Erythema nodosum is a condition that causes painful red bumps under the skin on a person’s shins. Sometimes the bumps also affect the ankles, knees, thighs, and forearms.

The condition is one of the most common forms of a rare inflammatory problem called panniculitis that affects subcutaneous fat under the skin.

A majority of erythema nodosum (EN) cases occur in people between the ages of 20 and 45 years old. Women are five times more likely than men to develop EN.

The condition is usually relatively harmless, but it can be a sign of an underlying infection, other inflammatory conditions, or an abnormal response to an allergen or medication.