Participants whose depression has improved after Phase 1 will be eligible to continue to Phase 2 of the study, which will determine the longer term effects of taking metformin with sertraline. Phase 2 of the study will last 24 weeks, with study visits occurring monthly. During the study visits, participants will complete questionnaires again, and medication will be collected and handed out. At Weeks 24 and 32, participants will provide blood samples by finger stick. After completing Phase 2, participants will again undergo repeat baseline testing.