Hydroxocobalamin vs cyanocobalamin: why the UK uses one and the US uses the other

The molecular difference

Vitamin B12 has a complex structure built around a corrin ring with a cobalt atom at its centre. The cobalt is bonded to four nitrogen atoms in the ring plane, a benzimidazole ligand from below, and a sixth ligand from above. That sixth ligand is the variable position and determines which form of B12 you have:

• Cyanocobalamin: Sixth ligand is a cyanide group (-CN). Most stable, easiest and cheapest to manufacture. Doesn't occur naturally in the body in any meaningful quantity — it's a synthetic form.
• Hydroxocobalamin: Sixth ligand is a hydroxyl group (-OH). Found in nature, also synthesised commercially. The cobalt-OH bond binds more tightly to plasma proteins than the cobalt-CN bond.
• Methylcobalamin: Sixth ligand is a methyl group (-CH3). One of the two active coenzyme forms in human metabolism.
• Adenosylcobalamin (also called cobamamide): Sixth ligand is 5-deoxyadenosyl. The other active coenzyme form.

In the body, any of these can be converted into the active forms (methyl and adenosylcobalamin) by enzymatic processing. Cyanocobalamin needs to have its cyanide group displaced first, which adds a small step but is metabolically trivial.

Pharmacokinetics: why the schedules differ

The clinically relevant difference is plasma retention. Hydroxocobalamin binds more avidly to plasma proteins (transcobalamin and others) and is excreted more slowly. Its plasma half-life is approximately 26 days. Cyanocobalamin binds less tightly and is excreted in urine more rapidly; plasma half-life approximately 6 days.

The practical effect: a 1 mg intramuscular dose of hydroxocobalamin maintains adequate serum B12 for approximately 8–12 weeks. The same dose of cyanocobalamin maintains levels for approximately 4 weeks.

That's why NHS practice and BNF first-line guidance is hydroxocobalamin every 3 months (after an initial loading schedule), while the US pharmacopoeia traditionally uses cyanocobalamin monthly. Both regimens deliver therapeutic B12 to patients with deficiency; the dosing intervals reflect the underlying pharmacokinetics.

BNF first-line listing

The British National Formulary lists hydroxocobalamin as the treatment of choice for B12 deficiency. The standard schedule:

• Loading: 1 mg intramuscular three times per week for two weeks (six injections). If neurological symptoms are present, continue every other day until no further improvement is seen.
• Maintenance: 1 mg intramuscular every three months.

For B12 deficiency due to dietary insufficiency (strict vegan diet) without absorption problems, the loading schedule is shorter (1 mg three times in the first week) and oral high-dose supplementation can sometimes replace injections thereafter — see the cyanocobalamin section below.

US/UK practice divergence

Why two countries with similar evidence bases and similar populations use different first-line products comes down to historical practice and supply chains. The US Pharmacopeia listed cyanocobalamin in the early 20th century when it was the cheapest stable form to manufacture, and the prescribing habit became entrenched. The UK adopted hydroxocobalamin in the 1960s when its longer half-life was recognised as clinically advantageous — fewer injections, lower healthcare contact, similar outcomes.

Recent US literature has reopened the question, with some American clinicians advocating switching to hydroxocobalamin to reduce injection burden. But for now the divergence persists, and there's no compelling clinical reason to disrupt either established practice. Both work.

When cyanocobalamin still has a role

Even in the UK, cyanocobalamin has niches:

• High-dose oral supplementation: Cyanocobalamin is the standard ingredient in oral B12 supplements (tablets and sublingual). Mass-action absorption (about 1% of a high oral dose absorbs passively, bypassing intrinsic-factor-mediated absorption) means 1000 mcg oral cyanocobalamin daily can sometimes replace injections in patients with mild absorption issues. Not first-line for established deficiency with neurological signs, but useful for maintenance in some scenarios.
• Strict vegans: Oral cyanocobalamin 50–100 mcg daily prevents deficiency in vegan diets at low cost.
• Pernicious anaemia patients who prefer monthly self-administered injections — some patients prefer the rhythm of monthly cyanocobalamin to quarterly hydroxocobalamin, especially if symptoms recur towards the end of the 12-week window.

Methylcobalamin and adenosylcobalamin: the 'active forms'

Online B12 marketing often pushes methylcobalamin as 'better' or 'more bioavailable' than hydroxocobalamin or cyanocobalamin. The truth is more nuanced.

Methylcobalamin and adenosylcobalamin are the active coenzyme forms used by your cells. But when you inject hydroxocobalamin or cyanocobalamin, your body readily converts them to the active forms intracellularly. The conversion is not rate-limiting in healthy people. So 'starting with the active form' offers no clinical advantage in routine B12 deficiency treatment.

The handful of clinical scenarios where methylcobalamin might be preferred are rare inborn errors of B12 metabolism — specific enzyme defects in the conversion pathway. For all the common causes of B12 deficiency (pernicious anaemia, dietary, metformin-related, post-gastric-surgery, ileal disease), hydroxocobalamin works perfectly well and is the evidence-based first-line.

Methylcobalamin is also less stable in solution (it's light-sensitive), more expensive, and not on the NHS formulary as an injection. There's no compelling reason to pay private prescription rates for it unless you have a specific indication.

Practical implications for our patients

At Trafford Clinic we use hydroxocobalamin for B12 injections. This is consistent with BNF and NICE Clinical Knowledge Summary guidance and what the NHS would prescribe.

For patients on metformin (who have higher B12 deficiency rates — see our metformin and B12 guide), for older adults, for vegans, and for patients with neurological symptoms suggesting deficiency, we recommend testing first (see our B12 deficiency symptoms guide) and then a loading course followed by quarterly maintenance.

For routine maintenance after loading, we offer scheduled appointments every 12 weeks. We see B12 patients across Greater Manchester: Rusholme, Manchester city centre, Old Trafford, Salford, Salford Quays, Sale, Altrincham, Stretford, Chorlton, Eccles, Whalley Range and Firswood.

How to book

For B12 testing or injections, book at traffordclinic.co.uk/vitamin-b12 or call 0161 258 6149. Our standalone serum B12 test is available across all our blood test sites — see Manchester blood tests for the full menu.

Empire Pharmacy is GPhC-registered (premises 1123966); Haroon Iqbal is an Independent Prescriber (reg. 2051093) with 26 years of pharmacy experience.

Related reading: Metformin and B12, B12 deficiency symptoms.