Metformin is an antidiabetic drug that may have, as demonstrated by developing evidence, considerable therapeutic application for the treatment of antipsychotic drug side effects. Studies published over the past 6 years have indicated that metformin can attenuate or reverse some of the weight gain induced by antipsychotic drugs (1–3) and that this effect may be additive to lifestyle intervention (4). The weight-loss effect of adjunctive metformin may be stronger in first-episode or drug-naive patients started on second-generation antipsychotic medications, especially those that are associated with the greatest weight gain, such as clozapine and olanzapine (5–7). These studies have also shown that metformin concomitantly improved measures of glucose metabolism, such as reduction in fasting glucose and insulin levels and measures of insulin resistance, in the same antipsychotic-treated patients (4, 6, 7). Although metformin may decrease weight and improve glucose metabolism in the same patient, it is not clear how closely these effects are correlated or whether they operate through the same or different underlying mechanisms.