Limbium is a medicine that directly affects the elves' Limbic System.
In the book series, Keeper of the Lost Cities, Sophie Foster has a horrible allergic reaction to only a drop of Limbium. Later, in Exile, Mr. Forkle gave her an entire ounce of Limbium and then gives her a modified human medicine to stop the reaction. This occurs because Sophie's abilities were malfunctioning and limbium was the only way to fix it. Doing so created a star-shaped scar on one of her hands.