Weight gain is a clinically important side effect of antipsychotic drug therapy. The aim of this study was to determine the effect of the antidiabetic drug metformin on antipsychotic-induced weight gain in non-diabetic patients. In a systematic literature review we identified 195 citations from which seven randomized, placebo-controlled studies (398 patients) were included in the final analysis. Studies in adults (n = 5) and in children (n = 2) were analysed separately. Compared with placebo, metformin treatment caused a significant body weight reduction in adult non-diabetic patients treated with atypical antipsychotics (4.8%, 95% CI 1.6 to 8.0) and in children (4.1%, 95% CI 2.2 to 6.0). There was evidence of substantial heterogeneity among studies, and when the analysis was restricted to patients with a manifest (>10%) body weight increase prior to randomisation metformin reduced weight by 7.5% (95% CI 2.9 to 12.0). The effect was larger in Asians (7.8%, 95% CI 4.4 to 11.2) than in Hispanics (2.0%, 95% CI 0.7 to 3.3). In conclusion, metformin has a pronounced weight-reducing effect in antipsychotic-treated patients, especially in those with a manifest weight gain. Although direct comparisons are lacking, the observed effect on body weight compares favourably with the effect of sibutramine and orlistat, approved for weight reduction. However, metformin is not approved for use in non-diabetic patients and it is still not generally advisable to recommend metformin to counteract antipsychotic-induced weight gain.