Although studies using PBMC demonstrated that antidepressants can induce BDNF promoter methylation [50,51], a 2015 study of buccal cells from 251 elderly patients with depression and 773 HC reported that chronic depression was associated with increased BDNF promoter methylation in elderly patients, but there were no significant interactions with antidepressants (SSRIs, TCAs, or other classes) [53]. Here, BDNF is linked to depressive disorder.