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A deficiency that insidiously affects nerves and blood formation — recognised early it is simple and fully correctable.
Also known as: vitamin B12 deficiency · cobalamin deficiency · pernicious anemia
Open in the tool — with search and all the links →This compilation is for information and replaces neither medical diagnosis nor treatment. If you have symptoms, see a doctor.
B12 is needed for making blood and for the insulating sheath of the nerves. It comes almost only from animal foods, and its uptake is roundabout: the stomach must make a helper, intrinsic factor, and only in the last stretch of the small bowel are the two taken up together. Hence two quite different roads into deficiency. One is intake: a purely plant-based diet without a supplement leads there reliably, though slowly, because the liver holds stores for years. The other is absorption — and that is commoner: a stomach lining that no longer makes the helper (once called pernicious anaemia), metformin, acid blockers over years, surgery on stomach or small bowel, Crohn’s disease. The catch is the order: nerves can suffer before the blood count changes, and folic acid can tidy the blood count while nerve damage continues. That is why, with unclear values, not only B12 is measured but also active holo-transcobalamin, methylmalonic acid and homocysteine. Refilling is by high-dose tablets or injections depending on the cause — lifelong with an absorption problem.
A long-standing shortage can damage the optic nerve: on both sides, slowly, with pale red and a blurred patch in the middle. Together with alcohol and smoking this is the commonest cause — and partly reversible if treated early.
When the B12 is neither clearly low nor clearly normal, the methylmalonic acid decides. It rises only with B12 deficiency and is measured before the first dose, because afterwards it falls quickly.
Folate and B12 work at the same place and make the same picture in the blood count. So both are always measured — and B12 first, because folate alone makes the warning sign disappear.
An unsteady gait, numb feet, anaemia — and a completely normal B12. Then copper is worth it. Copper deficiency makes the same picture and is therefore overlooked for years.
Meat, fish, eggs and milk are the natural B12 sources.
On a vegan diet a B12 supplement is essential — plant foods contain virtually none.
In pernicious anaemia or gut problems B12 is injected because intestinal uptake is missing.
Too little eaten, or too little absorbed? That decides whether tablets are enough and whether it is for months or for good.
It tidies the blood count while nerve damage continues. So with large red cells, always measure both.
Not as a course but permanently — as a supplement or through fortified foods. That holds in pregnancy and breastfeeding too, where the need rises.
A fish tapeworm takes B12 for itself in the small bowel. It comes from raw freshwater fish — rare in Europe but described. The hint is only worthwhile with a fitting diet; otherwise the cause almost always lies in the stomach or in the way of eating.
A tapeworm from raw freshwater fish takes the vitamin in the small bowel. Rare, but the one cause of B12 deficiency that ends with a single treatment.
From sixty-five, about one person in ten to five has a deficiency, because uptake in the stomach declines with age. The body holds a store for two to five years — so a deficiency shows late and improves under replacement only over weeks to months.
B12 refills the store — blood values rise within days, anaemia improves within weeks. Nerve damage takes months and reverses only partly; what has lasted longer than a year often stays. Injections are needed when the stomach uptake route is missing; otherwise high-dose tablets suffice, since a small share crosses the gut wall without that route. Folic acid alone corrects the blood count and hides the nerve damage while doing so.
Usually symmetrical and rising from below. Together with an unsteady gait in the dark it is the most important warning sign, because damage can remain here.
Often the first measurable nerve sign — long before any tingling. The tuning fork on the ankle is felt for a shorter time than before.
The papillae recede, the tongue looks polished and hurts when eating. A clearly visible sign that is often missed.
Forgetfulness, word-finding, irritability, low mood. In older people this is easily put down to age — which is why B12 belongs in the work-up.
The MCV rises, often before haemoglobin falls. Careful: with an iron deficiency at the same time the two effects cancel out and the MCV looks normal.
Anyone who has lost the stomach in whole or part, or the last stretch of small bowel, can no longer absorb B12 — regardless of how much they eat.
In pregnancy clearly established against neural-tube defects, otherwise mainly with deficiency. Can mask a B12 deficiency — watch both.
Pernicious anaemia and autoimmune thyroid disease regularly occur together — both are directed against the body’s own tissue. Anyone with the one has the other measured once.
Without stomach acid B12 is not released from food, and the body answers with rising gastrin. A high gastrin value therefore points to the cause — and an acid blocker over years causes the same deficiency.
Rarely covered on a vegan diet.
Without intrinsic factor, high oral doses also work — the injection is not always needed.
Folate can normalise the blood count while the nerve damage progresses — which is why B12 is measured first.