Inclusion Criteria:  Patients with unipolar depression clinically diagnosed as TRD, who are currently on Sertraline treatment (dose range = 100-200 mg/day) Patients aged 18-60 years of either sex  Exclusion Criteria:  Patients with Bipolar affective disorder Patient with TRD on antidepressants other than Sertraline History of psychoactive substance abuse or dependence Co-morbid psychiatric, major medical, or neurological disorders History of organicity or significant head injury Pregnant and lactating women