Vitamin B₁₂ deficiency, also known as cobalamin deficiency, is the medical condition in which the blood and tissue have a lower than normal level of vitamin B₁₂. Symptoms can vary from none to severe. Mild deficiency may have few or absent symptoms. In moderate deficiency, feeling tired, headaches, soreness of the tongue, mouth ulcers, breathlessness, feeling faint, rapid heartbeat, low blood pressure, pallor, hair loss, decreased ability to think and severe joint pain and the beginning of neurological symptoms, including abnormal sensations such as pins and needles, numbness and tinnitus may occur. Severe deficiency may include symptoms of reduced heart function as well as more severe neurological symptoms, including changes in reflexes, poor muscle function, memory problems, blurred vision, irritability, ataxia, decreased smell and taste, decreased level of consciousness, depression, anxiety, guilt and psychosis. If left untreated, some of these changes can become permanent. Temporary infertility, reversible with treatment, may occur. A late finding type of anemia known as megaloblastic anemia is often but not always present. In exclusively breastfed infants of vegan mothers, undetected and untreated deficiency can lead to poor growth, poor development, and difficulties with movement.

Causes are usually related to conditions that give rise to malabsorption of vitamin B₁₂ particularly autoimmune gastritis in pernicious anemia. Other conditions giving rise to malabsorption include surgical removal of the stomach, chronic inflammation of the pancreas, intestinal parasites, certain medications such as long-term use of proton pump inhibitors, H2-receptor blockers, and metformin, and some genetic disorders. Deficiency can also be caused by inadequate dietary intake such as with the diets of vegetarians, and vegans, and in the malnourished. Deficiency may be caused by increased needs of the body for example in those with HIV/AIDS, and shortened red blood cell lifespan. Diagnosis is typically based on blood levels of vitamin B₁₂ below 148–185 pmol/L (200 to 250 pg/mL) in adults. Diagnosis is not always straightforward as serum levels can be falsely high or normal. Elevated methylmalonic acid levels may also indicate a deficiency. Individuals with low or marginal values of vitamin B₁₂ in the range of 148–221 pmol/L (200–300 pg/mL) may not have classic neurological or hematological signs or symptoms, or may have symptoms despite having normal levels.

Treatment is by vitamin B₁₂ supplementation, either by mouth or by injection. Initially in high daily doses, followed by less frequent lower doses, as the condition improves. If a reversible cause is found, that cause should be corrected if possible. If no reversible cause is found, or when found it cannot be eliminated, lifelong vitamin B₁₂ administration is usually recommended. A nasal spray is also available. Vitamin B₁₂ deficiency is preventable with supplements, which are recommended for pregnant vegetarians and vegans, and not harmful in others. Risk of toxicity due to vitamin B₁₂ is low.

Vitamin B₁₂ deficiency in the US and the UK is estimated to occur in about 6 percent of those under the age of 60, and 20 percent of those over the age of 60. In Latin America, about 40 percent are estimated to be affected, and this may be as high as 80 percent in parts of Africa and Asia. Marginal deficiency is much more common and may occur in up to 40% of Western populations.

Vitamin B₁₂ deficiency appears slowly and worsens over time, and can often be confused with other conditions. It may often go unrecognized, as the body becomes used to feeling unwell.

Vitamin B₁₂ deficiency can lead to anemia, neurologic, and digestive dysfunctions. A mild deficiency may not cause any discernible symptoms, but at levels moderately lower than normal, a range of symptoms such as feeling tired, weak, lightheadedness, headaches, dizziness, rapid breathing, rapid heartbeat, cold hands and feet, low-grade fevers, tremor, cold intolerance, easy bruising and bleeding, pale skin, low blood pressure, sore tongue, upset stomach, loss of appetite, weight loss, constipation, diarrhea, severe joint pain, feeling abnormal sensations including numbness or tingling (pins and needles) to the fingers and toes, and tinnitus, may be experienced. A wide range of associated symptoms may include angular cheilitis, mouth ulcers, bleeding gums, hair loss and thinning, premature greying, a look of exhaustion and dark circles around the eyes, as well as brittle nails.

Severe vitamin B₁₂ deficiency can damage nerve cells. If this happens, vitamin B₁₂ deficiency may result in sense loss, loss of sensation in the feet, difficulty walking, poor balance, blurred vision, changes in reflexes, muscle weakness, decreased smell and taste, decreased level of consciousness, mood changes, memory loss, depression, irritability, anxiety, clumsiness, disorientation, psychosis and, in severe cases, dementia. Tissue deficiencies may negatively affect nerve cells, bone marrow, and the skin.

A further complication of severe deficiency is the neurological complex known as subacute combined degeneration of spinal cord. also myelosis funicularis, or funicular myelosis. This complex consists of the following symptoms: