BackgroundAlthough there is a consensus that clozapine is more effective than conventional antipsychotic drugs for treatment-resistant schizophrenia, there is great heterogeneity among results of relevant trials.AimsTo re-evaluate the evidence comparing clozapine with conventional antipsychotics and to investigate sources of heterogeneity.MethodIndividual studies were inspected with assessment of clinical relevance of results. Meta-regression analysis was performed to investigate sources of heterogeneity.ResultsTen trials were examined. Recent large-scale studies have not found a substantial advantage for clozapine, especially in terms of a clinically relevant effect. Meta-regression showed that shorter study duration, financial support from a drug company and higher baseline symptom score consistently predicted greater advantage of clozapine.ConclusionsIt may be inappropriate to combine studies in meta-analysis, given the degree of heterogeneity between their findings. The benefits of clozapine compared with conventional treatment may not be substantial.