Current depression treatments are not effective for approximately one-half of overweight people, and depression eventually returns in approximately one-third of those overweight people whose initial treatment is effective. One possible reason for these treatment failures is that overweight people often experience a condition called insulin resistance (IR), which can lead to type 2 diabetes, an associated disease that might also affect depression treatment. IR results in elevated blood sugar levels that may interfere with medications used to treat depression. Metformin, a medication commonly used to treat diabetes, reduces blood sugar levels. This study will examine whether taking metformin with the antidepressant medication sertraline will enhance the effectiveness of sertraline in people who are overweight and depressed.