Porphyria: One of a variety of hereditary diseases that are characterized by abnormalities in the reactions needed for the production of heme, an essential substance for the body, resulting in increased formation and excretion of chemicals called porphyrins. Most porphyrias affect the skin or the nervous system. Attacks may range from mild to severe. Besides having abdominal and nerve pain, the patient may suffer rapid heartbeat, mania, muscle cramps, muscle weakness, breathing problems, hallucinations, and coma. Acute attacks are often precipitated by the use of certain drugs, such as barbiturates, sulphonamides, and birth control pills; alcohol use; hormonal changes during menstruation or pregnancy; dieting or fasting; and infections. One type of porphyria, acute intermittent porphyria, may have affected members of the House of Hanover in England, including 'Mad' King George, who may have suffered attacks of porphyria instead of being 'mad.'