Porphyria is an illness characterized by the overproduction and accumulation of porphyrins, a type of molecule in the body. There are different types of porphyria, each of which is caused by deficiency in a certain enzyme, which results in a buildup of porphyrin at a specific site in the body. Porphyria types may be generally categorized as hepatic, affecting the nervous system; erythropoietic, affecting the skin; or a combination of the two. Porphyria may be genetic or acquired.

Hepatic porphyria, also known as acute porphyria, is a very serious condition. Symptoms may include mental illness, seizures, severe neuropathy, gastrointestinal complaints, tachycardia, and arrhythmia. Patients with hepatic porphyria also have an increased risk of liver cancer. Erythropoietic porphyria causes a number of skin conditions, including photosensitivity, blisters, swelling, itching, abnormal hair growth, and necrosis. Discoloration of the urine, teeth, and nails may also present.

All types of porphyria are rare, and the condition is often misdiagnosed, especially the hepatic varieties. Diagnosis is made through analysis of urine, blood, or stool samples, and tests must often be conducted at a specialized facility. In addition, the samples to be tested should be collected during a suspected attack, they must be stored properly, and multiple tests may be necessary.

There is no cure for porphyria, but it can be managed through diet and drug therapy. A high-carbohydrate diet can help stave off attacks in patients with hepatic porphyria, and drugs can be used to minimize the symptoms associated with attacks. The pain associated with hepatic porphyria is severe, so opiates are typically prescribed for pain management. Hormone treatment is sometimes used as well, though it can trigger attacks in some patients. Certain drugs cannot be used by porphyria patients under any circumstances, including any drugs that may have triggered the condition. Blood transfusion, the anti-malarial chloroquine, and sunscreen are the primary treatments for erythropoietic porphyria.

Biochemist David Dolphin speculated in a 1985 speech that erythropoietic porphyria cases may have been the basis for vampire legends, due to the sufferers' sensitivity to light and strange appearance. He also suggested that people with porphyria may have craved and ingested blood in the belief that it would alleviate their symptoms and that they have an aversion to garlic, but neither of these speculations has ever been substantiated. In fact, it was not known until fairly recently that porphyria is a disorder of the blood that can be treated with transfusions. Unfortunately, Dolphin's ideas gained a lot of publicity, and porphyria consequently carries somewhat of a stigma.