BACKGROUND: Many people with schizophrenia do not achieve a satisfactory treatment response with ordinary antipsychotic drug treatment. In these cases, various add-on medications are used, and valproate is one of these. OBJECTIVES: To examine whether:1. valproate alone is an effective treatment for schizophrenia and schizoaffective psychoses; and2. valproate augmentation of antipsychotic medication is an effective treatment for the same illnesses. SEARCH METHODS: We searched the Cochrane Schizophrenia Group's Study-Based Register of Trials (July 2002; February 2007; July 2012; March 04, 2016). We also contacted pharmaceutical companies and authors of relevant studies in order to identify further trials. SELECTION CRITERIA: We included all randomised controlled trials comparing valproate to antipsychotics or to placebo (or no intervention), whether as the sole agent or as an adjunct to antipsychotic medication for the treatment of people with schizophrenia or schizophrenia-like psychoses. DATA COLLECTION AND ANALYSIS: We independently inspected citations and, where possible, abstracts, ordered papers, and re-inspected and quality-assessed these. At least two review authors independently extracted data. We analysed dichotomous data using risk ratio (RR) and its 95% confidence intervals (CI). We analysed continuous data using mean differences (MD) and their 95% CI. We assessed risk of bias for included studies and used GRADE (Grading of Recommendations Assessment, Development and Evaluation) to create a 'Summary of findings' table. MAIN RESULTS: The 2012 update search identified 19 further relevant studies, most of which were from China. Thus the review currently includes 26 studies with a total of 2184 participants. All trials examined the effectiveness of valproate as an adjunct to antipsychotics. With the exception of two studies, the studies were small, the participants and personnel were not blinded (neither was outcome assessment), and most were short-term and incompletely reported.For this update we prespecified seven main outcomes of interest: clinical response (clinically significant response, aggression/agitation), leaving the study early (acceptability of treatment, overall tolerability), adverse events (sedation, weight gain) and quality of life.Adding valproate to antipsychotic treatment resulted in more clinically significant response than adding placebo to antipsychotic drugs (14 RCTs, n = 1049, RR 1.31, 95% CI 1.16 to 1.47, I = 12%, low-quality evidence). However, this effect was removed after excluding open RCTs in a sensitivity analysis. In terms of acceptability of treatment (measured by the number of participants leaving the study early due to any reason) valproate was just as acceptable as placebo (11 RCTs, n = 951, RR 0.76, 95% CI 0.47 to 1.24, I = 55%). Also overall tolerability (measured by the number of participants leaving the study early for adverse events) between valproate and placebo was similar (6 RCTs, n = 974, RR 1.33, 95% CI 0.90 to 1.97, I = 0).Participants in the valproate group were found to be less aggressive than the control group based on the Modified Overt Aggression Scale (3 RCTs, n = 186, MD -2.55, 95% CI -3.92 to -1.19, I = 82%, very low-quality evidence). Participants receiving valproate more frequently experienced sedation (8 RCTs, n = 770, RR 1.38, 95% CI 1.07 to 1.79, I = 0, low-quality evidence) but were no more likely to gain weight than those receiving placebo (4 RCTs, n = 427, RR 1.17, 95% CI 0.76 to 1.82, I = 0, low-quality evidence). No study reported on the important outcome of quality of life. AUTHORS' CONCLUSIONS: There is limited evidence, based on a number of trials, that the augmentation of antipsychotics with valproate may be effective for overall clinical response, and also for specific symptoms, especially in terms of excitement and aggression. However, this evidence was entirely based on open RCTs. Moreover, valproate was associated with a number of adverse events among which sedation and dizziness appeared significantly more frequently than in the control groups. Further randomised studies which are blinded are necessary before any clear recommendation can be made. Ideally these would focus on people with schizophrenia and aggression, on those with treatment-resistant forms of the illness and on those with schizoaffective disorders.