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# Hydroxocobalamin

A form of vitamin B12 for B12 deficiency that stays longer in the body, and the antidote that binds cyanide to form harmless cyanocobalamin.

Updated 2026-11-01

- Class: [Antianaemia agents (iron, folic acid, vitamin B12)](https://durgical.com/en/pharmacology/classes/antianaemia-agents/)
- Enzyme: Methionine synthase (B12 coenzyme); free cyanide in poisoning
- ATC code: B03BA03
- WHO Model List of Essential Medicines

Also called: vitamin B12a

✂**Methionine synthase (B12 coenzyme); free cyanide in poisoning**Hydroxocobalamin

ⓘ The figure shows human methionine synthase with hydroxocobalamin bound (cryo-EM); in cyanide poisoning the drug's cobalt instead captures cyanide in the blood.

**Target in 3D:** Methionine synthase (B12 coenzyme); free cyanide in poisoning. PDB structure 9SSS (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/9SSS) · Drag to rotate · pinch to zoom

## Mechanism of action

As a vitamin, hydroxocobalamin becomes the coenzymes methylcobalamin and adenosylcobalamin, restoring DNA synthesis and myelin. It binds plasma proteins more strongly than cyanocobalamin, so it stays longer in the circulation. As an antidote, the hydroxyl group on its cobalt is exchanged for cyanide: it binds cyanide to form cyanocobalamin, which is not toxic, freeing cytochrome oxidase so cells can use oxygen again.

- Cyanide blocks cytochrome oxidase; cells cannot use oxygen
- Hydroxocobalamin's cobalt grabs free cyanide
- Non-toxic cyanocobalamin forms
- As a vitamin it also restores B12 coenzymes

## Pharmacokinetics

**Absorption**
For deficiency it is injected intramuscularly; for cyanide it is infused intravenously

**Distribution**
Strongly bound to plasma proteins; stored in the liver

**Elimination**
Bile, with some enterohepatic recycling, and urine

## Indications

- Pernicious anaemia, with or without nervous system involvement [Pernicious anaemia →](https://durgical.com/en/conditions/pernicious-anaemia/)
- Other causes of B12 deficiency: malabsorption, gastrectomy, ileal resection, fish tapeworm, strict vegetarian diet [Megaloblastic anaemia (B12 or folate deficiency) →](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- Acute cyanide poisoning, including smoke inhalation from fires (high-dose intravenous product) [Cyanide poisoning →](https://durgical.com/en/conditions/cyanide-poisoning/)

## Adverse effects

### Common

- Pain at the injection site
- Mild transient diarrhoea
- Itching and transient rash

### Serious (rare, important)

- Anaphylaxis
- Hypokalaemia and thrombocytosis when severe megaloblastic anaemia starts to correct

## Contraindications

- Hypersensitivity to the drug (in life-threatening cyanide poisoning the benefit usually outweighs this)

## Key interactions

**Sodium thiosulfate**
Chemically incompatible: they are not given through the same intravenous line

**Folic acid alone**
Can correct the anaemia of B12 deficiency while nerve damage progresses; it is not a substitute

**Marrow-suppressing drugs (e.g. chloramphenicol)**
Blunt the response to vitamin B12

## What is monitored

- Reticulocyte count (rises within days if the diagnosis is right), haemoglobin and MCV
- Potassium and platelets early in treatment of severe megaloblastic anaemia
- In cyanide poisoning: blood pressure, lactate, acid–base status and level of consciousness

## Administration and safety

- Vitamin product: intramuscular only; avoid the intravenous route
- Cyanide antidote: a separate high-dose product reconstituted and infused intravenously, as early as possible
- Use a separate line from sodium thiosulfate
- Protect the vitamin product from light

## Nursing care

- In fires or suspected cyanide exposure: remove from the source, give oxygen and support breathing while the antidote is prepared
- Record the times of each antidote and keep thiosulfate on another line
- In anaemia: check potassium and platelets in the first days and the reticulocyte response

## Pregnancy and breastfeeding

For deficiency it is given when clearly needed; in cyanide poisoning the threat to mother and fetus outweighs the uncertain risk.

## Memory aid

Hydroxo «swaps OH for CN»: hydroxocobalamin + cyanide → cyanocobalamin (vitamin B12) leaves safely.

## Test yourself

Think of the answer, then tap the question to see it.

## What does hydroxocobalamin form when it binds cyanide?

Cyanocobalamin (vitamin B12), which is not toxic.

## Why is it preferred to cyanocobalamin for injections in deficiency?

It binds plasma proteins more and stays longer in the circulation.

## Which antidote must not share its intravenous line?

Sodium thiosulfate.

## How does its antidote action differ from that of sodium nitrite?

Nitrite works by producing methaemoglobin, which binds cyanide; hydroxocobalamin binds cyanide directly to form cyanocobalamin.

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## Related drugs

- [Cyanocobalamin](https://durgical.com/en/pharmacology/cyanocobalamin/)
- [Sodium thiosulfate](https://durgical.com/en/pharmacology/sodium-thiosulfate/)
- [Sodium nitrite](https://durgical.com/en/pharmacology/sodium-nitrite/)
- [Folic acid](https://durgical.com/en/pharmacology/folic-acid/)

## Related conditions

- [Pernicious anaemia](https://durgical.com/en/conditions/pernicious-anaemia/)
- [Megaloblastic anaemia (B12 or folate deficiency)](https://durgical.com/en/conditions/megaloblastic-anaemia/)
- [Cyanide poisoning](https://durgical.com/en/conditions/cyanide-poisoning/)

[← Back to the class: Antianaemia agents (iron, folic acid, vitamin B12)](https://durgical.com/en/pharmacology/classes/antianaemia-agents/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - HYDROXOCOBALAMIN injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=a3087a39-f218-4f16-a1e7-17c39bb2eb69) (DailyMed (U.S. National Library of Medicine), 2026); [Guía para el Manejo de Emergencias Toxicológicas](https://minsalud.gov.co/sites/rid/Lists/BibliotecaDigital/RIDE/DE/GT/guias-manejo-emergencias-toxicologicas-outpout.pdf) (Ministerio de Salud y Protección Social; Organización Panamericana de la Salud, 2017); [DailyMed - SODIUM THIOSULFATE injection, solution](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=82f55408-2de9-463d-85e8-b39c3b259f0b) (DailyMed (U.S. National Library of Medicine), 2026); Farmacología básica y clínica (Katzung), 16.ª ed., cap. 33 (McGraw Hill, 2025).

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Representative model, for educational purposes only.

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