You likely discovered it the way most people do — a brownish, net-like pattern on your thigh from a laptop used too long, on your shins from a space heater relied on through winter, or on your abdomen from a heating pad kept in place night after night.

If you have experienced a fixed drug eruption, you know the pattern: a round, darkening patch appears in the exact same spot every time you take a particular medication. It might be on your lip, your forearm, your groin, or your torso.