Pernicious anemia is a disease in which not enough red blood cells are produced due to a deficiency of vitamin B₁₂. Those affected often have a gradual onset. The most common initial symptoms are feeling tired and weak. Other symptoms of anemia may include shortness of breath, lightheadedness, headaches, sore red tongue, cold hands and feet, pale or yellow skin, chest pain, and an irregular heartbeat. The digestive tract may also be disturbed giving symptoms that can include nausea and vomiting, heartburn, upset stomach and loss of appetite. Symptoms of vitamin B₁₂ deficiency may include decreased ability to think, numbness in the hands and feet, memory problems, blurred vision, trouble walking, poor balance, muscle weakness, decreased smell and taste, poor reflexes, clumsiness, depression, and confusion. Without treatment, some of these problems may become permanent.

Pernicious anemia refers to a type of vitamin B₁₂ deficiency anemia that results from lack of intrinsic factor. 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 all or part of the stomach or small intestine; from an inherited disorder or illnesses that damage the stomach lining. When suspected, diagnosis is made by blood tests initially a complete blood count, 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.

Because pernicious anemia is due to a lack of intrinsic factor, it is not preventable. Pernicious anemia can be treated with injections or pills of vitamin B₁₂. If the symptoms are serious, injections are typically recommended initially. For those who have trouble swallowing pills, a nasal spray is available. Often, treatment may needed for life.

Pernicious anemia due to autoimmune problems occurs in about one per 1000 people in the US. 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.