The effect of antidepressants in people with severe and less severe depression has been extensively studied. A meta-analysis of all these studies showed that  antidepressants are only beneficial for people with severe depression. With less severe depression there is no benefit in people taking antidepressants compared to people taking a placebo. 

• Kirsch I, Deacon BJ, Huedo-Medina TB, Scoboria A, Moore TJ, Johnson BT. Initial severity and antidepressant benefits: a meta-analysis of data submitted to the Food and Drug Administration. PLoS Med. 2008 Feb;5(2):e45.