• Mean change between TC-5214 and placebo from DB baseline (Week 8) of the HAMD-17 score, at Week 16. [ Time Frame: 16 Weeks ] [ Designated as safety issue: No ]
  

• Number of Participants with Adverse Events [ Time Frame: 16 Weeks ] [ Designated as safety issue: Yes ]
  Treatment emergent adverse events (TEAEs) will be tabulated and summarized by presenting the incidence (number of subjects) in each treatment group.
  
  

This is a multi-center, double blind, randomized, placebo-controlled, parallel group, flexible dose titration study conducted in centers in the USA and India.

Following a washout period, subject will be treated with citalopram 20 mg once daily for 4 weeks, then with 40 mg once daily for 4 weeks. Subjects who tolerate 40 mg citalopram, but whose MADRS score is reduced 50% from baseline, but no lower than 17, will be considered partial or non-responders and will be randomized to receive either placebo or TC-5214 as Add:-on therapy.

TC-5214 or placebo will be started at 2 mg daily (1mg BID dosing). After 2 weeks treatment, medication can be increased to 4 mg (2mg BID) or continued unchanged. Dose escalation will depend on good tolerability and inadequate therapeutic response. After a further 2 weeks, medication can be increased to 8 mg (4mg BID) if felt appropriate by the investigator. Again, dose escalation will depend on good tolerability and inadequate therapeutic response. At any time during the double blind phase of the study, placebo or TC-5214 can be reduced to the last previous dose level following the emergence of unacceptable adverse event(s).

If a subject is prematurely discontinued from the study between Week 8 and Week 16 for any reason, the investigator will make every effort to perform all evaluations as per protocol, assuming the subject had reached the end of the double blind Add:-on treatment phase. These evaluations are to be made as soon as possible but within 2 weeks of discontinuation.

For the subjects completing the double blind phase of the study, there will be a follow-up visit 2-3 weeks after the last dose of trial medication. At this follow-up, any signs or symptoms of relapse will be evaluated.