Vitamin B12 deficiency anemia, of which pernicious anemia (PA) is a type, is a disease in which not enough red blood cells are produced due to a deficiency of vitamin B12. The most common initial symptom is feeling tiredness and weakness. Other symptoms may include shortness of breath, pale skin, feeling like one may pass out (lightheadedness), chest pain, rapid heartbeat, numbness in the hands and feet, poor balance, a frequent fall of blood pressure, loss of appetite, a smooth red tongue, poor reflexes, and in severe cases even depression and confusion. Without treatment some of these problems may become permanent.

Pernicious anemia refers to anemia that results from lack of vitamin B12. Lack of intrinsic factor is most commonly due to an autoimmune attack on the cells that create it in the stomach. It can also occur following the surgical removal of part of the stomach or from an inherited disorder. Other causes of low vitamin B12 include not enough dietary intake (which can be a risk in a vegan diet), celiac disease, or tapeworm infection. When suspected, diagnosis is made by blood and, occasionally, bone marrow tests. Blood tests may show fewer but larger red blood cells, low numbers of young red blood cells, low levels of vitamin B12, and antibodies to intrinsic factor.

Pernicious anemia can be treated with injections of vitamin B12. If the symptoms are severe, injections are typically recommended initially. For those who have trouble swallowing pills, a nasal spray is available. Often, treatment is lifelong.

Pernicious anemia due to autoimmune problems occurs in about one per 1000 people. Among those over the age of 60, about 2% have the condition. It more commonly affects people of northern European descent. Women are more commonly affected than men. With proper treatment, most people live normal lives. Due to a higher risk of stomach cancer, those with pernicious anemia should be checked regularly for this. The first clear description was by Thomas Addison in 1849. The term "pernicious" means "deadly", and this term came into use because before the availability of treatment the disease was often fatal.