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Research profile · Small molecule

Vitamin B12

Also known as Cyanocobalamin, Hydroxocobalamin, Cobalamin

Clinical — evidence level 4 of 4: Established clinical evidencePrescription only — UK regulatory status: Prescription-only medicine (UK)

Overview

A cobalt-containing vitamin essential to methylation and DNA synthesis. The injectable form is a prescription-only medicine in the UK.

Category
Small molecule
Highest evidence level identified
Established clinical evidence
Last scientifically reviewed
22 August 2026
Last regulatory review
22 August 2026

Mechanism under investigation

Vitamin B12 is a corrinoid — a cobalt atom held in a corrin ring — required by exactly two human enzymes: methionine synthase, which remethylates homocysteine to methionine and sustains the folate cycle, and methylmalonyl-CoA mutase in mitochondria. Deficiency therefore produces megaloblastic anaemia through impaired DNA synthesis and, independently, neurological damage through the methylation pathway — the reason neurological features can occur without anaemia. Absorption is the vulnerable step: gastric intrinsic factor and a specific ileal receptor are required, which is why pernicious anaemia and gastric or ileal disease produce deficiency regardless of diet, and why the injectable route exists.

Research areas

  • Nutritional biochemistry
  • Haematology

These are fields in which the compound has been investigated. Listing a field is not a statement that anything was demonstrated within it.

Current evidence

In-vitro evidence

Cells, tissues or biochemical systems outside a living organism.

The cofactor chemistry is settled biochemistry, characterised down to crystal structures of both dependent enzymes.

Animal evidence

Studies in living non-human animals.

Classical deficiency physiology across species underpins the human picture; the vitamin’s discovery era rests on it.

Human observational evidence

Studies observing people without assigning an intervention.

Very large: deficiency epidemiology, the neurological and haematological syndromes, and population work on marginal status in older adults, vegans and after gastric surgery.

Clinical-trial evidence

Studies assigning an intervention to human participants.

Replacement in deficiency is among the oldest established treatments in medicine. The modern trial question is route: the Cochrane comparison found oral high-dose B12 achieved outcomes comparable to intramuscular in the populations studied, with the evidence graded low-to-moderate certainty — relevant because it undercuts the assumption that injection is inherently superior for most people.

Study limitations

  • The decisive evidence is for treating deficiency; trials of B12 in replete people — for energy, mood or performance — are consistently negative or absent.
  • Serum B12 is an imperfect marker, so parts of the observational literature blur who was actually deficient.
  • The oral-versus-intramuscular Cochrane evidence, while directionally consistent, is limited in size and certainty.
  • Injection-clinic marketing runs far ahead of any controlled evidence of benefit in people without a deficiency or an absorption problem.

UK regulatory context

Prescription-only medicine (UK)

A medicine that may lawfully be supplied in the UK only against a prescription from an appropriate practitioner.

Source: Hydroxocobalamin 1mg/ml Solution for Injection — Summary of Product Characteristics (opens medicines.org.uk in a new tab) (electronic Medicines Compendium)

Regulatory classification is jurisdiction-specific. A compound authorised elsewhere is not thereby authorised in the UK, and a supplier’s description of a substance does not determine how it is classified in law. See safety and regulation for the framework and the authoritative sources.

Safety and interpretation

The classification follows the route, not the molecule: oral B12 is a lawful food supplement and pharmacy product, while every UK injectable B12 is a prescription-only medicine, licensed for deficiency states such as pernicious anaemia. B12 itself has low toxicity — excess is excreted — and hydroxocobalamin at massive doses is even an antidote in cyanide poisoning. The safety questions around non-medical B12 injections are therefore about the act, not the vitamin: unlicensed injection outside the medicines framework, sterility, and the substitution of injections for the diagnosis a deficiency actually needs, since B12 deficiency has causes that matter to find.

References