Pernicious anemia (also known as Biermer's anemia or Addison's anemia) is a form of megaloblastic anemia caused by the body's inability to absorb the vitamin B12 and/or folic acid. On the whole, anemia is a condition in which the body lacks a sufficient amount of red blood cells or hemoglobin, resulting from disruptions in their production or a loss of blood. In cases of megaloblastic anemia, red blood cells do not function as such; they are overly-large (megaloblasts) and immature, and inefficient in transporting oxygen throughout the body. Because the body contains large stores of vitamin B12, it can take many years before the disease manifests itself. Before it was learned that vitamin B12 could treat pernicious anemia, it was very often fatal.

The most common cause of pernicious anemia is the loss of stomach cells that create intrinsic factor, the protein responsible for the body's absorption of vitamin B12. This is considered an autoimmune disorder and frequently occurs comorbidly with other autoimmune disorders such as celiac disease, but can also be a complication of partial or total gastrectomy. Other causes of pernicious anemia, namely those resulting from folate deficiency, include the increased need during pregnancy, insufficient dietary intake (seen often within vegan individuals) and alcohol abuse.

General symptoms of pernicious anemia include dizziness, fatigue, pale skin, poor concentration, loss of coordination, and shortness of breath. Additionally, digestive disturbances such as constipation, diarrhea, loss of appetite, and nausea and/or vomiting may occur. Some serious and potentially fatal complications of untreated pernicious anemia are depression, changes in personality, numbness or tingling in the extremities (neuropathy), an increased risk for stomach cancer and polyps, arrhythmia, or an enlarged heart.

Thankfully, when caught in the early stages pernicious anemia is easily treated and prognosis is good. Treatment involves the administration of vitamin B12, usually taken intravenously but occasionally orally in large doses. Lifelong treatment is required to minimize the risk for serious and permanent neurological complications.