Several concerns remain to be answered regarding the association between BDNF levels and its (1) relationship with MDD severity, remission, and relapse; (2) direct effects vs. augmented expression of BDNF after antidepressants; (3) long-term antidepressant therapy maintenance; (4) response to other treatment options (e.g., physical activity and psychotherapy); (5) variation according to age, sex, and ethnicity, among other important questions; and (6) ideal measurement of BDNF. Here, BDNF is linked to major depressive disorder.