In conclusion, future research must first better address laboratory techniques for peripheral BDNF peripheral detection with a high correlation with brain concentrations; second, it must minimize important heterogeneity factors such as the inclusion of different MDD phenotypes, different quality of patient's environment, the presence of child trauma, and genetic Val66Met polymorphisms; and third, it must explore potential direct BDNF effects depicted from rapid-acting antidepressants such as ketamine. Here, BDNF is linked to major depressive disorder.