Vitamin B12

Pronunciation: VYE-ta-min B-twelve.

Generic name
cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin)
Drug class
Water-soluble vitamin (B-complex); dietary supplement and, in injectable prescription form, a haematinic/neurotrophic agent
Form
Oral: typically 500-1000 mcg tablets/capsules/sublingual. Injectable: 100 mcg/mL or 1000 mcg/mL (cyanocobalamin, US) or 1 mg/mL (hydroxocobalamin, UK/EU/Australia/New Zealand)
Route
Oral (tablet, capsule, sublingual, gummy) for supplement use; intramuscular or subcutaneous injection for prescription treatment of diagnosed deficiency
Legal status
Over-the-counter (oral dietary/food supplement forms) OR Prescription-only (injectable cyanocobalamin/hydroxocobalamin for diagnosed deficiency) -- the legal status genuinely depends on the formulation and intended use, not on the country alone

Availability and supply rules vary by country

Trade names by country: United States · United Kingdom · Canada · Australia · New Zealand

Australia: Vitamin B12 oral supplements (AUST L listed, various brands) (6), HYDROXO-B12 / VITA-B12 / COBAL-B12 / NEO-B12 injection (generic hydroxocobalamin) (6) · Full directory

What cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin) is and how it works

Vitamin B12 is an essential water-soluble vitamin needed for healthy red blood cell formation, nerve function, and DNA synthesis. It is available in two genuinely different ways: as an over-the-counter oral dietary supplement that anyone can buy without a prescription for general nutritional support, and as a prescription-only injectable medicine used specifically to treat a diagnosed vitamin B12 deficiency or pernicious anaemia under medical supervision. (1) (2)

If you are taking an oral vitamin B12 supplement for general health or a known mild dietary gap (for example, if you follow a vegan diet), it is available without a prescription and does not need routine blood monitoring for most healthy adults. If your clinician has diagnosed you with vitamin B12 deficiency or pernicious anaemia, you may instead be prescribed injectable vitamin B12 (cyanocobalamin or hydroxocobalamin), which works faster and bypasses any absorption problem in your gut -- this requires a diagnosis, a prescription, and ongoing follow-up, and in true pernicious anaemia is usually needed for life.

Mechanism of action: how cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin) works

Vitamin B12 (cobalamin) is a water-soluble vitamin and essential cofactor for methionine synthase and L-methylmalonyl-CoA mutase, required for DNA synthesis, red blood cell maturation, and myelin maintenance in the nervous system. Absorption from food requires intrinsic factor, a glycoprotein secreted by gastric parietal cells; injectable administration bypasses this absorption step entirely. (1) (2)

What cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin) is used for

General nutritional support (OTC oral supplement)

Oral vitamin B12 supplements are used to support general nutritional intake, particularly for people with lower dietary intake (such as those following a vegan or strict vegetarian diet) or mild, non-severe deficiency risk. This use does not require a prescription or a specific diagnosis. (1)

Pernicious anaemia and diagnosed B12 deficiency (prescription injectable)

Injectable cyanocobalamin or hydroxocobalamin is used to treat diagnosed vitamin B12 deficiency, most importantly pernicious anaemia (deficiency caused by lack of intrinsic factor, a stomach protein needed to absorb B12 from food), and deficiency from other malabsorption causes such as gastrointestinal surgery (e.g. gastrectomy), certain bowel diseases, or fish tapeworm infestation. (2) (3)

Tobacco amblyopia and Leber's optic atrophy (UK-specific use, hydroxocobalamin only)

In the UK, hydroxocobalamin injection is also indicated for tobacco amblyopia (a specific type of vision loss) -- note this is the opposite of cyanocobalamin, which must not be used in Leber's hereditary optic atrophy because it can worsen it. (3)

Before taking cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin)

Do not take / not appropriate for

  • Do not use injectable cyanocobalamin or hydroxocobalamin if you have had an allergic reaction to vitamin B12, cobalt, or any ingredient in the product. (all countries) (2) (3)
  • Cyanocobalamin should not be used in early Leber's hereditary optic (eye nerve) disease -- it has caused severe, swift optic atrophy in this specific rare condition. (all countries) (2)

Tell your clinician about

  • Whether you have been diagnosed with pernicious anaemia or another cause of vitamin B12 deficiency, versus taking B12 as a general supplement.
  • Any history of allergic reaction to vitamin B12 or cobalt.
  • Any personal or family history of Leber's hereditary optic atrophy.
  • Whether you take metformin, a proton pump inhibitor (PPI), or an H2-receptor blocker long-term, since these can reduce your body's vitamin B12 status over time.
  • Whether you are or could be pregnant, or are breastfeeding.
  • Any other supplements or medicines you take, including folic acid.
(2) (3) (1)

How to take cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin)

  • Oral vitamin B12 supplements are taken by mouth as directed on the product label, typically once daily, with or without food.
  • Injectable vitamin B12 (cyanocobalamin or hydroxocobalamin) is given by a healthcare professional as an intramuscular or subcutaneous injection, following the specific schedule your prescriber sets for your diagnosis.
  • Do not switch yourself from a prescribed injectable regimen to an oral supplement, or stop injections, without talking to your prescriber -- in true pernicious anaemia, stopping treatment allows the anaemia and nerve damage to return.
(2) (3) (1)

Procedures and treatment changes

Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (2) (3) (4) (5) (6) (7)

Missed dose

For an oral supplement, simply take it when you remember, or skip it and resume your normal schedule if it is nearly time for the next dose -- do not double up. For a prescribed injectable regimen, contact your prescriber's clinic for guidance on rescheduling a missed injection appointment.

Overdose

Oral vitamin B12 has a very low potential for toxicity and no established upper intake level, so a single extra dose is unlikely to cause harm, though you should still contact a poison control service or clinician if a large accidental ingestion occurs (e.g. by a child). Seek urgent medical attention for any signs of an allergic reaction after an injectable dose. (1) (2)

Side effects

Side effects and their urgency vary; report severe, persistent, or concerning symptoms.

Oral supplement: generally very well tolerated; mild diarrhoea, itching, or headache have been reported occasionally.

What to notice: Oral supplement: generally very well tolerated; mild diarrhoea, itching, or headache have been reported occasionally.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (1)

Injectable: injection-site pain or redness.

What to notice: Injectable: injection-site pain or redness.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (2) (3)

Injectable: mild, transient itching, feeling of body swelling, or diarrhoea have been reported.

What to notice: Injectable: mild, transient itching, feeling of body swelling, or diarrhoea have been reported.

What to do: Discuss persistent or troublesome symptoms with the treating clinician or pharmacist. (3)

Serious warnings

Signs of a serious allergic reaction to injectable vitamin B12 — emergency

Symptoms: Swelling of the face, lips, tongue, or throat; difficulty breathing; widespread rash or hives; sudden collapse.

Action: Seek emergency medical help immediately -- anaphylactic shock and death have been reported after injected vitamin B12, though this is rare. (2)

Vision changes in a person with Leber's hereditary optic disease — high caution

Symptoms: Sudden or worsening vision loss.

Action: Cyanocobalamin must not be used in this condition -- it has caused severe, rapid optic nerve damage; tell your prescriber immediately about any family history of this condition before starting injectable B12. (2)

Irregular heartbeat during early treatment of severe anaemia — high caution

Symptoms: Palpitations, weakness, or an irregular heartbeat, especially in the first days to weeks of treatment for severe megaloblastic anaemia.

Action: Seek prompt medical assessment -- rapid correction of severe anaemia can cause a sudden drop in blood potassium (hypokalaemia), which has been linked to dangerous heart rhythm problems; your prescriber will monitor your potassium levels closely during this period. (2) (3)

Untreated pernicious anaemia (stopping treatment) — high caution

Symptoms: Return of tiredness/anaemia symptoms, or new numbness, tingling, balance problems, or memory/thinking changes.

Action: Do not stop prescribed injections on your own -- in true pernicious anaemia, stopping treatment allows the anaemia to return and can lead to irreversible spinal cord and nerve damage. Contact your prescriber with any concerns about your treatment. (2)

Interactions

  • Long-term use of metformin or acid-reducing medicines (proton pump inhibitors or H2-receptor blockers) is associated with reduced vitamin B12 absorption and lower B12 status over time -- your clinician may check your B12 levels periodically if you take these long-term. (1)
  • Chloramphenicol (an antibiotic) may reduce the effectiveness of hydroxocobalamin's response in treating anaemia. (3)
  • Oral contraceptives can modestly lower serum vitamin B12 concentrations, though this is generally not considered clinically significant. (3)
  • Methotrexate and pyrimethamine can interfere with folic acid and vitamin B12 diagnostic blood test results, not with the vitamin's clinical effect. (2)
  • Colchicine, para-aminosalicylic acid, and heavy long-term alcohol use may impair the body's ability to absorb vitamin B12 from food. (2)
  • High-dose folic acid can mask (hide) the blood-count signs of vitamin B12 deficiency while allowing nerve damage to progress unrecognised -- do not substitute folic acid for a prescribed B12 treatment. (2)

Pregnancy, breastfeeding, kidney/liver function, age

Pregnancy

Vitamin B12 requirements increase during pregnancy (around 2.6 micrograms/day from dietary sources per US guidance). Injectable hydroxocobalamin should not be used to treat megaloblastic anaemia of pregnancy specifically (a different underlying mechanism applies) -- discuss any diagnosed deficiency in pregnancy with your prescriber. (1) (2) (3) (4) (5) (6) (7)

Breastfeeding

Requirements are slightly higher during breastfeeding (around 2.8 micrograms/day from dietary sources per US guidance). Injectable hydroxocobalamin is considered unlikely to harm a nursing infant at normal therapeutic doses. (1) (2) (3) (4) (5) (6) (7)

Children

Vitamin B12 requirements are lower in infants and children and increase with age; any diagnosed deficiency or pernicious anaemia in a child should be managed by a paediatric specialist. (1) (2) (3) (4) (5) (6) (7)

Elderly

Older adults are at higher risk of vitamin B12 deficiency due to reduced stomach acid production (atrophic gastritis) impairing absorption of B12 from food, even without pernicious anaemia -- periodic checking of B12 status is often reasonable in this group. (1) (2) (3) (4) (5) (6) (7)

Hepatic_impairment

No specific dose adjustment is described in the sources reviewed for liver impairment. (1) (2) (3) (4) (5) (6) (7)

Renal_impairment

No specific dose adjustment is described in the sources reviewed for kidney impairment. (1) (2) (3) (4) (5) (6) (7)

Monitoring

Before starting

  • Confirmation of the diagnosis (vitamin B12 deficiency or pernicious anaemia) before starting injectable treatment -- not simply assumed
  • Baseline full blood count
  • Baseline potassium level in patients with severe anaemia

During treatment

  • Periodic vitamin B12 blood level checks, especially in long-term metformin, PPI, or H2-blocker users, and in older adults.
  • Potassium monitoring during the first days to weeks of treatment for severe megaloblastic anaemia.
  • Full blood count monitoring until values normalise, then periodically during lifelong maintenance treatment for pernicious anaemia.
(2) (3) (1)

Storage

Store oral vitamin B12 supplements at room temperature, away from moisture and light, as directed on the product label. Injectable cyanocobalamin/hydroxocobalamin is stored and handled by healthcare professionals according to the specific product's labelling (commonly protected from light, at controlled room temperature or refrigerated depending on the product).

Professional information

Clinician-facing context only, sourced from NIH ODS, the current US FDA cyanocobalamin injection prescribing information, and the UK emc hydroxocobalamin SmPC; verify against the current full label in the relevant jurisdiction before prescribing.

United States

Indication: Pernicious anaemia and other B12-deficiency malabsorption states (injectable cyanocobalamin)

Dose summary: 100 mcg IM/SC daily x6-7 days, then alternate days x7 doses, then every 3-4 days x2-3 weeks, then 100 mcg monthly for life once haematologic values normalise.

Renal context: Not specifically established in the source reviewed for this dossier. (2)

United Kingdom

Indication: Pernicious/macrocytic anaemia, tobacco amblyopia, Leber's optic atrophy (injectable hydroxocobalamin)

Dose summary: Without neurological involvement: 250-1000 mcg IM alternate days x1-2 weeks, then weekly until normalised, then 1000 mcg every 2-3 months maintenance. With neurological involvement: 1000 mcg alternate days initially, same maintenance.

Renal context: Not specifically established in the source reviewed for this dossier. (3)

Mechanism of action

Hepatic guidance

No specific hepatic dose adjustment described in the sources reviewed.

Overdose note

Vitamin B12 has a wide margin of safety with no established tolerable upper intake level for oral intake; injectable overdose experience is limited and management is supportive. (1) (2)

Frequently asked questions

Do I need a prescription for vitamin B12?

It depends on the form. Oral vitamin B12 supplements (tablets, capsules, sublingual) are sold over-the-counter without a prescription in the US, UK, EU, Canada, Australia, and New Zealand. Injectable vitamin B12 (cyanocobalamin or hydroxocobalamin), used to treat a diagnosed deficiency or pernicious anaemia, is prescription-only in all of these markets. (1) (2) (3) (6) (7) (5)

Is there a maximum safe dose of vitamin B12?

For oral vitamin B12, no tolerable upper intake level has been set because it has a very low potential for toxicity -- the body does not store large excess amounts. This does not apply in the same way to prescription injectable B12, which is dosed by a clinician according to your specific diagnosis. (1)

Why do some people need B12 injections instead of tablets?

People with pernicious anaemia lack intrinsic factor, a stomach protein needed to absorb B12 from food or standard oral tablets, so injections that bypass the gut are used instead. People with certain gut surgeries or malabsorption conditions may also need injections for the same reason. (2) (3)

Can metformin or acid-reducing medicines affect my vitamin B12?

Yes -- long-term use of metformin, proton pump inhibitors, or H2-receptor blockers is associated with reduced vitamin B12 absorption. If you take these long-term, your clinician may periodically check your B12 levels. (1)

What is the mechanism of action of cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin)?

Vitamin B12 (cobalamin) is a water-soluble vitamin and essential cofactor for methionine synthase and L-methylmalonyl-CoA mutase, required for DNA synthesis, red blood cell maturation, and myelin maintenance in the nervous system. Absorption from food requires intrinsic factor, a glycoprotein secreted by gastric parietal cells; injectable administration bypasses this absorption step entirely. (1) (2)

What is the generic name of Vitamin B12?

The generic name is cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin). (1)

What class of drug is cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin)?

Vitamin B12 is classed as: Water-soluble vitamin (B-complex); dietary supplement and, in injectable prescription form, a haematinic/neurotrophic agent. (1)

References

  1. Vitamin B12 -- Health Professional Fact SheetU.S. National Institutes of Health, Office of Dietary Supplements (ODS) · United States · accessed 2026-07-30 · source 1
  2. CYANOCOBALAMIN injection, USP -- prescribing informationU.S. Food and Drug Administration / DailyMed (NIH), prescribing information for cyanocobalamin injection, USP · United States · accessed 2026-07-30 · source 2
  3. Hydroxocobalamin 1mg/ml Solution for Injection -- Summary of Product Characteristicselectronic medicines compendium (emc), United Kingdom · United Kingdom · accessed 2026-07-30 · source 3
  4. Directive 2002/46/EC of the European Parliament and of the Council on the approximation of the laws of the Member States relating to food supplementsEuropean Union -- Directive 2002/46/EC on food supplements (as amended) · European Union · accessed 2026-07-30 · source 4
  5. Cyanocobalamin Injection USP (Drug Product Database, DIN 01987003 and related DINs) and Vitamin B12/Cyanocobalamin (Natural Health Products Ingredients Database)Health Canada -- Drug Product Database and Natural Health Products Ingredients Database · Canada · accessed 2026-07-30 · source 5
  6. Vitamin B12 / hydroxocobalamin injection -- ARTG entries and TGA guidance on how vitamins are regulated in AustraliaTherapeutic Goods Administration (TGA), Australia -- Australian Register of Therapeutic Goods (ARTG) and TGA vitamin-regulation guidance · Australia · accessed 2026-07-30 · source 6
  7. Neo-B12 Injection and Hydroxocobalamin Panpharma -- New Zealand Data SheetsMedsafe (New Zealand regulator) · New Zealand · accessed 2026-07-30 · source 7

Brand names by country

Single-ingredient cyanocobalamin (also includes hydroxocobalamin, methylcobalamin, adenosylcobalamin) names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.

Priority countries

United States

  • Nascobal500 ug · par health usa, llc
  • Non-proprietary name2 mg/mL · xseer pharmaceuticals

United Kingdom

  • Vitamin B12 oral supplements (generic, various brands) (3)OTC food supplement no prescription · various oral doses · multiple OTC supplement manufacturers
  • Cobalin-H / hydroxocobalamin injection (generic) (3)MHRA licensed emc smpc confirmed prescription injectable · 1 mg/mL injection · multiple UK marketing authorisation holders

Canada

  • Vitamin B12 oral supplements (NHP-licensed, various brands) (5)Health Canada Natural Health Product NPN confirmed · various oral doses · multiple NHP licence holders
  • Cyanocobalamin injection (generic, DIN-listed) (5)Health Canada Drug Product Database DIN confirmed prescription · 1000 mcg/mL injection · multiple Health Canada-authorised manufacturers

Australia

  • Vitamin B12 oral supplements (AUST L listed, various brands) (6)TGA AUST L listed confirmed · various oral doses · multiple TGA-listed sponsors
  • HYDROXO-B12 / VITA-B12 / COBAL-B12 / NEO-B12 injection (generic hydroxocobalamin) (6)TGA ARTG registered confirmed prescription injectable · 1 mg/mL injection · Aspen Pharmacare Australia Pty Ltd; Amdipharm Mercury Australia Pty Ltd; Pfizer Australia Pty Ltd

New Zealand

  • Vitamin B12 oral dietary supplements (various brands) (7)dietary supplement regulations 1985 confirmed no prescription · various oral doses · multiple supplement manufacturers
  • Neo-B12 Injection / Hydroxocobalamin Panpharma (7)Medsafe approved confirmed prescription injectable · 1 mg/mL (1000 microgram/mL) injection · multiple Medsafe-approved sponsors, e.g. Panpharma
View brands in additional countries.

European Union

Vitamin B12 oral food supplements (various EU brands) (4), Hydroxocobalamin/cyanocobalamin injection (nationally authorised generics, various EU brand names) (4)

This page provides general reference information about vitamin B12 (both the over-the-counter supplement and the prescription injectable forms) and is not a substitute for advice from your clinician or pharmacist. Diagnosis, prescription-injectable treatment, and all dosing decisions for a diagnosed deficiency should be made with your prescriber.

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