Just two micrograms of vitamin B12 a day is enough to keep the human body going — less than a sliver of a grain of salt. Yet despite that tiny requirement, deficiency is widespread and often goes undetected for years. The problem is not that the condition is rare; it is that its symptoms look almost nothing like a vitamin shortage and everything like normal ageing.
Fatigue, weakness, breathlessness, tingling in the hands and feet, poor balance, memory lapses and brain fog — every one of these can be shrugged off as the price of getting older. Because the symptoms develop gradually, people rarely question them. The result is that most cases are never recognised until the damage has been done.
Why B12 deficiency is so easy to miss
The reason B12 deficiency is so frequently dismissed lies in the nature of its progression. Unlike a sudden infection or an acute injury, the decline happens over months or years. The body can store B12 for two to five years, so a person can be running low on the vitamin long before any symptom appears. When symptoms do emerge, they are so non‑specific that they are routinely blamed on age, stress or overwork.
The irony is that the people most affected — older adults — are also those whose complaints are most likely to be written off as inevitable wear and tear. In the UK, around 6% of people under 60 are deficient, but that figure jumps to 20% in those over 60. Among the over‑75s, roughly one in ten is affected, and among those aged 65 to 74 it is one in 20. Yet many of these individuals never receive a diagnosis.
Even when patients do seek medical help, the standard B12 blood test can miss up to 26% of deficiencies, particularly in cases where neurological symptoms are present but serum levels appear normal. Researchers have found that markers such as holotranscobalamin and methylmalonic acid may give a more accurate picture, especially for cognitive decline, but these tests are not routine.
A 2026 study offered a biological explanation for why fatigue and brain fog can appear before anaemia shows up in blood work. Low B12 levels were found to interfere with DNA inside mitochondria and reduce energy production in muscle cells, meaning the energy deficit is real even when red blood cells look normal.
Symptoms: from fatigue to neurological damage
The earliest signs of B12 deficiency are persistent tiredness, weakness and breathlessness. These are caused by anaemia — a shortage of healthy red blood cells. Without enough B12, the bone marrow produces oversized cells that cannot carry oxygen efficiently, leaving the body feeling exhausted.
But the condition does not stop at fatigue. Neurological symptoms can develop before any anaemia is detected. Patients report pins and needles or numbness in the hands and feet — often in a symmetrical “glove and stocking” pattern — as well as poor balance, difficulty walking and loss of coordination. Cognitive effects include memory problems, confusion, trouble understanding or judging situations, and what is commonly described as brain fog. Some people develop depression, anxiety, irritability or personality changes.
Other symptoms include headaches, feeling faint, pale or yellowish skin, heart palpitations, tinnitus, indigestion, loss of appetite, weight loss, diarrhoea, mouth ulcers, a sore red tongue, muscle weakness, disturbed vision and, in severe cases, incontinence.
If left untreated, the neurological damage can become irreversible. Severe anaemia can also lead to heart failure and lung complications. There is evidence that B12 deficiency can cause temporary infertility, and in pregnant women it increases the risk of neural tube defects in the baby.
Who is most at risk
B12 is found naturally almost exclusively in meat, fish, eggs and dairy, so vegans and vegetarians are at significantly elevated risk. Estimates suggest that up to 62% of pregnant vegan women and 52% of vegan men are deficient, while up to 40% of vegetarians may be affected.
But diet is only part of the story. The most common cause of severe B12 deficiency in the UK is pernicious anaemia, an autoimmune condition in which the immune system attacks the stomach cells that produce a protein called intrinsic factor. Without intrinsic factor, the body cannot absorb B12 from food, regardless of how much is eaten. Pernicious anaemia is more common in women around the age of 60 and in people with a family history of the condition or other autoimmune disorders.
Age itself plays a major role. As people get older, the stomach produces less acid, which is needed to release B12 from food during digestion. Some older adults also develop autoimmune gastritis, a silent condition that damages the stomach lining and further reduces intrinsic factor production.
Certain medications can interfere with B12 absorption, including proton pump inhibitors for acid reflux, anticonvulsants, and metformin for type 2 diabetes. Other causes include chronic alcoholism, pancreatic failure, excessive urination, and nitrous oxide abuse, which can make laboratory tests inaccurate.
Conditions affecting the intestines, such as Crohn’s disease and coeliac disease, can also hinder B12 uptake. In all these cases, the underlying problem is malabsorption — the body cannot get the vitamin into the bloodstream no matter how much is consumed.
Treatment: straightforward once identified
Diagnosing B12 deficiency is straightforward. Anyone experiencing persistent tiredness, brain fog or any of the neurological symptoms listed above can ask their GP for a blood test. In the UK, a serum level below 180 nanograms per litre is generally considered deficient, though some guidelines use 200 ng/L. Because standard tests are not always definitive, doctors may also check for anti‑intrinsic factor antibodies to confirm pernicious anaemia, or measure methylmalonic acid and homocysteine levels for a more sensitive assessment.
Once diagnosed, treatment is effective. For absorption problems such as pernicious anaemia or neurological symptoms, B12 injections are the standard approach. The preferred form in the UK is hydroxocobalamin. Initial treatment often involves injections every other day for two weeks, then lifelong maintenance injections every two to three months. When neurological symptoms are present, injections may be needed every two months to prevent progression.
For deficiencies caused purely by diet — most often in vegans and vegetarians — daily oral B12 tablets may be sufficient. If oral supplements do not raise levels after two months, injections are usually started. Lifelong supplementation is typically required for vegans and for anyone with an absorption disorder.
Dietary advice alone is rarely enough if absorption is impaired, but increasing consumption of B12‑rich foods such as meat, fish, eggs, dairy and fortified products is recommended for those with dietary shortages. In cases where anaemia is present, folic acid supplements may be prescribed alongside B12, but doctors must ensure B12 levels are normal first, because folic acid can mask a deficiency and allow neurological damage to continue unnoticed.
The main challenge, health experts stress, is not the complexity of the condition but the simple fact that what feels like ageing may be something entirely treatable — and that recognising the difference is the only way to stop permanent harm.