Vitamin B12
What it is
Vitamin B12 (cobalamin) is a water-soluble vitamin essential for neurological function, red blood cell formation, and DNA synthesis. It is found almost exclusively in animal-derived foods, including meat, fish, shellfish, eggs, and dairy. Plant foods do not contain biologically active B12 in nutritionally meaningful quantities, with the exception of some fortified foods. This makes B12 the nutrient of greatest concern for people following vegan diets.
B12 absorption follows a two-stage process. In the stomach, hydrochloric acid and digestive enzymes release B12 from food proteins. It then binds to intrinsic factor, a glycoprotein produced by parietal cells in the stomach lining. The resulting complex is absorbed in the terminal ileum. Any condition that impairs stomach acid production, intrinsic factor secretion, or ileal function can lead to deficiency regardless of dietary intake.
Supplemental B12 is available in several forms. Cyanocobalamin is the most extensively studied and most widely used in supplements and food fortification. Methylcobalamin and adenosylcobalamin are the two active coenzyme forms found in human tissues. Hydroxocobalamin is used clinically for intramuscular injection.
What the evidence shows
Treatment of confirmed deficiency with oral supplementation is where the evidence is clearest. A critical and clinically underappreciated finding is that high-dose oral B12 can be effective even when intrinsic factor-mediated absorption is impaired. Approximately one per cent of any oral dose is absorbed by passive diffusion across the gut wall, independently of intrinsic factor. At doses of 1,000 to 2,000 mcg daily, this passive mechanism delivers sufficient B12 to correct and maintain status in most individuals. Multiple trials in patients with pernicious anaemia and other absorption disorders have demonstrated this, and it is reflected in current clinical guidance. Intramuscular therapy remains appropriate in severe deficiency with neurological involvement, where adherence is uncertain, or where faster supervised repletion is clinically necessary.
Prevention of deficiency in at-risk populations has strong evidence. In vegans and vegetarians who supplement consistently, deficiency can be reliably prevented. The evidence base here draws on a combination of intervention studies and well-characterised mechanistic understanding of the absorption pathway.
Homocysteine reduction in deficient individuals is a consistent biochemical effect of B12 supplementation and is well-documented across multiple trials. However, homocysteine is best understood as a mechanistically relevant biomarker rather than a validated clinical surrogate. Large well-powered trials including HOPE-2 and VITATOPS demonstrated that B-vitamin supplementation that successfully lowered homocysteine did not reduce major cardiovascular events, which means that lowering homocysteine cannot be assumed to translate to clinical benefit. The biochemical effect is reliable; its clinical significance for patient-important outcomes is not established.
Neurological and cognitive improvement in early deficiency has moderate evidence. Supplementation in individuals with confirmed deficiency and early neurological manifestations may improve neurological and cognitive function, particularly when treatment is initiated early. Recovery is often partial and its extent depends on the duration and severity of deficiency, and the age of the patient. Irreversible neurological damage can occur with prolonged untreated deficiency, and the window for full recovery narrows with time. This makes early identification and treatment clinically important.
Fatigue in confirmed deficiency with haematologic involvement has moderate evidence. When fatigue is a manifestation of B12-related megaloblastic anaemia, supplementation and haematologic correction may improve symptoms, with the effect running primarily through correction of anaemia. The evidence here is largely indirect, running through correction of anaemia rather than fatigue-specific trials. Fatigue is nonspecific and response to B12 supplementation should not be assumed without biochemical confirmation of deficiency.
Cognitive function or energy in replete adults has insufficient evidence. In individuals with normal B12 status, supplementation does not produce consistent benefits on cognitive performance or energy levels. The effects attributed to B12 supplementation in popular media are almost entirely deficiency-correction effects, not pharmacological effects beyond normal physiology.
What the evidence does not show
B12 supplementation does not improve cognitive performance, energy, or mood in individuals without deficiency. The marketing of B12 as an energy supplement is not supported by evidence in replete individuals.
Homocysteine lowering through B12 supplementation does not demonstrably reduce cardiovascular events. Large controlled trials have consistently found no significant cardiovascular benefit despite effective homocysteine reduction. Homocysteine is likely a marker of metabolic dysfunction rather than a causal factor in cardiovascular disease.
High-dose oral B12 does not replace intramuscular therapy in all clinical situations. Severe deficiency with marked neurological involvement, significant adherence concerns, or complex malabsorption may still warrant intramuscular treatment, and this is a clinical judgement rather than a rule.
Form and dose considerations
Cyanocobalamin is the form used in the majority of clinical trials and the form most commonly found in supplements and fortified foods. It is converted to the active coenzyme forms in the body. It is stable, inexpensive, and well-evidenced for deficiency correction and prevention. In standard supplemental doses, the cyano group released during metabolism does not represent a toxicological concern in healthy individuals.
Methylcobalamin is one of the two active coenzyme forms and does not require enzymatic conversion. It is the predominant form in human blood and cerebrospinal fluid. Some practitioners prefer it on the basis that it provides the active form directly. However, direct head-to-head clinical trials comparing methylcobalamin to cyanocobalamin for patient-important outcomes are limited, and current evidence does not consistently demonstrate clinical superiority of one form over the other for most people. Methylcobalamin is a reasonable choice; the case for its routine superiority over cyanocobalamin is not established by trial evidence. Observed pharmacokinetic differences between forms, including differences in tissue distribution, have not been shown to produce consistent differences in patient-important clinical outcomes.
Hydroxocobalamin has a longer half-life and is the preferred form for intramuscular injection in clinical settings. Oral hydroxocobalamin is available but less commonly used.
For deficiency correction, doses of 1,000 to 2,000 mcg daily are typically used in oral supplementation protocols. For deficiency prevention in vegans, lower daily doses in the range of 25 to 100 mcg or higher weekly doses are commonly recommended, though optimal prevention doses remain an area of discussion. Dose requirements vary by individual absorption capacity, particularly in older adults where gastric acid production declines with age.
Who the evidence applies to
Vegans have essentially no dietary B12 intake and require consistent supplementation or regular consumption of B12-fortified foods. Deficiency can develop silently over years because hepatic stores are substantial and serum levels may remain normal for some time after dietary intake ceases.
Older adults are at elevated risk due to age-related decline in gastric acid production, which impairs the release of B12 from food proteins. Critically, this affects food-bound B12 but not free supplemental B12, which is why supplementation can correct deficiency in older adults even when their dietary intake appears adequate.
Metformin users face increased risk of B12 depletion through a mechanism involving impaired calcium-dependent ileal absorption of the B12-intrinsic factor complex. Proton pump inhibitors and H2 receptor antagonists reduce gastric acid production and can impair the release of B12 from food proteins with long-term use, typically over one to two years or more. Short-term use is unlikely to produce clinically significant depletion. De Jager et al. (2010) demonstrated a dose-dependent reduction in serum B12 with long-term metformin use. Regular monitoring of B12 status is clinically warranted in this group.
Individuals with pernicious anaemia, atrophic gastritis, or prior gastric surgery have structural impairment of the absorption pathway and require supplementation regardless of dietary intake.
Generally healthy omnivores with adequate dietary intake and intact gastrointestinal function are at low risk of deficiency and the evidence does not support routine supplementation in this group.
Testing and diagnosis
Serum B12 is the most accessible initial test for B12 status, but it has recognised diagnostic limitations, particularly in the borderline range. Serum B12 does not directly measure tissue sufficiency and can be normal even when functional deficiency is present. Where serum B12 is borderline or equivocal, or where clinical suspicion is high, additional markers including methylmalonic acid, holotranscobalamin, plasma homocysteine, and haematologic indices including mean corpuscular volume should be considered depending on clinical context. Diagnosis is a clinical judgement integrating multiple markers rather than a single test result.
A clinically important and underappreciated category is subclinical or functional deficiency, where serum B12 falls within the normal range but elevated methylmalonic acid or homocysteine indicates impaired intracellular B12 function. This is particularly common in older adults and may be associated with early neurological or cognitive symptoms before anaemia develops. Standard serum B12 testing alone may miss this category.
This matters for the individualisation argument: an individual's true B12 status cannot always be reliably determined from serum B12 alone, and the threshold for further investigation should be informed by their risk profile including diet, age, medications, and gastrointestinal history.
Safety and contraindications
Vitamin B12 has a very strong safety profile. No upper tolerable intake level has been established because excess is renally excreted. High-dose supplementation is generally well tolerated.
An important interaction: high-dose folic acid can correct the haematologic manifestations of B12 deficiency, specifically megaloblastic anaemia, while neurological damage continues to progress undetected. This is clinically significant, particularly in older adults who may be taking folate-containing supplements or eating folate-fortified foods. B12 status should be assessed before initiating high-dose folate supplementation in populations at risk of B12 deficiency.
Persistently elevated serum B12 should prompt clinical evaluation for an underlying cause rather than being attributed to supplement use, as it can be associated with certain haematological and hepatic conditions.
What can reasonably be concluded
Vitamin B12 is a nutrient where individual assessment matters considerably more than population-level recommendations. The same dietary and supplementation advice that is appropriate for a vegan or an older adult on metformin is unnecessary for a healthy omnivore with adequate status.
For confirmed deficiency, high-dose oral supplementation is effective and well-evidenced for most individuals, including many with impaired intrinsic factor-mediated absorption. Intramuscular therapy remains the appropriate choice in severe or rapidly progressive deficiency and in situations where oral compliance is uncertain.
The widespread marketing of B12 as an energy or performance supplement is not supported by evidence in individuals without deficiency. Where such effects occur following supplementation, they reflect correction of an unrecognised deficiency rather than a pharmacological effect beyond normal physiology. Where evidence is limited or outcomes are uncertain, conclusions in this entry should be treated as provisional and subject to revision as the evidence base develops.