Myositis can be caused by an injury, infection, medicines or illicit drugs. It also can be caused by cancer or a disease in which the immune system attacks body tissues, called autoimmune disease.

Symptoms include muscle weakness, pain and swelling. Symptoms also can include falls, trouble breathing or swallowing, or a rash.

Treatment may include a steroid medicine, physical therapy and exercise. Treatment also can involve treating the disease that's causing the myositis or stopping the medicine or illicit drug use causing it.