Vitamins
Vitamin B12
Vitamin B12 is an essential, water-soluble vitamin. Your body needs it for normal nervous-system function, red blood cell formation and DNA synthesis.
Vitamin B12 refers to a family of related cobalt-containing compounds called cobalamins. Its active coenzyme forms are methylcobalamin and adenosylcobalamin. Your body can convert other forms, such as cyanocobalamin and hydroxocobalamin, into these active forms. Certain microorganisms produce B12; you need to obtain it through your diet. It occurs mainly in animal-source foods and is also added to fortified foods and supplements. Cyanocobalamin is one specific form of B12, not an exact synonym for the whole vitamin family.
Vitamin B12: important for nerves and blood cells
Vitamin B12 is an essential, water-soluble vitamin. Your body needs it for normal nervous-system function, red blood cell formation and DNA synthesis. It also contributes to normal energy-yielding metabolism and has a role in cell division. This does not mean that extra B12 supplements improve performance when your B12 status is adequate.
The name refers to a family of cobalt-containing compounds called cobalamins. The active forms are methylcobalamin and adenosylcobalamin. Your body can convert other forms, such as cyanocobalamin and hydroxocobalamin, into these active forms.
Where to find vitamin B12
Vitamin B12 occurs naturally mainly in fish, meat, eggs and dairy products. Fortified foods and supplements also provide B12. Check the label of fortified products to see whether B12 has been added and in what amount. Ordinary unfortified plant foods are not reliable sources. If you follow a vegan diet, paying particular attention to your B12 supply is therefore important.
Absorption and stores: intake is not the whole picture
B12 bound to food proteins must first be released during digestion. The main absorption pathway uses intrinsic factor produced in the stomach and the final part of the small intestine. Inadequate B12 status can therefore result from either low intake or impaired absorption.
Your body stores B12, particularly in the liver. When intake becomes inadequate, deficiency symptoms can take several years to appear. Having no symptoms does not necessarily mean that your B12 status is adequate. Higher-risk groups include older adults, people with certain gastrointestinal disorders or surgery, and people following vegan or some vegetarian diets. Discussing assessment with a healthcare professional may be useful if you have these risks.
What extra supplements can do — and what remains uncertain
- Tiredness: B12 contributes to the reduction of tiredness and fatigue. However, evidence for a benefit from extra supplements in unexplained fatigue without overt B12 deficiency is insufficient.
- Memory and thinking: A systematic review found no demonstrated cognitive benefit from supplementation in adults without overt B12 deficiency or advanced neurological disease. This finding does not apply to treatment of confirmed deficiency.
- Cardiovascular health: Normal homocysteine metabolism does not automatically mean protection against cardiovascular disease. Results from B-vitamin combinations cannot be attributed to B12 alone; evidence for a specific B12 benefit is limited.
- Choice of B12 form: Generally better absorption of methylcobalamin than cyanocobalamin from supplements has not been established. This also does not prove that all forms are equivalent in every situation.
Safety and medicines
No tolerable upper intake level has been established for B12. Its recognised potential for toxicity is low, but this does not prove that unlimited intake is safe or useful. Use long-term high doses only for an identified need and with appropriate advice. Metformin, proton-pump inhibitors and H2-receptor antagonists can affect B12 status. Discuss testing with your clinician when appropriate, and do not stop prescribed medicines yourself.
How nutrition works
Energy metabolism
Nervous system
Homocysteine metabolism
Psychological function
Red blood cell formation
Immune system
Tiredness and fatigue
Cell division
Cognitive function
In adults without overt vitamin B12 deficiency or advanced neurological disease, a systematic review found no demonstrated cognitive benefit from B12 supplementation. [5]
Cardiovascular health
Statements & evidence
Vitamin B12 is a water-soluble vitamin. Its different forms are called cobalamins. [3]
Cobalamins are cobalt-containing compounds. Methylcobalamin and adenosylcobalamin are the biologically active forms of vitamin B12.
Vitamin B12 occurs naturally mainly in animal foods. B12-fortified foods provide another source; ordinary unfortified plant foods are not reliable sources. [3]
Natural sources include fish, meat, eggs and dairy products. For fortified foods, check the label to see whether vitamin B12 has been added.
Absorbing vitamin B12 from food requires several digestive steps. The main absorption pathway uses intrinsic factor produced in the stomach and the final part of the small intestine. [6]
B12 bound to food proteins must first be released. The amount absorbed depends on factors including its source, dose and how well this absorption pathway functions.
The body stores vitamin B12, particularly in the liver. Consequently, deficiency symptoms can take several years to appear when intake becomes inadequate. [6][3]
How long these stores last varies between individuals. Having no symptoms therefore does not necessarily mean that B12 intake or status is adequate.
EFSA sets Adequate Intakes for vitamin B12 of 4 µg per day for adults, 4.5 µg per day during pregnancy and 5 µg per day during lactation. [6]
These are dietary reference values for healthy population groups, not doses for treating deficiency. The evidence was insufficient to derive an Average Requirement, so these values do not establish a precise individual requirement.
Older adults, people with certain gastrointestinal disorders or surgery, and people following vegan or some vegetarian diets are at greater risk of inadequate vitamin B12 status. [3]
Inadequate status can result from low intake or impaired absorption. If you belong to a higher-risk group, discussing assessment of your B12 status with a healthcare professional may be useful.
A tolerable upper intake level could not be derived for vitamin B12 because suitable data on adverse effects were lacking. [6]
The absence of an established upper limit is well documented. It does not mean that unlimited intake is proven safe or useful.
Generally better absorption of methylcobalamin than cyanocobalamin from supplements has not been established. [3]
NIH ODS reports no evidence that absorption from supplements differs by B12 form. This does not establish superiority of one form, nor does it prove that all forms are equivalent in every situation.
Safety information
Upper level
No tolerable upper intake level established
No tolerable upper intake level (UL) has been established for vitamin B12, which has a low recognised potential for toxicity. This does not prove that every dose is safe for any duration. Use long-term high doses only for an identified need and with appropriate advice. [8][9][3]
Upper level
BfR recommendation for food supplements
BfR recommends a maximum of 25 µg of vitamin B12 per recommended daily dose of an individual food supplement. This is a product-formulation recommendation, not a toxicity-derived UL or a ceiling for prescribed deficiency treatment. [10]
Caution
Consider known B12 or cobalt allergy
B12 preparations can cause allergic reactions, particularly in people with known cobalt allergy. Skin reactions may be immediate or delayed. Cobalt contact allergy alone does not automatically rule out necessary treatment; a previous severe reaction requires individual clinical assessment and attention to the specific product information. Seek emergency help immediately for breathing difficulty, swelling of the mouth or throat, or collapse. [11][12][13]
Side effect
Side effects of B12 injections
Hydroxocobalamin injections for deficiency can cause injection-site pain, swelling or itching, as well as nausea, vomiting, diarrhoea, headache, dizziness or flushing. Seek medical advice for persistent or troublesome symptoms and new palpitations. These effects should not automatically be attributed to oral preparations. [14]
Side effect
Acne-like skin reactions have been reported
Acne-like eruptions have been reported, including after high-dose parenteral hydroxocobalamin. Evidence is limited: their frequency and reliable dose thresholds are unknown. Discuss new skin changes during treatment with your clinical team. [9]
Caution
Potassium monitoring in severe anaemia
During intensive treatment of severe megaloblastic anaemia with cyanocobalamin injections, potassium levels can fall dangerously. Initial treatment therefore requires clinical monitoring. This is a treatment-specific precaution, not a recognised risk from ordinary amounts of B12 in food. [13]
Interaction
Metformin can lower B12 levels
Metformin can lower B12 levels and cause deficiency, particularly with higher doses, longer treatment or additional risk factors. Testing is appropriate when deficiency is suspected, and periodic monitoring may be considered in people at risk. Do not stop metformin yourself: it can usually continue while the deficiency is corrected. [15]
Interaction
Acid-suppressing medicines can affect B12 absorption
Proton-pump inhibitors and H2-receptor antagonists can reduce the release and absorption of food-bound B12. If you take these medicines long term, discuss your B12 status with your clinician when appropriate. Do not stop prescribed medicines yourself. [3]
Interaction
Nitrous oxide inactivates vitamin B12
Nitrous oxide can inactivate B12 and cause neurological or haematological harm. With underlying B12 deficiency, even a single anaesthetic exposure can be problematic; a normal measured B12 concentration does not reliably exclude this effect. Tell the anaesthetic team if you have known deficiency or relevant risk factors. [16]
Caution
Cyanocobalamin in Leber hereditary optic neuropathy
If you have Leber hereditary optic neuropathy, seek specialist advice on the choice of B12 preparation. US cyanocobalamin injection labelling warns of severe, rapid optic atrophy in early Leber disease. This warning should not be generalised to every form of B12. [13]
Caution
Choose injection formulations carefully in vulnerable patients
For premature infants and people with impaired kidney function, the specific injection formulation needs checking. Some cyanocobalamin injections contain benzyl alcohol with warnings for premature infants; certain labels also warn of aluminium accumulation during prolonged parenteral use with impaired kidney function. These are product-specific risks, not evidence that ordinary oral B12 amounts damage the kidneys. [13]
Caution
Deficiency treatment during pregnancy and breastfeeding
Hydroxocobalamin can be used for B12 deficiency during pregnancy and breastfeeding. Have the dose and treatment plan determined clinically; this does not endorse unnecessary megadoses. If a breastfed infant is also deficient, maternal treatment does not replace the infant's own treatment. [17]
Caution
Intravenous cyanide antidote is a separate context
Intravenous hydroxocobalamin for cyanide poisoning (Cyanokit) is administered in gram doses in emergency medical care. Documented risks include hypersensitivity, temporary increases in blood pressure, acute kidney injury, and interference with laboratory tests and dialysis equipment. These risks should not be transferred to ordinary B12 supplements or routine deficiency treatment. [18]
Frequently asked questions
Which foods contain vitamin B12?
Vitamin B12 occurs naturally mainly in fish, meat, eggs and dairy products. B12-fortified foods are another source; check the label to see whether B12 has been added.
How much vitamin B12 do I need each day?
EFSA sets an Adequate Intake of 4 µg daily for adults, 4.5 µg during pregnancy and 5 µg during breastfeeding. These are dietary reference values for healthy population groups, not supplement doses or doses for treating deficiency.
What should I know about vitamin B12 if I follow a vegan diet?
Ordinary unfortified plant foods are not reliable B12 sources, and a vegan diet increases the risk of inadequate B12 status. Fortified foods and supplements provide B12. Discussing assessment of your B12 status with a healthcare professional may be useful.
Do vitamin B12 supplements help with tiredness?
Vitamin B12 contributes to the reduction of tiredness and fatigue, but this does not mean that extra supplements relieve all tiredness. Evidence for a benefit in unexplained fatigue without overt B12 deficiency is insufficient.
Is methylcobalamin better than cyanocobalamin?
Generally better absorption of methylcobalamin than cyanocobalamin from supplements has not been established. Your body can convert cyanocobalamin into active B12 forms, but this does not prove that all forms are equivalent in every situation.
More entries
Folate
Folate is a water-soluble B vitamin. Its active forms take part in DNA synthesis and amino-acid metabolism.
Vitamin A
Vitamin A is an essential, fat-soluble vitamin. Your diet supplies it as preformed vitamin A or as precursors from plant foods.
Vitamin C
Vitamin C is an essential, water-soluble vitamin that you need to obtain through your diet. Its principal chemical form is L-ascorbic acid.
Vitamin D
Vitamin D is a fat-soluble vitamin. The term mainly covers vitamins D₂ and D₃; your body can produce D₃ in the skin through exposure to UVB radiation.
Vitamin K
Vitamin K is an essential, fat-soluble vitamin. Your body needs it to activate proteins involved in blood clotting and bone metabolism.
Vitamin B6
Vitamin B6 is a water-soluble vitamin. Its active forms support many metabolic processes, particularly amino-acid metabolism.
Sources
- 01European Commission · 2012Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods, other than those referring to the reduction of disease risk and to children's development and healthOpen source
- 02EFSA Journal · 2009Scientific Opinion on the substantiation of health claims related to vitamin B12 and red blood cell formation (ID 92, 101), cell division (ID 93), energy-yielding metabolism (ID 99, 190) and function of the immune system (ID 107) pursuant to Article 13(1) of Regulation (EC) No 1924/2006DOI 10.2903/j.efsa.2009.1223
- 03National Institutes of Health, Office of Dietary SupplementsVitamin B12 - Health Professional Fact SheetOpen source
- 04Cochrane · 2017Homocysteine-lowering interventions (B-complex vitamin therapy) for preventing cardiovascular eventsOpen source
- 05Nutrients · 2021Effects of Vitamin B12 Supplementation on Cognitive Function, Depressive Symptoms, and Fatigue: A Systematic Review, Meta-Analysis, and Meta-RegressionOpen source
- 06EFSA Journal · 2015Scientific Opinion on Dietary Reference Values for cobalamin (vitamin B12)EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA)DOI 10.2903/j.efsa.2015.4150
- 07European Parliament and Council of the European Union · 2011Regulation (EU) No 1169/2011 on the provision of food information to consumersOpen source
- 08European Food Safety AuthoritySummary tables of tolerable upper intake levels for vitamins and minerals, version 8Open source
- 09Scientific Committee on Food; EFSA Scientific Panel on Dietetic Products, Nutrition and Allergies · 2006Tolerable Upper Intake Levels for Vitamins and MineralsOpen source
- 10German Federal Institute for Risk AssessmentMaximum levels for the addition of vitamin B12 to foods including food supplementsOpen source
- 11Medicines and Healthcare products Regulatory AgencyVitamin B12 (hydroxocobalamin, cyanocobalamin): advise patients with known cobalt allergy to be vigilant for sensitivity reactionsOpen source
- 12NHSSide effects of cyanocobalaminOpen source
- 13US National Library of Medicine, DailyMedDailyMed - CYANOCOBALAMIN injection, solutionOpen source
- 14NHSSide effects of hydroxocobalaminOpen source
- 15Medicines and Healthcare products Regulatory AgencyMetformin and reduced vitamin B12 levels: new advice for monitoring patients at riskOpen source
- 16Medicines and Healthcare products Regulatory AgencyNitrous oxide: neurological and haematological toxic effectsOpen source
- 17NHSPregnancy, breastfeeding and fertility while having hydroxocobalaminOpen source
- 18European Medicines AgencyCyanokit, INN-hydroxocobalamin: product informationOpen source
- 19European Food Safety Authority · 2017Dietary Reference Values for nutrients: Summary reportOpen source
- 202024Elevated Vitamin B12, Risk of Cancer, and Mortality: A Systematic ReviewPubMed 38953509Open source
- 21Cochrane · 2018Oral vitamin B12 compared with intramuscular vitamin B12 for vitamin B12 deficiencyOpen source
- 22Bundesinstitut für RisikobewertungVitamin B12 – bei pflanzenbasierter Ernährung besonders auf eine angemessene Versorgung achtenOpen source