Quick answer: All four supplemental B12 forms — cyanocobalamin, methylcobalamin, hydroxocobalamin, and adenosylcobalamin — convert to the same two active coenzymes inside your cells. Cyanocobalamin is the cheapest and most stable; the body excretes its tiny cyanide moiety harmlessly. Methylcobalamin’s “active form” superiority is largely marketing — head-to-head clinical evidence is sparse. Hydroxocobalamin is the form NHS clinicians use for injections in pernicious anaemia. If you suspect B12 deficiency, get a GP blood test rather than self-supplementing.
The four forms — what they are and what your body does with them
Vitamin B12 — cobalamin — is a family of cobalt-containing molecules. Methylcobalamin and adenosylcobalamin are the two active intracellular coenzymes: methylcobalamin in the cytoplasm for methionine synthase (which recycles homocysteine to methionine), adenosylcobalamin in the mitochondria for methylmalonyl-CoA mutase. Cyanocobalamin and hydroxocobalamin are stable forms the body converts into those active coenzymes after absorption. The body interconverts all four readily — the supplement form does not bypass intracellular conversion to a meaningful clinical extent in healthy adults.
Cyanocobalamin (CN-Cbl)
Cyanocobalamin is the synthetic, stable, low-cost form used in most fortified foods, supermarket multivitamins (typically 2.5 µg, 100% UK NRV), and the NHS oral high-dose tablets prescribed for diet-related deficiency. The cyanide group bound to the cobalt atom is what makes the molecule shelf-stable; once absorbed, the cyanide moiety is split off, detoxified to thiocyanate, and excreted in trivial amounts (the dose of cyanide in a 1,000 µg tablet is roughly 20 µg — orders of magnitude below any toxicology threshold).
Methylcobalamin (Me-Cbl)
Methylcobalamin participates directly in the homocysteine-to-methionine reaction. It is sold in supplements as “the active form”, “the bioavailable form”, or “the methylated form”, at typical UK retail prices three to four times higher per microgram than cyanocobalamin. The marketing claim is that it bypasses an intracellular conversion step and is therefore more bioavailable or effective. The biology is real — methylcobalamin is the active coenzyme — but the clinical superiority claim runs ahead of the trial evidence (Paul & Brady 2017, Integr Med). Head-to-head clinical-outcome trials between methylcobalamin and cyanocobalamin in healthy adults are sparse; what trials exist describe modest pharmacokinetic differences, not consistently better clinical outcomes for routine supplementation.
Hydroxocobalamin (OH-Cbl)
Hydroxocobalamin is the form NHS clinicians use for intramuscular injections in B12 deficiency and pernicious anaemia. Per the NHS B12 deficiency anaemia treatment page, “hydroxocobalamin is preferred to an alternative called cyanocobalamin” because it remains in the body longer — longer plasma half-life and tissue retention mean fewer injections to maintain steady-state levels.
The standard NHS regimen for non-diet-related B12 deficiency is loading-phase injections every other day for around two weeks, followed by maintenance every two to three months for life (every two months where neurological symptoms are present). Hydroxocobalamin appears in some oral supplements but is much rarer than methylcobalamin or cyanocobalamin in the UK retail aisle.
Adenosylcobalamin (Ado-Cbl)
Adenosylcobalamin is the second intracellular active coenzyme — the mitochondrial one used by methylmalonyl-CoA mutase. It appears in a small number of “dual-form B12” supplements that combine methylcobalamin and adenosylcobalamin, on the rationale that giving both active coenzymes covers both reactions directly. The trial evidence base for adenosylcobalamin specifically — and for combination methyl-plus-adenosyl products versus well-dosed single-form alternatives — is thin and does not currently establish a clinical advantage in healthy adults.
The “methylcobalamin is more active” claim — what’s true, what’s marketed
The “active form” framing is a real biological concept misapplied to a marketing question. Two things are true at once: methylcobalamin and adenosylcobalamin are the active coenzymes the enzymes actually use, and that conversion is not a clinical bottleneck for most people. The cells that need B12 — red-cell precursors, nerve cells, every dividing cell — convert any of the supplemental forms to the active coenzymes effectively. Healthy adults given oral cyanocobalamin develop normal intracellular methylcobalamin and adenosylcobalamin pools; the conversion step is not rate-limiting.
Paul and Brady’s 2017 review in Integrative Medicine (Encinitas) — widely cited in the methyl-versus-cyano debate — argues for natural-form preference and notes rare genetic polymorphisms (e.g. in MTRR, MTR) that could in principle affect intracellular conversion. The review’s conclusion is forward-leaning compared with the formal evidence base; head-to-head clinical-outcome trials remain sparse. NHS, NICE Clinical Knowledge Summaries, and BNF guidance for routine B12 supplementation do not currently distinguish between the forms on the basis of clinical superiority.
For a UK shopper deciding between an £8 methylcobalamin and a £3 cyanocobalamin: both will correct ordinary dietary inadequacy. The price gap is real; the clinical-outcome gap, in healthy adults without specific genetic or absorption issues, is not well established.
Cyanocobalamin and the cyanide myth
The most common consumer concern about cyanocobalamin is the cyanide group bound to the cobalt atom. The framing — “you’re swallowing cyanide” — is misleading.
The cyanide is bound covalently to the cobalt and dissociates only after intracellular conversion. The amount released from a typical 1,000 µg tablet is around 20 µg of cyanide — orders of magnitude below any toxicological concern, and well within the body’s normal capacity to detoxify cyanide via the rhodanese enzyme system, which converts it to thiocyanate for urinary excretion. The same enzyme system handles the much larger cyanide loads from cassava, almonds, and stone-fruit kernels.
A narrow clinical exception: in Leber hereditary optic neuropathy (LHON) and a handful of rare mitochondrial conditions where cyanide handling is impaired, cyanocobalamin is specifically contraindicated and hydroxocobalamin is preferred. This is a clinical-pathway decision, not a routine consumer concern.
Sublingual, spray, lozenge — does the route actually matter?
UK supplement shelves carry sublingual tablets, oral sprays, lozenges, and standard tablets. The marketing argument for sublingual and spray formats is that B12 is absorbed across the oral mucosa and bypasses intestinal absorption (which depends on intrinsic factor, the gastric glycoprotein required for B12 uptake in the terminal ileum). Pharmacokinetic studies show comparable plasma B12 rises between equivalent doses of sublingual and standard oral B12 — both rely heavily on the same ~1% passive-diffusion absorption, and there is no robust head-to-head clinical-outcome data showing one format is meaningfully more effective.
The better-evidenced question is high-dose oral versus injection. The 2005 Cochrane review by Vidal-Alaball and colleagues (CD004655), updated in 2018, concluded that high-dose oral B12 (1,000–2,000 µg/day) achieves comparable haematological and short-term neurological outcomes to intramuscular B12 in many cases of mild-to-moderate deficiency — the ~1% passive-diffusion route delivers enough at high doses to bypass the saturated intrinsic-factor pathway. This is why typical UK B12 supplements are dosed at 500–1,500 µg rather than the 1.5 µg NHS RNI: a 1,000 µg tablet absorbs about 10 µg, several multiples of the daily requirement.
NHS pernicious anaemia treatment vs supplemental B12
Pernicious anaemia is an autoimmune condition in which the parietal cells of the stomach are destroyed, intrinsic-factor production fails, and dietary B12 absorption collapses. NHS practice for confirmed pernicious anaemia is hydroxocobalamin intramuscular injection on the regimen above. Self-treating suspected pernicious anaemia with over-the-counter oral B12 risks under-treatment, and the neurological consequences (subacute combined degeneration of the spinal cord) can be partially or fully irreversible if missed.
If you have ongoing fatigue, peripheral neuropathy, unexplained cognitive changes, or a known autoimmune condition, the right step is a GP visit and a blood test (full blood count, serum B12, folate, sometimes methylmalonic acid and homocysteine as functional markers), not a higher-dose supplement.
Vegan / vegetarian B12 — the algae trap
Plant foods do not reliably contain bioavailable B12. The vitamin is synthesised by bacteria; trace cobalamin-like compounds appear in some seaweeds, fermented soy products, and mushrooms, but bioavailability and assayable activity vary widely. Many cobalamin-like compounds in seaweed are inactive analogues that bind the same transport proteins as real B12 and can interfere with active B12 uptake (Watanabe et al. 2014, Nutrients). Dried purple laver (nori) appears to contain measurable bioavailable B12, but batch-to-batch consistency makes it an unreliable sole source.
NHS guidance for vegans is unambiguous: use B12-fortified foods (some plant milks, nutritional yeast, breakfast cereals) or a supplement as the dependable source. The form question (methyl versus cyano versus hydroxo) is secondary to taking any reliable B12 source consistently.
What the GB Nutrition and Health Claims Register actually allows
The GB Nutrition and Health Claims (NHC) Register lists eight authorised function claims for vitamin B12 — the only ones a UK supplement label can make:
- Vitamin B12 contributes to the reduction of tiredness and fatigue.
- Vitamin B12 contributes to normal psychological function.
- Vitamin B12 contributes to normal functioning of the nervous system.
- Vitamin B12 contributes to normal energy-yielding metabolism.
- Vitamin B12 contributes to normal homocysteine metabolism.
- Vitamin B12 contributes to the normal function of the immune system.
- Vitamin B12 contributes to normal red blood cell formation.
- Vitamin B12 has a role in the process of cell division.
Two things matter here. First, every claim is a function-of-adequacy claim — they describe what B12 is required for in normal physiology, not promises that supplementation in a non-deficient adult will produce supra-normal effects. The “reduction of tiredness and fatigue” claim does not authorise framing B12 as an “energy booster” for adults whose B12 status is already adequate. Second, no GB-authorised claim distinguishes between cobalamin forms — methylcobalamin, cyanocobalamin, hydroxocobalamin, and adenosylcobalamin can all carry the same claims at the 15% NRV (≥0.375 µg) threshold per portion.
Talk to your pharmacist or GP if
- You have ongoing fatigue, numbness, tingling, or memory changes you suspect might be B12 deficiency. High-dose oral B12 (and high-dose folate) can normalise the haematological picture without correcting an underlying pernicious anaemia, masking the diagnosis while neurological damage progresses. Get a blood test first.
- You take metformin long-term. Metformin reduces B12 absorption over years; NICE Clinical Knowledge Summaries lists it among drug-related causes of B12 deficiency. An annual B12 check is a reasonable conversation to have with your prescriber.
- You take a long-term proton-pump inhibitor (PPI) or H2 blocker. Reduced gastric acid impairs B12 release from food protein; food-derived B12 is more affected than free supplemental B12.
- You are vegan, breastfeeding, or pregnant. Adequacy matters for the developing nervous system; talk to a midwife, GP, or pharmacist before deciding on format and dose.
- You have a confirmed diagnosis of pernicious anaemia or any cause of malabsorption. NHS injection pathway is the standard of care — supplements do not replace it.
- You take high-dose folate (≥1 mg/day). Get B12 status checked first.
You can report any suspected side effect from a supplement or medicine to the MHRA via the Yellow Card scheme: yellowcard.mhra.gov.uk.
Related Camden Medicals reading
Sources and further reading
- NHS — Vitamin B12 or folate deficiency anaemia: Treatment (hydroxocobalamin preferred over cyanocobalamin)
- NHS — Vitamins and minerals: B vitamins (incl. B12)
- NICE Clinical Knowledge Summaries — Anaemia: B12 and folate deficiency
- BNF — Cyanocobalamin
- BNF — Hydroxocobalamin
- gov.uk — GB Nutrition and Health Claims Register (Vitamin B12 authorised claims)
- Paul & Brady (2017) — Comparative Bioavailability and Utilization of Particular Forms of B12 Supplements, Integr Med (Encinitas), PMID 28223907
- Watanabe, Yabuta, Bito & Teng (2014) — Vitamin B12-containing plant food sources for vegetarians, Nutrients, PMID 24803097
- Vidal-Alaball et al. (2005) — Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency, Cochrane CD004655, PMID 16034940
- Wang, Li, Qin, Song, Vidal-Alaball & Liu (2018) — Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency (updated Cochrane review), PMID 29543316
- MHRA Yellow Card scheme
Camden Medicals editorial. Information, not medical advice. Talk to your pharmacist or GP before starting any supplement, especially if you take prescribed medicines, are pregnant or breastfeeding, or have an existing condition. Suspected B12 deficiency or pernicious anaemia should be investigated by a GP, not self-treated.