Usnic Acid is derived from an algae or fungus that grows on certain species of trees and is sometimes referred to as Old Man's Beard because of the way that it hangs from the branches to the ground. Usnic acid is often used in supplements that are intended to promote weight loss, and although this type of supplement is found to be beneficial for some folks, others have had very dangerous side effects. Use particular caution when using usnic acid as a fat burner or diet aid, and check with your health care provider before you begin a regimen that includes usnic acid. Usnic acid can cause a potential toxicity within your body that can be life threatening.