Peripheral BDNF levels were not significantly altered in BD participants in a bipolar mixed episode compared to healthy controls (g = 0.09, 95 % CI −0.57 to 0.75, P = 0.787, 3 between-group comparisons, n = 213); however, only three studies [57, 72, 75] were included in this sub-group and the 95 % CI was extremely large and, consequently, this analysis is likely to be underpowered (Table 1, Additional file 1: Figure S2). This evidence concerns the gene BDNF and Behcet disease.