This article contains plot details from the latest book.
Limbium is a medicine that directly affects the elvin limbic system, which is the emotional center of the brain, and the root point of any special ability. The first time limbium was mentioned (Keeper of the Lost Cities), it is described as a "rare mineral that could supposedly clear [one's] mind." As a result, it is the main ingredient in Nogginease.
In the first book, Sophie Foster has a horrible allergic reaction to a drop of limbium in the Nogginease that Dex gave her. 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. In Legacy, Sophie lets Mr. Forkle give her limbium to fix her inflicting, which affects her more than the second time she is given limbium.