The stable synthetic form of vitamin B12 used in most supplements and in food fortification. It carries a cyanide group that the body removes and excretes, in an amount far below any toxic threshold.
B12 is the vitamin where supplementation genuinely and reliably matters — for the people who need it. Deficiency causes megaloblastic anaemia and a progressive neurological syndrome — numbness, unsteady gait, memory and mood changes — that can become permanent if left long enough. The at-risk groups are well defined and large: people eating no animal foods, adults over about 50 with reduced stomach acid, anyone on long-term metformin or proton pump inhibitors, people with pernicious anaemia or after gastric or ileal surgery. In all of these, B12 works decisively. In everyone else, taking 1,000 µg of B12 does nothing except produce expensive urine — and the "energy" marketing attached to B12 injections and sprays is aimed squarely at people with normal levels.
Sold as
Tablet, Sublingual tablet, Spray, Nasal gel, Injection (medical use), Fortificant in plant milks and nutritional yeast
Also known as
Cyanocobalamin, Vitamin B12, Cobalamin
Regulation: No Tolerable Upper Intake Level has been established — no adverse effects have been associated with high oral intake in healthy people. B12 injections are a prescription medicine used for pernicious anaemia and severe deficiency, though oral high-dose regimens have been shown to work comparably in trials.
What the research used
The amounts, forms and durations published trials of Vitamin B12 (cyanocobalamin) actually administered.
These are the amounts published trials administered — not a recommendation, and not a dose to copy. Trial participants were screened, supervised and given a characterised preparation. What is in a retail bottle is frequently not what was studied, and the amount that suits one person is not general advice.
Studied for Recommended Dietary Allowance, adults
2.4 µg
Strong
2.4 µg/day, rising slightly in pregnancy and breastfeeding. Adults over 50 are advised to get it from fortified foods or supplements because food-bound B12 requires stomach acid to release it.
Studied for Correcting deficiency by mouth
1000–2000 µg
Strong
Tablet, daily · Ongoing, with blood monitoring
High oral doses work even in pernicious anaemia because roughly 1% is absorbed by passive diffusion, bypassing intrinsic factor. Randomised trials found oral and intramuscular regimens comparable for correcting blood measures.
Studied for Typical supplement for vegans
25–250 µg
Moderate
Daily tablet, or 2,000 µg weekly
Absorption via intrinsic factor saturates at 1.5–2 µg per dose, so large single doses are absorbed inefficiently — which is why daily small doses and weekly large doses are both used.
This is general information about food, not medical advice. It cannot diagnose, treat or replace guidance from a clinician who knows your history.
When it was taken
Each window says whether a trial compared it against an alternative, or whether it is convention and pharmacology. Most timing advice is the second kind.
1 of 2 timings below were actually compared in a trial. The rest reflect convention or pharmacology.
🕐Any time
Moderate
Intrinsic-factor-mediated absorption saturates at 1.5–2 µg per dose and takes several hours to reset, which is why splitting or spacing doses gets more in than one large dose. That comes from absorption physiology; no trial has compared times of day.
Timing was not directly compared. This reflects convention or how the compound behaves in the body, not a trial showing this window works better.
⏳On an empty stomach
Moderate
Supplemental B12 is not protein-bound, so it does not need stomach acid to be released and is absorbed with or without food. This is the reason supplements work in people with low stomach acid who cannot use B12 from meat.
Timing was tested. A trial compared this window against another and reported a difference.
The evidence in depth
Best supported: Strong
What each association actually rests on, and the papers behind it. A rating describes the state of the research, not how promising the supplement sounds. How we rate evidence.
In people who are deficient, B12 resolves the anaemia and the fatigue that comes with it, and this is one of the most reliably reproduced findings in clinical nutrition. In people with normal B12 status, supplementation does not improve energy — the marketing of B12 shots and sprays for tiredness is aimed at a group in whom it has never been shown to work.
What may be going on: B12 is the cofactor for methionine synthase and methylmalonyl-CoA mutase. Without it, DNA synthesis in dividing red-cell precursors fails, producing large, ineffective red cells and anaemia.
Strong evidence. Consistent findings across multiple good-quality human studies.
B12 deficiency causes a neurological syndrome including memory and mood disturbance, and correcting it early can halt or partly resolve the damage. Supplementation in people with normal B12 has not improved cognition in randomised trials.
Moderate evidence. Several human studies point the same way, with real caveats.
Maternal B12 deficiency is associated with poorer infant outcomes and with deficiency in exclusively breastfed infants, which can cause serious neurological problems. Supplementation in deficient mothers raises infant status; benefit in replete mothers has not been demonstrated.
Moderate evidence. Several human studies point the same way, with real caveats.
Randomised controlled trial · European journal of clinical nutrition · 2020 · PMID 31296936
Deep research mode adds each paper’s own conclusion and stated limitations.
Things to know
Interactions, contraindications and dose limits, most serious first. Concentrated extracts interact with medication in ways the whole plant does not, so this section carries more weight here than it does on a food page.
Worth knowing
Folic acid can hide the anaemia while nerve damage progresses
Because folate corrects the blood picture of B12 deficiency without correcting the deficiency, a person taking folic acid can have a normal blood count while neurological damage advances. Anyone with unexplained numbness, unsteadiness or memory change who takes folate should have B12 measured rather than assume the blood count rules it out.
Long-term metformin reduces B12 absorption in a substantial proportion of users, and proton pump inhibitors and H2 blockers reduce the acid needed to release B12 from food. Nitrous oxide — recreational or anaesthetic — inactivates B12 directly and has caused acute neurological injury in people with borderline status.
Worth knowing
Deficiency is a diagnosis, not a self-assessment
Blood B12 is an imperfect test; methylmalonic acid and homocysteine add information. Starting supplements before testing can normalise the result and obscure the underlying cause — pernicious anaemia, coeliac disease and ileal disease all need identifying in their own right.
Worth knowing
Cyanocobalamin in kidney disease and tobacco amblyopia
The cyanide moiety is trivial for most people, but hydroxocobalamin is generally preferred in advanced kidney disease, in Leber hereditary optic neuropathy and in tobacco-related optic neuropathy, where cyanide handling is impaired or implicated.
Worth knowing
Sprays, patches and lozenges are not better absorbed
Sublingual and spray formats are marketed as bypassing the gut. Comparative studies have found no advantage over swallowed tablets, which already work through passive diffusion at high doses.
In the food catalogue
The whole-food form, where one exists. The evidence for a concentrated extract rarely transfers to the food, or the other way round — these are cross-links, not equivalents.