A 12-week, randomized controlled trial (RCT) will be conducted to explore the effect of add-on sulforaphane (SFN) to selective serotonin reuptake inhibitors (SSRIs) for major depressive disorder (MDD). This study also aims to explore the value of niacin skin flush response test in MDD. One hundred adults diagnosed with MDD will be recruited. Then all the patients will be randomly assigned to SSRI only group and SSRI plus SFN group. Clinical symptoms and side-effects will be evaluated using the 17-Hamilton Depression Rating Scale (17-HDRS), the Hamilton Anxiety Scale (HAMA), Treatment Emergent Symptom Scale (TESS), and Asberg's Rating Scale for Side Effects (SERS) at baseline and weeks 2, 4, 8 and 12 after treatment. Fifty healthy subjects will be recruited as control group. For all subjects, testing of niacin skin flush response and serum levels of nuclear factor erythroid-2-related factor 2(Nrf-2), p-Nrf2, Heme Oxygenase-1 (HO-1), malondialdehyde (MDA) and erythrocyte glutathion peroxidase (GPX) will be performed at baseline and endpoint. The primary outcome is the reduction rate in 17-HDRS total score from baseline to the end of the study. The secondary outcomes include changes in niacin skin flush response test and levels of serum markers. All the data will be analyzed by SPSS software.

Inclusion Criteria:

1. aged 18-60 years;
2. diagnosed with moderate or severe depressive disorder according to the criteria of the DSM-5;
3. total score of 17-HDRS≥17;
4. never receiving electroconvulsive therapy (ECT)/modified ECT (MECT)/rTMS or other treatments in the past month;
5. having sufficient audio-visual ability and comprehension;
6. signed informed consent statements.

Exclusion Criteria:

1. serious or active somatic illness (abnormal index values were more than twice the limit of normal);
2. a history of mania/hypomania;
3. current high risk of suicide (score of item 3 of 17-HDRS ≥ 3);
4. pregnant or lactating women, or planning pregnant women;
5. taking immunosuppressants or vitamins recently.