IL6 and major depressive disorder: When only high quality studies (NOS-score ⩾ 6) were included in the analysis, the association between IL-6 and MDD remained statistically significant and stable after completion of the first five studies (N = 21 studies, d = 0.60, 95%CI = 0.42–0.78, total N(MDD) = 781, total N(non-MDD) = 711, p < 0.0001) (Table 1).