BDNF and bipolar disorder: In addition, serum/plasma BDNF levels were significantly decreased in patients with acute depressive episodes of bipolar disorder (Cohen’s d = −1.16; 95% CI = −1.79 to −0.54; p < 0.0001; I2 = 83.4%; N = 6, n = 117) and acute manic episodes of bipolar disorder (Cohen’s d = −0.77; 95% CI = −1.10 to −0.44; p < 0.0001; I2 = 50.0%; N = 8, n = 156).