Vitamin B12 (cobalamin)

Water-soluble B-vitamin; cofactor of methionine synthase and methylmalonyl-CoA mutase

ATC code: B03BA-VITAMIN-B12(Vitamin B12 (umbrella: cyanocobalamin, methylcobalamin, hydroxocobalamin, adenosylcobalamin))

Brand names: Nature Made B12, Solgar Methylcobalamin, Jarrow Methyl B12, Nature's Bounty B12

Updated 24 Sep 2026 · Methodology

Indications

Vegan and vegetarian diet: B12

First line

Vegan and long-term vegetarian diets without fortified foods are a major indication for preventive B12 intake. Without supplementation deficiency develops in some people within 1–2 years and in others over 5–10 years. NIH ODS 2024 and SEEN recommend 25–100 mcg of cyanocobalamin per day or 2000 mcg once weekly in vegans. Any iHerb form works; cyanocobalamin is cheaper and has the longest evidence base. Annual or biannual monitoring of B12, homocysteine and methylmalonic acid is appropriate. If you are vegan and not taking B12, discuss testing and a regimen with a clinician.

Sources

ICD-10: E53.8 · ICD-11: 5B5F

Vitamin B12 (cobalamin) deficiency

First line

Vitamin B12 is the cornerstone of treatment for confirmed deficiency. AEMPS and approve oral 1000–2000 mcg/day and intramuscular 1000 mcg on a schedule: daily for 1–2 weeks, then weekly until normalisation, then maintenance. The oral route works even with reduced absorption – a fraction is absorbed by passive diffusion. NIH ODS 2024 and British Society for Haematology 2021 accept high-dose oral therapy as equivalent to injections in most patients. Monitoring: serum B12, methylmalonic acid and homocysteine at 3 months.

Sources

ICD-10: E53.8 · ICD-11: 5B5F

Marketing claims without evidence base

The drug is promoted for these uses outside international guidelines. Each entry below is analyzed against AEMPS, FDA, EMA, Cochrane and major RCTs.

Anti-aging and longevity

Not recommended

Vitamin B12 (cobalamin) is a water-soluble vitamin needed for blood formation and nerve function. For anti-aging and longevity it gets marketed as a supplement that supposedly slows aging and extends lifespan. In people without a B12 deficiency, this use has no convincing evidence behind it: when blood levels are normal, extra B12 does not extend life or reverse aging. The vitamin itself is harmless at usual doses, but it does not act on aging this way. What actually supports healthy aging are sleep, diet, physical activity, not smoking, and managing chronic conditions. Do not take B12 in place of a treatment with proven benefit, since swapping out real care means losing valuable time.

Sources

Chronic fatigue

Not recommended

Vitamin B12 (cobalamin) is a water-soluble vitamin needed for blood formation and nerve function. For chronic fatigue it is often taken and marketed as an energy booster, yet no convincing benefit has been shown for this use in people who are not B12-deficient. When fatigue persists, the useful step is to find its cause with a doctor: check sleep, thyroid, iron levels, mood, and other conditions. The vitamin itself is harmless here, but it should not replace proper evaluation and treatment with proven benefit, or the real cause may be missed.

Sources

ICD-10: R53 · ICD-11: MG22

Mood stabilization

Not recommended

Vitamin B12 (cobalamin) is a water-soluble vitamin the body needs for the nervous system and blood formation. It is sometimes promoted as a supplement for mood stabilization and emotional swings. There is no convincing evidence that B12 helps with this: in people with normal vitamin levels, taking extra B12 does not even out mood. The vitamin itself is harmless at usual doses, but ongoing mood swings respond to psychotherapy and treatment chosen by a doctor. Do not use B12 in place of therapy with proven benefit, or you may miss the care you actually need.

Sources

How to read tiers A–F

AStrong evidence – first or second line in international guidelines
BRecommended in selected cases
CStudy data contradict each other
DBenefit not proven, no documented harm
FDocumented harm, or risk without benefit

Mechanism of action

Cobalamin drives two key reactions. Cytoplasmic methylcobalamin transfers a methyl group to homocysteine, converting it to methionine and then to S-adenosylmethionine, the main methyl donor for DNA, myelin and neurotransmitters. Mitochondrial adenosylcobalamin works with methylmalonyl-CoA mutase and feeds branched-chain amino acids and odd-chain fatty acids into the Krebs cycle. Of the four forms on iHerb, cyanocobalamin and hydroxocobalamin convert intracellularly into the active methyl- and adenosyl-cobalamin; pre-formed active forms cost more but for most patients have no clinical advantage over cyano.

Safety

Contraindications

Hypersensitivity to cobalt or to a specific cobalamin form
Leber's hereditary optic neuropathy – cyanocobalamin accelerates progression; hydroxo- or methylcobalamin are used instead
Polycythaemia vera (relative contraindication – B12 stimulates erythropoiesis)

Serious adverse effects

Anaphylactic reactions to parenteral forms (especially cyanocobalamin)
Unmasking of iron and folate deficiency during anaemia correction
Thrombocytosis and hypokalaemia with rapid replenishment of severe deficiency

Common adverse effects

Injection-site pain and redness
Rash and pruritus
Urine discolouration (pink tint with hydroxocobalamin)
Mild nausea at high oral doses

Pregnancy: can you take Vitamin B12 (cobalamin)?

AEMPS summary of product characteristics for Optovite B12 (cyanocobalamin solution for injection), section 4.6: vitamin B12 can be given to pregnant women whenever needed, following the dosing schedules described in the summary. A daily amount of 4 micrograms is recommended.

Breastfeeding: can you take Vitamin B12 (cobalamin)?

Compatible. Hale L1. B12 transfers into breast milk at physiological levels; deficiency in a vegan lactating mother causes infant deficiency with developmental delay risk. eLactancia rates the risk as minimal.

Calculate RID

Reference information, not a clinical decision. Discuss feeding pauses or changes with your physician or an IBCLC.

Drug–drug interactions

2 pairs found. Sorted from critical to minor.

ModerateMetforminBiguanides. Oral antihyperglycemic agents

Mechanism

The metformin label states it outright: long-term use reduces absorption and serum levels of vitamin B12, and it advises considering this cause in megaloblastic anaemia. The mechanism is calcium-dependent in the terminal ileum, and the effect grows with dose and years of treatment.

Symptoms

Paraesthesia in the feet, unsteady gait, loss of vibration sense. In diabetes the picture is mistaken for diabetic polyneuropathy and treated as such for years while the deficiency continues.

Management

Measure B12 at least yearly in anyone more than four years on metformin, and whenever neuropathy or anaemia appears. A low value does not force metformin withdrawal: supplement B12 orally and keep the antidiabetic. Test as well in patients who also take a proton pump inhibitor, since the two mechanisms add up.

Sources

Pair details →Metformin →

ModerateOmeprazoleProton pump inhibitors

Mechanism

Food-bound vitamin B12 comes attached to protein and needs acid and pepsin to be released. Without acidity that release fails, and NIH ODS names gastric acid inhibitors among medications that affect B12 status. Supplemental B12 arrives already free and barely depends on pH, which is why the answer is to supplement rather than stop the drug.

Symptoms

Fatigue, tingling in hands and feet, difficulty concentrating, a smooth, sore tongue. Macrocytic anaemia comes late: neurological symptoms can precede an abnormal blood count.

Management

After 2–3 years on an inhibitor, measure serum B12 and, if borderline, methylmalonic acid. High-dose oral B12 works even without gastric acid, so the intramuscular route is rarely needed for this reason. Vegetarians and people over 65 start with less reserve and deserve earlier testing.

Sources

Pair details →Omeprazole →

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Frequently asked

What is Vitamin B12 (cobalamin) used for?

Vitamin B12 (cobalamin) is evaluated for the following indications with varying evidence strength: Vegan and vegetarian diet: B12 (evidence tier A), Vitamin B12 (cobalamin) deficiency (evidence tier A), Chronic fatigue (evidence tier D). See the full indication matrix with dosing and citations above on this page.

What are the side effects of Vitamin B12 (cobalamin)?

Common side effects of Vitamin B12 (cobalamin) (≥ 1 in 100): Injection-site pain and redness, Rash and pruritus, Urine discolouration (pink tint with hydroxocobalamin), Mild nausea at high oral doses. See the Safety section for uncommon and serious reactions.

Is Vitamin B12 (cobalamin) safe during pregnancy?

Is Vitamin B12 (cobalamin) compatible with breastfeeding?

Who should not take Vitamin B12 (cobalamin)?

Vitamin B12 (cobalamin) is contraindicated in: Hypersensitivity to cobalt or to a specific cobalamin form; Leber's hereditary optic neuropathy – cyanocobalamin accelerates progression; hydroxo- or methylcobalamin are used instead; Polycythaemia vera (relative contraindication – B12 stimulates erythropoiesis). Full list in the Safety section.

Other drugs in this group

Cyanocobalamin, Folic acid, Hydroxocobalamin, Iron oral (salts: sulfate, bisglycinate, fumarate, gluconate, polymaltose; heme iron polypeptide, lactoferrin), Methylcobalamin, Levomefolate (L-methylfolate, 5-MTHF)