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Background: Multiple sclerosis is a chronic disease considered the major cause of neurological disability in young adults worldwide. While depression is considered a determinant factor of impaired quality of life and poorer prognosis among patients with multiple sclerosis, it is very often dismissed and undertreated by physicians. Depression has been related to treatment with some immunomodulatory drugs, such as IFNβ. Data from patients who committed suicide during the pivotal study of interferon used as a disease modifying treatment in multiple sclerosis support this association. Moreover, there is plenty of evidence of neuropsychiatric toxicity caused by the use of IFNα as a treatment for other medical conditions. Although this link still remains relatively unknown, the presence of warnings regarding the possible relationship between depression and IFNβ led to restriction in medical indications in these patients. The purpose of this paper is to try to understand the reasons for an increased prevalence in depression in multiple sclerosis and to examine the impact that IFNβ treatment has on their mood., Methods: We performed a literature search on MEDLINE and Google Scholar databases applying PRISMA guidelines for systematic reviews. Studies were included if the participants were diagnosed with MS and prescribed IFNβ as the main treatment. We excluded non-english and full-text non available papers, as well as the articles where mental health was assessed exclusively as a feature of quality of life. The sample includes articles from 1980 to 2014, although filtration by year of publication was not applied and contains data from IFNβ-1a and IFNβ-1b. The Cochrane Collaboration Tool assessing risk of bias was used to determine the quality of the studies., Results: Ten studies met full criteria for inclusion and final data extraction. The articles have heterogeneity regarding the samples, the methodology used and the expression of the results. Only three studies support the evidence of a relationship between depression and interferon, which is statistically significant in some patients at the beginning of the treatment. They suggest that only patients on IFNβ treatment with a past history of depression may develop a major depression episode during the first six months. The remaining articles reviewed (including BENEFIT, BEYOND, and LTF trials) suggest the absence of an association., Conclusion: The reviewed studies conclude that there is not a clear relationship between IFNβ and depression. A history of depression is a risk factor for developing depression during the first 6 months of treatment, nevertheless, it is not sufficient to contraindicate it. The development of new strategies is crucial for early detection of depressive symptoms. An adequate treatment can both improve the mood and deal with the neurological disease by increasing treatment adherence and interfering with inflammation. Chronic destructive brain changes and serotonergic depletion due to inflammatory factors have been proposed as the underlying cause of depression in these patients. It is suggested that these patients would have fewer functional reserve remaining to deal with stressful life events, which could precipitate a depressive disorder., (Copyright © 2017 Elsevier B.V. All rights reserved.)