Tehran University of Medical Sciences Case Reports in Clinical Practice 2538-2683 1 2 2016 09 24 34 38 EN Hamed Cheraghmakani Neurologist, Assistant Professor, Department of Neurology, Bu Ali Sina Hospital, Mazandaran University of Medical Sciences, Sari, Iran Maryam Mobini Rheumatologist, Associate Professor, Diabetes Research Center, Mazandaran University of Medical Sciences, Sari, Iran Mohammad Baghbanian Neurologist, Assistant Professor, Department of Neurology, Bu Ali Sina Hospital, Mazandaran University of Medical Sciences, Sari, Iran 2016 09 13 2016 09 13 Interferon (IFN) beta is the most widely prescribed disease-modifying drug for multiple sclerosis (MS). However, some adverse reactions are observed in course of IFN-beta therapy. This article presents two cases of female patients diagnosed with relapsing-remitting MS who developed inflammatory musculoskeletal manifestations, following IFN-beta 1a therapy. In the first patient recurrent arthritis developed a week after initiation of IFN-beta, which improved few weeks after a switch to glatiramer acetate. The second patient developed recurrent arthritis 1 month after IFN-beta 1a therapy who suffered painful arthritis despite discontinuation of the medication. Both patients were seropositive for anti- cyclic citrullinated  peptide;  the first  patient  was a  positive  rheumatoid factor (RF) and the second patient was both positive RF and anti-Ro. The role of IFN-beta in the setting of inflammatory musculoskeletal disease remains unclear. To minimize its side effects, review of these antibodies may be required in patients who are candidates for this therapy. https://crcp.tums.ac.ir/index.php/crcp/article/view/82 https://crcp.tums.ac.ir/index.php/crcp/article/download/82/12