 Patients will be recruited at outpatient clinic of AKUH from among those who have received a diagnosis of major depressive disorder by a qualified psychiatrist there. After this they will be explained the research study in a simple language and will be registered after the informed consent. At this point a unique ID will be assigned to them which will be in the form of an alpha numeric code and attached to each of these ID will be a computer number which is obtained from a number series used for similar trials The research officer then will administer an Urdu version of the HAM-D scale. A blood sample of each participant will be drawn by a phlebotomist at Aga Khan University Hospital clinical laboratory. Cold chain will be maintained throughout. Those who will be fulfilling the eligibility criteria will then be recruited to the study. After recruitment these numbers will be randomized to the two arms of the study by the computer itself. The procedure will be performed by a data entry operator who will have no affiliation with the research team and will be hired on per diem basis for this purpose only. Final recruitment may take place in the next visit after the B12 levels are back. The entire process will be monitored by the research officer who will be supervised by a consultant psychiatrist at AKUH clinic who is also principal investigators. The research officer as well as the supervisor will be blind to the arm allocation and will know only codes such as Arms A and B. Participants in the monotherapy arm will only receive the SSRI. Those in B12 arm will concomitantly be administered B12 intramuscular injectable as 1000 mcg every week for 06 weeks . HAM-D scoring will be monitored at baseline and 12 weeks after the baseline. 