Porphyria is a disorder in which the body produces too much of the chemical porphyrin. Porphyrin is used to make heme, the part of blood that carries oxygen. Heme also gives blood its color. Any circulating porphyrin the body does not use is excreted in urine and stool. When the body produces and excretes too much porphyrin, as happens with porphyria, not enough heme remains to keep a person healthy.

Porphyria affects either the nervous system or the skin. When porphyria affects the nervous system, it can cause chest pain, abdominal pain, muscle cramps, weakness, hallucinations, seizures, purple-red-colored urine, or mental disorders like depression, anxiety, and paranoia. When porphyria affects the skin, blisters, itching, swelling, and sensitivity to the sun can result.

Porphyria is an inherited condition. Attacks of the disease can be triggered by drugs (barbiturates, tranquilizers, birth control pills, sedatives), chemicals, certain foods, and exposure to the sun.

Porphyria is diagnosed through tests on blood, urine, and stool. It can be treated with medicines to relieve symptoms, a drug called hemin (which is like heme), or a high-carbohydrate diet.

Porphyria has been speculatively linked with the vampirism myth, based on a number of superficial resemblances between the symptoms of porphyria and attributes of mythical vampires. However, these claims are based on a misunderstanding of the nature of porphyria as well as faulty understanding of vampire folklore. Before this, porphyria was suggested as a possible explanation for werewolf beliefs but is inadequate there as well for similar reasons.

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