Histamine intolerance is a disorder of histamine homeostasis due to decreased intestinal degradation of this amine; it is mainly caused by a deficiency of the enzyme diamine oxidase, which leads to its accumulation in plasma and the development of adverse health effects. Two metabolic pathways are known for histamine in humans. Histamine-N-methyltransferase is the enzyme responsible for the ring methylation of histamine, found primarily in the liver and kidney, and ultimately converts histamine to 1-methylhistamine.