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Vitamin B12

Methylcobalamin / cyanocobalamin / B12 shots · Foundation

StrongFor the population and outcome described, multiple large human trials broadly agree.

Essential, cheap, and dramatic to fix when deficient, placebo-tier energy shots when you are not.

Years
How long liver stores last before deficiency appears, which is why it is caught late

What it is

A vitamin your body cannot make and only animal foods naturally supply (fortified foods also provide it reliably), needed for red blood cells, nerve insulation and DNA synthesis. Deficiency is sneaky: it builds over years and the neurological damage can outlast the fix, which is why the at-risk groups matter more here than the supplement does.

How it works

B12 absorption is a two-key system, stomach acid frees it from food, then a protein called intrinsic factor carries it across the gut wall. Anything that breaks either key (age, metformin, acid blockers, gut surgery) causes deficiency regardless of diet. High-dose oral B12 partly bypasses this via slow passive diffusion, which is why big oral doses work even for absorption problems.

Which form to buy

CyanocobalaminCheap, proven repletion

The most-studied and most stable form, used in the majority of trials and fortified foods. The trace cyanide is negligible at supplement doses.

Perfectly good and the cheapest, the default.

MethylcobalaminThe premium shelf

An active coenzyme form, retained somewhat better; the claim that it is meaningfully superior is not well established in outcome trials.

Fine, but the price premium buys theory more than proof.

HydroxocobalaminInjections

The form typically used clinically for injections, with longer retention.

Relevant if a clinician is treating a diagnosed deficiency.

Sublingual tablets & spraysPreference

Studied against oral tablets and found broadly equivalent, the under-the-tongue route is not the advantage it is sold as.

Use if you prefer them; do not pay extra expecting more.

B12 injectionsDiagnosed deficiency / absorption failure

The route for pernicious anemia and post-surgical malabsorption. Also widely sold at wellness clinics to people with normal levels.

Medical when indicated; theater when it is not.

What it actually does, graded

Corrects deficiency and its anemiaStrong

The core use, entirely uncontroversial, blood counts normalize and neurological progression stops.

Prevents deficiency in at-risk groupsStrong

Supplementation is standard advice for vegan diets and after certain surgeries. For metformin and acid-blocker users, guidance emphasises TESTING and monitoring rather than automatic supplementation.

Energy when you are NOT deficientNone shown

The biggest myth in the category: there is no good evidence of an energy benefit in people who are not deficient. (The honest caveat: a 2021 review found so few qualifying fatigue trials that the question is under-studied rather than exhaustively disproven.)

Cognitive protection in replete adultsNone shown

Large trials have not shown benefit where B12 status was already normal.

Pros and cons

Pros
  • Cheap, safe, and water-soluble with no established upper limit
  • Deficiency is genuinely common in identifiable groups, so it has real people to help
  • Status is measurable with a blood test rather than guesswork
Cons
  • No demonstrated energy, endurance or cognitive benefit if you are not low, and most buyers are not
  • Deficiency can be masked by folate, letting nerve damage progress quietly
  • The "B12 shot for energy" industry sells to people with normal levels

What to expect

  • If deficient: energy often improves within weeks. Nerve symptoms may begin improving after roughly two weeks but can take months.
  • Months 1-3: blood counts normalize on a repeat test.
  • Nerve symptoms may remain incomplete if deficiency ran for years, the argument for testing early rather than guessing.
  • If you were not deficient: nothing. Expect nothing, and you will not be disappointed.

Interactions

MonitorMetforminLong-term use meaningfully lowers B12, periodic testing is the standard recommendation, not an exotic precaution.
MonitorPPIs and H2 blockersLess stomach acid means less B12 freed from food; years of use is the risk window.
CautionFolate / folic acid supplementsHigh folate can normalize the blood picture while B12 nerve damage continues underneath, a reason not to self-treat vague fatigue with a B-complex.
MonitorGLP-1 medicationNot a drug interaction, simply that much lower food intake can mean much lower B12 intake over time.

Is it for you?

Probably worth it if
  • Vegan or near-vegan eating
  • Long-term metformin or acid-blocker use
  • Over 50, or any condition affecting stomach or gut absorption
  • On a GLP-1 with sharply reduced food intake
Probably skip it if
  • You eat animal foods or fortified foods regularly and have no absorption risk factor, though testing settles it better than assuming
  • You are buying it for energy with normal levels, that effect does not exist

Who should avoid it

  • Anyone with a known cobalt allergy, sensitivity reactions are a documented caution
  • Anyone with early Leber hereditary optic neuropathy, cyanocobalamin labelling warns against use
  • And generally: do not use B12 to self-treat unexplained fatigue instead of getting tested

Research on this supplement

Papers about this supplement, not proof of the claims above. A title says what was studied, which is sometimes a negative result or a different question entirely.

  • Efficacy of different routes of vitamin B12 supplementation for the treatment of patients with vitamin B12 deficiency: A systematic review and network meta-analysis
    Ir J Med Sci, 2024
  • A systematic review and meta-analysis of functional vitamin B12 status among adult vegans
    Nutr Bull, 2024
  • Elevated Vitamin B12, Risk of Cancer, and Mortality: A Systematic Review
    Cancer Invest, 2024

What the evidence says

Deficiency correction is textbook medicine with unambiguous benefit, and deficiency is genuinely common in vegans, long-term metformin users, and older adults (absorption, not intake, is usually the problem). In replete people, extra B12, including the injection ritual, has repeatedly failed to beat placebo for energy.

What studies used

Supplement studies commonly use 500-1,000 mcg/day oral; deficiency treatment is a clinical decision. Published studies had not established a timing dependence.

Reported from published studies and product labelling — a record of what was used in research, not a recommendation. What is right for you is a conversation with a clinician.

Watch out for

  • Long-term metformin use is associated with B12 deficiency; markedly reduced food intake from any cause may warrant individualized assessment
  • High-dose oral treats most people effectively (passive diffusion absorbs a small fraction even without intrinsic factor) though injections remain the choice for severe neurologic disease or significant malabsorption

Evidence base

S21,984 research papers
established literature

Large human evidence base: several phase 3 or 4 trial reports, plus multiple meta-analyses or systematic reviews.

This letter measures how much human research exists, not whether it works for you — the tier above (Strong) is the read on quality.

Research (12)

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Where next

Every supplement in the catalogThe full shelf, grouped and evidence-tiered.→The compound indexSteroids, nootropics and research chemicals.→The peptide indexInjectable peptides and what trials used.→