Participants will be assigned to one of 3 groups: levomilnacipran, duloxetine or placebo. The duration of study treatment for participants in the first 2 groups will range from 22-32 days. These participants will receive escalating doses of levomilnacipran or duloxetine over a period of 14-23 days followed by down-titrated doses over the next 8 days. The investigators will offer a flexible interval for dose escalation in participants receiving levomilnacipran and duloxetine to minimize side effects, improve tolerability, and maximize subject retention. Participants in the placebo group will receive the levomilnacipran-matched placebo capsule for 14 days.