2.71, 95% CI 0.30 to 24.08; I² = 0%, very low quality evidence), or leaving the study early (2 RCTs, n = 45, RR 0.49, 95% CI 0.10 to 2.51; I² = 34%, very low quality evidence). Clozapine was associated with lower levels of craving for cannabis (1 RCT, n = 28, MD 7.00, 95% CI 2.37 to 11.63, very low quality evidence).For risperidone versus olanzapine we found no clear differences in the reduction of positive psychotic symptoms (1 RCT, n = 37, MD -1.50, 95% CI -3.82 to 0.82, very low quality evidence), reduction in cannabis use (1 RCT, n = 41, MD 0.40, 95% CI -4.72 to 5.52, very low quality evidence), craving for cannabis (1 RCT, n = 41, MD 5.00, 95% CI -4.86 to 14.86, very low quality evidence), parkinsonism (1 RCT, n = 16, MD -0.08, 95% CI -1.21 to 1.05, very low quality evidence), or leaving the study early (2 RCT, n = 77, RR 0.68, 95% CI 0.34 to 1.35; I² = 0%, very low quality evidence).For risperidone versus perphenazine, we found no clear differences in the number of participants leaving the study early (1 RCT, n = 281, RR 1.05, 95% CI 0.92 to 1.20, low-quality evidence).For risperidone versus quetiapine, we found no clear differences in the number of participants leaving the study early (1 RCT, n = 294, RR 0.96, 95% CI 0.86 to 1.07, low-quality evidence).For risperidone versus ziprasidone, we found no clear differences in the number of participants leaving the study early (1 RCT, n = 240, RR 0.96, 95% CI 0.85 to 1.10, low-quality evidence).For many comparisons, important outcomes were missing; and no data were reported in any study for metabolic disturbances, global impression of illness severity, quality of life or mortality. There is not sufficient good-quality evidence available to determine the effects of risperidone compared with other antipsychotics in people with a dual diagnosis. Few trials compared risperidone with first-generation agents, leading to limited applicability to settings where access to second-generation agents is limited, such as in low- and middle-income countries. Moreover, heterogeneity in trial design and measurement of outcomes precluded the use of many trials in our analyses. Future trials in this area need to be sufficiently powered but also need to conform to consistent methods in study population selection, use of measurement scales, definition of outcomes, and measures to counter risk of bias. Investigators should adhere to CONSORT guidelines in the reporting of results.