Limbium is a substance that can be found in certain medicines 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. Elwin and Kesler make sure that after that none of the elixirs they made for Sophie had limbium in it. Later, in Exile, Mr. Forkle gave her an entire ounce of Limbium and then injected her with 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 Physic give her limbium in the form of a green pill to fix her inflicting. It has a greater effect this time as the limbium targets both her heart and mind, causing her heart to stop beating momentarily.