Erythema nodosum is a fairly uncommon, but distinctive, clinical syndrome. It consists of a reaction in the skin and deeper tissues that almost always occurs on the shins. It is more common in women than in men. Usually, several tender red nodules are present on both legs. There may be fever, aching joints, and fatigue. Over a period of weeks, the nodules become less tender, turn purplish, and heal without scarring. Rarely, they recur periodically.
The real significance of erythema nodosum lies in the possibility of a serious underlying disease. Although in some cases no such underlying cause is found, most are associated with either a drug that has been taken, an inflammatory condition elsewhere in the body or an infection.
Drugs that can cause erythema nodosum include penicillin (in all its forms), sulfa drugs, and oral contraceptives. The disease's association with oral contraceptives and sulfa, a drug component that is commonly used to treat urinary tract infections, may be why it is seen more frequently in women.
Sarcoid, an inflammatory condition of unknown cause that can affect almost any organ, and inflammatory bowel disease (cause also unkown) may both also be associated with erythema nodosum.
Finally, all kinds of infections, ranging from the common, like strep throat, to the uncommon, like coccidioidomycosis (a fungal disease mostly seen in the southwestern U. S.), may be associated with erythema nodosum. Chlamydia, tuberculosis, and other bacterial and fungal infections may also be associated with erythema nodosum, but fungal infections involving only the skin, such as athlete's foot, are not.
I have used the word "associated" because erythema nodosum is not an allergic reaction to these conditions or drugs, nor does it contain any infectious organisms such as tuberculosis bacilli or strep germs. Why some people develop these odd reactions to illness, drugs, or infections that many of us have or take, is completely unknown. Since the erythema itself is not dangerous and heals without treatment, doctors will spend most of their time and effort trying to rule out an underlying cause. Some experts suggest treating all cases with penicillin (if penicillin was not the underlying cause), because strep infection is frequently the underlying condition.