Major depressive disorder (MDD) is rapidly growing among the population. A widely believed neurobiological explanation is that the symptoms arise due to an imbalance of the neurotransmitter serotonin. Therefore, the most provided antidepressant is currently selective serotonin reuptake inhibitors (SSRI), which increase the serotonin in the synaptic cleft by inhibit the reuptake of serotonin. There are medications which challenge the serotonin hypothesis such as tianeptine. Tianeptine increases the reuptake of serotonin in the synaptic cleft and thus decreasing the serotonin levels. The thesis has three aims: First, to investigate what mechanisms tianeptine and SSRI work upon. Second, compare the efficiency of SSRI and tianeptine. Third, if the two agents display any differences in adverse side effects. A systematic review and search through relevant databases were made to obtain results. The main findings of this thesis were the two agents act differently of many aspects of the brain mechanisms and neurochemistry such as the cannabinoid system, expression of different cell types and their dependence of protein kinase. Even so, the results show that both agents are equally efficient in treating the depressive symptoms in the larger context, although some interesting findings are seen when zooming in. Anxiety is often comorbid with depression and even though both tianeptine and SSRI are shown to reduce these symptoms during chronic administration, SSRI can produce an anxiogenic effect in the beginning. Another noteworthy finding was that tianeptine showed to be clinically significant, but so did placebo. The third aim investigated the differences in side-effects between these two agents, and both agents were equally safe in number of adverse side-effects. Though tianeptine showed to have some slight advantages in manners of sexual dysfunction and the item 3 on the CGI scale.