The purpose of this study is to evaluate the effects of Vortioxetine (Lu AA21004), once daily (QD), compared with escitalopram on sexual functioning.

The drug being tested in this study is called Lu AA21004. Lu AA21004 is being tested to treat people who have major depressive disorder (MDD). This study will look at sexual function in people who take Lu AA21004.

The study will enroll approximately 440 patients. Participants will be randomly assigned (by chance, like flipping a coin) to one of the two treatment groups-which will remain undisclosed to the patient and study doctor during the study (unless there is an urgent medical need):

• Lu AA21004 10 to 20 mg
• Escitalopram 10 to 20 mg - This is a comparator drug that is also used to treat MDD.

All participants will be asked to take one tablet at the same time each day throughout the study. All participants will be asked to answer questionnaires at each study visit.

This multi-centre trial will be conducted in the United States and Canada. The overall time to participate in this study is 14 weeks. Participants will make 7 visits to the clinic, and will be contacted by telephone 21 days after last dose of study drug for a follow-up assessment.

Inclusion Criteria:

1. Is a man or a woman aged between 18 and 55 years, inclusive, who is currently being treated with selective serotonin reuptake inhibitor (SSRI) monotherapy (only citalopram, paroxetine, or sertraline allowed) for at least 8 weeks, which was prescribed to treat a major depressive episode (MDE), according to the DSM-IV-TR criteria.
2. Is currently stable; and has a Clinical Global Impression Scale-Severity of Illness Scale (CGI-S) score of ≤3.
3. Is currently experiencing treatment-emergent sexual dysfunction (TESD; defined as a Changes in Sexual Functioning Questionnaire (CSFQ-14) total score ≤41 for women and ≤47 for men), considered to be attributable to the current SSRI monotherapy and is suitable for a switch.

Exclusion Criteria:

1. Has previously participated in a Lu AA21004 clinical study.
2. Has 1 or more the following: any current psychiatric disorder other than MDD as defined in the Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition, Text Revision (DSM-IV-TR; as assessed by the Mini International Neuropsychiatric Interview Version 6.0.0); current or history of manic or hypomanic episode, schizophrenia, or any other psychotic disorder, including major depression with psychotic features, mental retardation, organic mental disorders, or mental disorders due to a general medical condition as defined in the DSM-IV-TR; current diagnosis of alcohol or other substance abuse or dependence (excluding nicotine or caffeine) as defined in the DSM-IV-TR that has not been in sustained full remission for at least 2 years prior to Screening (subject must also have negative urine drug screen prior to Baseline); presence or history of a clinically significant neurological disorder (including epilepsy); neurodegenerative disorder (Alzheimer disease, Parkinson disease, multiple sclerosis, Huntington disease, etc); or any Axis II disorder that might compromise the study.
3. Has sexual dysfunction associated with an etiology other than SSRI treatment or current MDE (e.g., due to a medical condition, such as diabetes or hypertension, a medication, a genital anatomical deformity, or alcohol abuse).
4. Is nonsexually active or anticipates decreasing frequency of sexual activity (ie, sexual activity anticipated to lead to orgasm or that would normally lead to orgasm, which can include sexual intercourse, oral sex, masturbation, sexual fantasies, and/or thinking of sexual activity) during the course of the study below the level at study initiation.
5. Is a male with a history of premature ejaculation in the past year.
6. Has had major relationship changes during the preceding SSRI treatment period or plans to have major relationship changes during the course of the study.
7. Has a sexual partner(s) who plans to initiate treatment for sexual dysfunction during the study.