Objective: To evaluate the effectiveness of adjunctive Thiamine (vitamin B1) with standard treatment in patients with Major Depressive Disorder (MDD) by comparing with standard treatment.

Methods: Randomized control trial in patients with MDD aged 18 – 60 years old, treated at both inpatient and outpatient wards of Nakhonsawan Rajanakarindra Psychiatric Hospital during September 2018 to June 2019. Fifty patients were randomly divided into Thiamine group and control group by draw lots. The Thiamine group received 300 mg/day of Thiamine with standard treatment and the control group received only standard treatment. Outcomes were evaluated by using Hamilton Rating Scale for Depression (HRSD), Montreal Cognitive Assessment (MoCA) and Reaction Time from Continuous Performance Test (CPT). Frequency, mean, percentage and standard deviation were used to examine personal data. Independent T-Test and Pearson Chi Square were used to compare the proportion of personal data. Independent T-Test was used to compare the changes of dependent variable between groups. Paired T-Test was used to compare the changes of dependent variable within a group.

Result: From the 4^th week to the end of the study, the depressive severity score from HRSD in the Thiamine group was significantly decreased more than in the control group (p=0.004, 0.031 and 0.035). At 12^th week the cognitive score from MoCA in the Thiamine group was significantly increased more than in the control group(p=0.007). From 8^th week to the end of study, the reaction time from CPT in the Thiamine group was significantly decreased more than in the control group. (p=0.047 and 0.003).The only adverse event found was nausea and not severe.

Conclusion: Using adjunctive Thiamine with standard treatment was more effective than standard treatment alone in patients with Major Depressive Disorder without any serious adverse event.