Erythema nodosum is an inflammatory disorder that involves tender, red bumps (nodules) under the skin. In about half of cases, the exact cause of erythema nodosum is unknown. Some cases may occur with infections. Some of the more common infections are: streptococcus (most common), cat scratch disease, chlamydia, coccidioidomycosis, hepatitis B, histoplasmosis, leptospirosis, mononucleosis (EBV), mycobacteria, mycoplasma, psittacosis, syphilis, tuberculosis, tularemia, and yersinia.
Erythema nodosum may occur with sensitivity to certain medications, including: antibiotics including amoxicillin and other penicillins, sulfonamides, sulfones, birth control pills, and progestin. Erythema nodosum is most common on the shins. It may also occur on other areas of the body such as buttocks, calves, ankles, thighs, and arms.
The lesions begin as flat, firm, hot, red, painful lumps that are about an inch across. Within a few days, they may become purplish in color. Over several weeks, the lumps fade to a brownish, flat patch.