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evidence · mechanism · dose
Supplements/Vitamin

Vitamin B12

aka Methylcobalamin · Cyanocobalamin

Essential cobalamin for methylation, red blood cell formation, and myelin maintenance.

Energy ProductionBrain & CognitiveGeneral Wellness

Overview

Water-soluble cobalt-containing vitamin found almost exclusively in animal products; deficiency is common in vegans and older adults.

Natural sources
  • Beef liver
  • Clams
  • Sardines
  • Eggs
  • Fortified plant milks
Common forms
  • Methylcobalamin (active)
  • Cyanocobalamin (stable)
  • Hydroxocobalamin (injectable)
  • Adenosylcobalamin

Forms & variants of Vitamin B12

Different chemical forms of Vitamin B12 behave differently. Browse each form's mechanism, bioavailability, and best-use context.

Methylcobalamin
Methylcobalamin (B12)

Active methyl-form of B12 — directly usable in the methylation cycle.

Adenosylcobalamin
Adenosylcobalamin (B12)

Mitochondrial coenzyme form of B12 — partners with methylcobalamin for full B12 status.

Hydroxocobalamin
Hydroxocobalamin (B12)

Long-acting injectable form of B12 — used clinically for deficiency and cyanide poisoning.

Mechanism of action

Cofactor for methionine synthase (methylation cycle) and methylmalonyl-CoA mutase (fatty acid metabolism).

Pathways
  • · Methylation (homocysteine → methionine)
  • · Myelin synthesis
Enzymes
  • · Methionine synthase
  • · Methylmalonyl-CoA mutase

Effects on the body

Decreases
Homocysteine
Supports
Red blood cell formation
Supports
Cognitive function (if deficient)

Organ system effects

Brain & CNS

Required for myelin; deficiency causes peripheral neuropathy.

Evidence-based benefits

Resolves megaloblastic anemia

Strong evidence

Reverses neuropathy in deficient patients

Strong evidence

Improves fatigue (only if deficient)

Moderate evidence

Potential side effects

Common
    Rare
    • · Acne (rare)
    Serious

      Drug interactions

      Metformin
      Long-term metformin lowers B12 — monitor levels.
      moderate
      PPIs (long-term)
      Reduce absorption of dietary B12.
      moderate

      Supplement interactions

      Pairs well with
      • Folate
        Methylation cycle partners.
      Avoid combining with

      No specific avoidances listed.

      Dosing & bioavailability

      Typical dose
      500–1000 mcg oral
      Upper safe limit
      None established
      Best form
      Methyl or hydroxy
      Timing
      Morning

      Where to buy

      Affiliate-style search links to reputable retailers. We don't endorse specific brands — verify third-party testing (USP, NSF, Informed Sport) before purchase.

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      Scientific evidence (10)

      Systematic reviewNutr Rev · 2013

      B12 deficiency prevalence in vegetarians

      Pawlak R, et al.

      Review of B12 status across diet patterns.

      Key findings

      Up to 86% of vegans showed deficiency without supplementation.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — Resolves megaloblastic anemia

      Live PubMed search

      Live results for "Vitamin B12 Resolves megaloblastic anemia" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "Resolves megaloblastic anemia". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — Reverses neuropathy in deficient patients

      Live PubMed search

      Live results for "Vitamin B12 Reverses neuropathy in deficient patients" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "Reverses neuropathy in deficient patients". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — Improves fatigue

      Live PubMed search

      Live results for "Vitamin B12 Improves fatigue" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "Improves fatigue". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — randomized controlled trial

      Live PubMed search

      Live results for "Vitamin B12 randomized controlled trial" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "randomized controlled trial". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — meta-analysis

      Live PubMed search

      Live results for "Vitamin B12 meta-analysis" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "meta-analysis". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — pharmacokinetics

      Live PubMed search

      Live results for "Vitamin B12 pharmacokinetics" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "pharmacokinetics". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — safety adverse events

      Live PubMed search

      Live results for "Vitamin B12 safety adverse events" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "safety adverse events". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewPubMed (NLM) · 1970

      PubMed: Vitamin B12 — dose response

      Live PubMed search

      Live results for "Vitamin B12 dose response" across the PubMed/MEDLINE peer-reviewed index.

      Key findings

      Opens current human and preclinical literature matching Vitamin B12 and "dose response". Filter by RCT, meta-analysis, or year on the results page.

      Open study
      Systematic reviewGoogle Scholar · 1970

      Google Scholar: Vitamin B12 — clinical evidence

      Live Scholar search

      Cross-database citation index for Vitamin B12 as it relates to "clinical evidence".

      Key findings

      Includes preprints, theses, and journal articles. Useful for tracking citation counts and follow-on work.

      Open study
      Educational use only. This profile is not medical advice. Consult a qualified healthcare professional before starting any supplement, especially if you take prescription medication or have a chronic condition.