BACKGROUND: Quetiapine is an atypical antipsychotic with, theoretically, a low propensity for movement disorder adverse effects. It is used for the treatment of schizophrenia and other psychoses. OBJECTIVES: To determine the effects of quetiapine for schizophrenia in comparison to placebo, and other antipsychotics. SEARCH STRATEGY: Electronic searches of the Cochrane Schizophrenia Group's Register of Trials (February 2003), Biological Abstracts (1982-2000), CINAHL (1982-2000), the Cochrane Library (2000, Issue 1),EMBASE (1980-2000), MEDLINE (1966-2000), PsycLIT (1974-2000), SIGLE on CD (1980-1997), SocioFile (1974-1997) and many conference proceedings and hand searches of specific journals were undertaken. We contacted AstraZeneca Pharmaceuticals for information regarding unpublished trials. The review was updated in February 2003. SELECTION CRITERIA: All randomised controlled trials where adults with schizophrenia or similar illnesses were assigned to quetiapine, placebo or other neuroleptic drugs and where clinically relevant outcomes were reported. DATA COLLECTION AND ANALYSIS: Citations and, where possible, abstracts were inspected independently by reviewers, papers ordered, re-inspected and quality assessed. We independently extracted data. We analysed data using fixed effects relative risk (RR) and estimated the 95% confidence interval (CI). Only homogeneous data were interpreted as favouring treatment or control. Where possible we calculated the number needed to treat (NNT) or number needed to harm statistics (NNH). We calculated relative risk (RR) for dichotomous data, and weighted mean differences (WMD) for continuous data. MAIN RESULTS: Despite the fact that 3443 people were randomised in 12 quetiapine studies, there are almost no data on service utilisation, economic outcomes, social functioning and quality of life. Over half of those within the quetiapine versus placebo comparison were lost to follow up (53% quetiapine vs 61% placebo, n=716, 4RCTs, RR 0.84 CI 0.7 to 0.9, NNT 11 CI 7 to 55) so it is impossible to interpret any ratings of global or mental state within this comparison with confidence. People allocated quetiapine, however, did not have more movement disorders than those given placebo (n=395, 2 RCTs, RR needing medication for EPSE 0.62 CI 0.3 to 1.2). The same applies to the comparison of >/= 250 mg/day quetiapine with < 250 mg/day quetiapine (49% dropout >/= 250 mg/day vs 58% < 250 mg/day, n=1066, 3 RCTs, RR 0.84 CI 0.8 to 0.9, NNT 11 CI 7 to 29). It should be noted that two deaths occurred in the higher dose group (n=618, 1 RCT, RR 0.1 CI 0.0 to 2.1). When quetiapine was compared with typical antipsychotics, about 36% of both groups failed to complete the short-term studies (n=1624, 6 RCTs, RR 0.87 CI 0.8 to 1.0). Average change in global state was heterogeneous and equivocal (n=762, 3 RCTs, WMD in short term 0.19 CI 0.00 to 0.38, I squared 76%). Mental state measures were also equivocal (n=1247, RR not improved 0.97 CI 0.9 to 1.1) including specific measures of negative symptoms (n=305, 1 RCT, MD change in SANS short term 0.94 CI -0.2 to 2.0). Movement disorders were less prevalent for those allocated quetiapine (n=1117, 4 RCTs, RR needing medication for extrapyramidal adverse effects 0.47 CI 0.4 to 0.6, NNT 4 CI 4 to 5, I squared 88%). Dry mouth (n=649, 2 RCTs, RR short term 2.85 CI 1.5 to 5.6, NNH 17 CI 7 to 65) and sleepiness (n=959, 3 RCTs, RR 1.51 CI 1.1 to 2.2, NNH 18 CI 8 to 181) may also be more prevalent for people given quetiapine compared with the older drugs. In the quetiapine versus risperidone comparison, over 30% of people left the study before completion (n=728, 1 RCT, RR 0.94 CI 0.7 to 1.2). Four people, all treated with quetiapine, died during the study (n=728, 1 RCT, RR 2.86 CI 0.2 to 52.8). Continuous mental state measures did not show clear differences between the two drugs (n=637, 1 RCT, MD PANSS 1.2 CI -2.0 to 4.4). However, considerably fewer people given quetiapine needed medication for extrapyramidal side effects compared with those allocated to risperidone (n=712, 1 RCT, RR 0.27 CI 0.2 to 0.5, NNT 11 CI 10 to 16). Quetiapine caused more dizziness (n=728, 1 RCT, RR 1.85 CI 1.0 to 3.3, NNH 18 CI 7 to 487), more dry mouth (n=728, 1 RCT, RR 2.11 CI 1.2 to 3.8, NNH 14 CI 6 to 82) and more sleepiness than risperidone (n=728, 1 RCT, RR 2.03 CI 1.4 to 2.9, NNH 7 CI 4 to 17). REVIEWERS' CONCLUSIONS: Quetiapine is effective for the treatment of schizophrenia, but it is not much different from first-generation antipsychotics and risperidone with respect to treatment withdrawal and efficacy. In comparison to first-generation antipsychotics and risperidone, quetiapine has a lower risk of movement disorders but higher risks of dizziness, dry mouth and sleepiness. More clearly reported pragmatic randomised controlled trials should be carried out to determine its position in everyday clinical practice. Studies of medium and long-term effects, including cost-effectiveness, quality of life, social functioning and service utilisation, in comparison with the effects of typical and atypical antipsychotics should be priority areas.