IL18 and major depressive disorder: In our study, bidirectional MR analysis revealed a significant inverse causal relationship between MDD and genetically predicted IL-18 levels, with a higher genetic predisposition to MDD, correlating with reduced IL-18 levels (β: −0.40; 95% confidence interval (CI): −0.69 to −0.11; P = 7.09 × 10−3).