Multiple sclerosis is a chronic disease of the central nervous system, characterised by two pathophysiological processes taking place independently from the onset: inflammatory-demyelinating and neurodegenerative. During the course of the disease, disability accumulates over time as a result of relapses and progressive neurodegeneration associated with the so-called smouldering lesions. Treating patients with multiple sclerosis and other co-occurring autoimmune conditions is a major challenge for physicians. Therefore, it is rational to choose a disease-modifying therapy that has a positive therapeutic effect in both comorbidities. The use of combination therapies in the treatment of multiple sclerosis is not currently recommended. Studies conducted to assess the effectiveness of combining standard disease-modifying therapies with corticosteroids, methotrexate, azathioprine, or cyclophosphamide have yielded ambiguous or negative results. Selecting an appropriate disease-modifying therapy for patients with co-occurring multiple sclerosis and another autoimmune disease requires close cooperation of many specialists (neurologist, rheumatologist, gastroenterologist, dermatologist, immunologist). The decision should always be made on an individual basis and focus on achieving monotherapy that is effective in both diseases. The neurologist should know the impact of disease-modifying therapies used in other autoimmune diseases on the course of multiple sclerosis. The article reviews the literature and examines the impact of disease-modifying therapies used in the treatment of multiple sclerosis on the course of other common autoimmune diseases, as well as the effect of disease-modifying therapies used in other autoimmune diseases on the course of multiple sclerosis.