The exact mechanism of action of different modifying treatments in the evolutionary course of multiple sclerosis (MS) remains unknown, but it is assumed that they act upon the cells involved in acquired immunity. One effect of these treatments is the development of lymphopenia, which carries inherent safety risks. This study was conducted to understand the alterations that teriflunomide (TERI) and dimethyl fumarate (DMF) exert upon white blood cells in a series of patients with MS. This study included a total of 99 patients; 44 treated with DMF and 55 patients treated with TERI. Blood counts were evaluated at baseline and every 6 months in order to track the absolute leukocyte, lymphocyte, and neutrophil counts. Twelve months after starting treatment, we observed a significant decrease in leukocytes (21.1%), lymphocytes (39.1%), and neutrophils (10%) in the DMF group. In the TERI group, leukocytes decreased by 11.1%, lymphocytes by 8.1%, and neutrophils by 15.7%. Both TERI and DMF produced a significant decrease in leukocytes during the first year of treatment and this was mainly related with a decrease in neutrophils in the TERI group and a decrease in lymphocytes in the DMF group.
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