Erythema nodosum: An inflammatory reaction that occurs deep in the skin and is characterized by the presence of tender, red, raised lumps or nodules that range in size from 1 to 5 centimeters and are most commonly located over the shins but occasionally on the arms or other areas. The causes of erythema nodosum include medications (such as sulfa-related drugs, birth control pills, estrogens, iodides, and bromides), strep throat, cat scratch fever, fungal diseases, infectious mononucleosis, sarcoidosis, Behcet's syndrome, inflammatory bowel disease (Crohn's disease and ulcerative colitis), and normal pregnancy. In many cases, no cause can be determined. Erythema nodosum may be self-limited. If treatment is needed, the underlying condition is treated, and treatment is simultaneously directed toward the erythema nodosum itself. Treatment can include anti-inflammatory drugs and cortisone given by mouth or injection. Colchicine is sometime used effectively to reduce inflammation.