Treating Insulin Resistance as a Strategy to Improve Outcome in Refractory Bipolar Disorder

This is a 26-week randomized, double-blind, parallel group prospective study of the effectiveness of treating insulin resistance (IR) to improve mood in patients with IR and treatment-resistant bipolar depression (TRBD). The investigators will compare the effects of treating IR (with metformin) versus placebo on outcome in each patient. The primary outcome will be change in Montgomery-Ǻsberg Depression Rating Scale (MADRS) scores. Patients' current optimized mood stabilizing treatment as usual (TAU, according to the Canadian Network for Mood and Anxiety Treatments \[CANMAT\] or American Psychiatric Association \[APA\] guidelines) must remain unchanged for a period of at least 4 weeks prior to and throughout the study. Patients will undergo a baseline assessment and then be randomized to treatment with metformin or placebo with titration to full dose after 2 weeks. Patients will remain on full treatment for 24 weeks thereafter (total trial duration of 26 weeks for each patient). In those patients with TRBD assigned to treatment with the insulin sensitizer metformin, a significant improvement in depression symptoms will be mediated by the conversion of IR to insulin sensitivity. Subjects: We aim to enrol 110 subjects with IR and TRBD from 2 sites: the primary site in Halifax, Nova Scotia, Canada, and a second site in Pittsburgh, Pennsylvania, USA.