This study will use a prospective, randomized, double-blind, placebo-controlled design to investigate the effects of probiotic (lactic acid bacillus) supplementation over 6 weeks on patients with major depressive disorder (MDD). The primary aim is to assess changes in depressive symptoms using the Hamilton Depression Rating Scale (HAMD-17). Secondary objectives include evaluating changes in serum IDO and serum BDNF levels, cognitive scores, and the Montgomery-Ã…sberg Depression Rating Scale (MADRS), as well as assessing the safety of the supplementation. 

Participants will be clinically diagnosed with major depressive disorder and must have been on stable antidepressant therapy for at least the past 4 weeks. They will be screened in the psychiatry outpatient department at AIIMS, New Delhi. Eligible participants meeting the inclusion criteria will be randomly assigned to either the treatment group, receiving probiotic (lactic acid bacillus) supplementation, or the placebo group, receiving a matching placebo. Both groups will continue their ongoing antidepressant regimen for 6 weeks. 

At enrollment, demographic data, depression scores, and antidepressant treatment-related data will be collected. Blood samples will be taken to measure depression-related markers (BDNF and IDO). After 6 weeks of supplementation, all baseline parameters will be reassessed. Changes in depression scores from baseline to post-supplementation will be calculated and presented as mean (SD). Any adverse events during the intervention will be monitored and reported to the corresponding authorities.