CYP2C19 and major depressive disorder: In light of the global prevalence of MDD, the variability in treatment response between individuals, and the CYP2C19 enzyme’s role in metabolizing antidepressant drugs, this systematic review aimed to determine the CYP2C19 genetic variants’ frequency variation in different populations with major depressive disorder and to understand how these polymorphisms influence MDD clinical characteristics and the response to antidepressants.