Major depressive disorder (MDD) is the most common psychiatric disorder mainly characterized by depressed mood, loss of interest, feelings of worthlessness, and a high risk of suicide. Accumulated evidence suggests that both environmental and genetic factors are involved in the etiology of MDD although the pathogenesis of MDD remains unknown. Mutations in genes involved in brain serotonin synthesis, have been identified in patients. Brain serotonin deficiency has been hypothesized to play a role in a wide range of psychiatric diseases, including MDD. Several efficacious treatments for MDD are available, including different forms of psychotherapy and antidepressant medication. Recently, selective reuptake inhibitors (SSRIs, SNRIs,) have become the first-line antidepressant drug treatment of depression and replaced tricyclic antidepressants (TCA) and monoamine oxidase inhibitors (MAOI) due to fewer side-effects and ease of use.