The aim of this study is to evaluate the effectiveness, safety, and tolerability of metformin treatment in children and adolescents suffering from weight gain secondary to use of atypical antipsychotic medications. In this 12 week, open-label study we will investigate metformin's effects on weight control and/or weight loss. We hypothesize that metformin would prevent further weight gain or lead to weight loss, resulting in amelioration of one of the most significant side effects of atypical antipsychotic use.