Vitamin B12

Cobalamin · Vitamin B12 supplementation · Cyanocobalamin · Methylcobalamin

Snapshot

Indications	Contraindications
Vitamin B12 deficiency¹ Pernicious anemia² Post-gastric bypass status³ Metformin-associated deficiency⁴ Homocysteine lowering⁵	Coronary stenting² Cobalamin hypersensitivity⁶ Soy-containing products⁷

Vitamin B12 (cobalamin) is a water-soluble, cobalt-containing vitamin that acts as a cofactor in homocysteine and fatty acid metabolism, supporting red blood cell formation and nervous system function¹. People take it to correct or prevent low B12 status, which drives its main uses: deficiency anemia and neurologic symptoms, pernicious anemia, malabsorption after gastric bypass, and deficiency linked to metformin or acid-suppressing medicines^{8, 9}. Typical patients are older adults, vegans and vegetarians, and those with gastrointestinal disease.

In deficiency, a 2025 clinical review describes oral supplementation as noninferior to intramuscular dosing, with injection reserved for severe or neurologic cases⁸. In people without deficiency, a meta-analysis found no effect on cognition or depressive symptoms¹⁰. A 2026 Cochrane review in children found supplementation reduced deficiency with little effect on growth or cognition¹¹. Cyanocobalamin is the most common supplement form, alongside methylcobalamin, hydroxocobalamin, and adenosylcobalamin; see Form comparison¹.

High folic acid intake can mask B12 deficiency, so deficiency status is worth establishing first². Per the Memorial Sloan Kettering fact sheet, B12 may raise in-stent restenosis risk after coronary stenting².

Last updated October 16, 2026. Read how we make monographs.

In the full monograph

81 sources, 66 studies

When to useThe situations practitioners use it for, each with the reason and the study behind it.
SafetyWho should avoid it, who needs extra care, and the side effects reported in studies.
InteractionsThe medicines and supplements it interacts with, the effect of each, and what labeling says to watch.
Protocol notesThe doses used in studies, how and when they were taken, and how the forms on the market compare.
How it worksWhat it is and how it works in the body, in plain terms.
Published studiesThe studies behind each use, popular claims checked against them, and trials still under way.
Word of mouthWhat patients and practitioners report online, kept separate from the clinical evidence.
Clinical perspectiveA closing read of where the evidence is solid, where it is thin, and what has not been studied.

Snapshot

In the full monograph

Sources