Multiple sclerosis is a disorder of the nervous system affecting over 500,000 Europeans. It affects three times as many women as men, with the diagnosis typically occurring in patients aged in their 20s and 30s, and is more prevalent in Northern Europe. In recent years progress has been observed in the therapy of patients with this disease. At present there are ten medicines on the market which reduce the annualised relapse rate and progression of disability. Dimethyl fumarate is a new oral drug which significantly reduces the risk of multiple sclerosis relapse, slows the progression of disability and decreases the number of new magnetic resonance imaging demyelinating lesions. In comparison with the first-line medicines used so far such as interferon beta-1b, interferon beta-1a and glatiramer acetate, dimethyl fumarate is characterised with a higher clinical and radiological efficacy. In clinical trials it showed over 56% reduction of the relapse rate, 76% reduction of the number of new/growing T2-weighted lesions as well as stopping the progression of disability. At the same time, dimethyl fumarate has a good safety profile. During the clinical trials a small proportion of patients reported adverse reactions. Similarly to second-line drugs, dimethyl fumarate requires blood lymphocyte level monitoring. In the DEFINE and CONFIRM studies the total number of lymphocytes in patients treated with dimethyl fumarate decreased by 30% after one year and subsequently remained at a consistent level within the norm. In the majority of patients (76%) no lymphopenia was observed over the first 12 months of therapy. Only a small proportion of patients (2%) experienced severe lymphopenia for 6 months or longer. To date two cases of progressive multifocal encephalopathy were reported in patients treated with dimethyl fumarate who had chronic lymphopenia. This medicine is an attractive option for patients treated with first-line drugs.