Half-Life The time required for a concentration of a substance in a body fluid (usually blood plasma) to decrease by half. Hepatotoxicity Toxicity of the liver, associated bile duct, and gall bladder. Hepatotoxin A systemic poison whose target organ is the liver. Hydrolysis The chemical process in which water is used to split a substance into smaller molecules. The hydrogen and hydroxyl parts of a water molecule bond to opposite locations on a chemical bond at the site where the split occurs. Hydrophilic Water loving.  Substance that has strong polar groups that readily interacts with water. Hypersensitivity A state of altered immune reactivity in which the body reacts with an exaggerated response to a foreign agent. Hypoxia A partial reduction in the oxygen concentration supplied to cells or tissues.