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This is a three-center, randomized, double-blind, fixed dose study designed to assess the efficacy, safety, and tolerability of a switch to vilazodone for sexual dysfunction associated with use of a selective serotonin reuptake inhibitor (SSRI) and serotonin-norepinephrine reuptake inhibitor (SNRI)compared to switching to sertraline in patients with Major Depressive Disorder (MDD).

Vilazodone is a newly introduced, FDA approved antidepressant that is a combined serotonin specific reuptake inhibitor and serotonin 1A receptor partial agonist. In contrast to the SSRIs and SNRIs, appears to have low adverse effects on sexual functioning when compared to placebo.

The design will be a randomized, double blind study with patients being randomized to switching to either vilazodone or sertraline. This will consist of a 1-week Screening phase, a 2-week Cross Taper phase, an 8-week Treatment phase, and an optional 2-week Down taper phase. The total duration of each patient's participation will be 13 weeks.

Seventy-two patients will be randomized at the Baseline visit to either vilazodone or sertraline, 24 at each of the 3 sites. Patients will be recruited over 12 months.

Inclusion Criteria:

1. Patients must be able to understand the nature of the study, agree to comply with the prescribed dosage regimens, report for regularly scheduled office visits, and communicate to study personnel about adverse events and concomitant medication use;
2. Patients with major depressive disorder who are being treated with a selective serotonin reuptake inhibitor (citalopram, escitalopram, fluvoxamine, or paroxetine) or serotonin-norepinephrine reuptake inhibitor (desvenlafaxine, duloxetine, venlafaxine) for a minimum of 8 weeks.
3. The current episode of MDD is in remission (MADRS score < 10 and CGI score of 1 or 2)
4. The duration of the current MDD episode is less than 2 years
5. Presence of antidepressant-associated sexual dysfunction (i.e., absence of sexual dysfunction prior to becoming depressed and presence of significant dysfunction while on the SSRI or SNRI despite being in remission from the depression).
6. Patient is at least 18 years old and not more than 65 years old
7. Patients must have the opportunity for sexual activity during the study period (in the form of availability of a suitable partner for sexual activity and/or openness to masturbation)
8. Patients must be willing to attempt some sexual activity (including masturbation) at least once every two weeks during the study

Exclusion Criteria:

1. Patients who have previously failed to respond to or to tolerate either vilazodone or sertraline.
2. Patients with a history of severe discontinuation symptoms on tapering off the current antidepressant
3. Patients with other known causes of sexual dysfunction
4. Use of prohibited medications during the study period