BDNF in serum or plasma is increasingly used as a putative biomarker for depression, and some studies revealed that lower levels of BDNF in the blood of depressed patients are improved after long-term treatment of certain antidepressants (Shimizu et al., 2003; Gazal et al., 2012; Vanicek et al., 2019). Here, BDNF is linked to major depressive disorder.