Cyano vs methyl vs hydroxo — comparing B12 supplement forms

Walk down any supplement aisle and you’ll find three or four different forms of B12 — usually priced in that order from cheapest to most expensive — each with marketing claiming it’s the “active,” “natural,” or “superior” choice. Here is the honest version.

The four forms

Form	Chemistry	Marketed as	Real-world role
Cyanocobalamin	cyanide upper ligand	”standard,” “affordable”	The default supplement. Stable, cheap, heavily studied.
Methylcobalamin	methyl upper ligand	”active,” “bioavailable,” “natural”	Already in the active coenzyme form used by methionine synthase.
Hydroxocobalamin	hydroxyl upper ligand	”long-lasting,” “clinical”	Clinical injectable; highest tissue retention.
Adenosylcobalamin	5’-deoxyadenosyl upper ligand	”active (mitochondrial)“	The active coenzyme for methylmalonyl-CoA mutase.

What the body actually uses

The body only uses two forms of B12 at the enzyme level: methylcobalamin (for methionine synthase) and adenosylcobalamin (for methylmalonyl-CoA mutase).

When you eat or supplement any form — including cyanocobalamin — the body cleaves off the upper ligand, transports the core cobalamin molecule, and attaches whichever ligand is needed at the time. This conversion is efficient in healthy adults.

Which means: the “active” marketing on methyl- and adenosyl-cobalamin is technically true (they are the coenzyme forms) but practically misleading. Healthy people convert cyanocobalamin to the same active forms without issue.

When does the form actually matter?

• Kidney disease: In advanced chronic kidney disease, the cyanide group (trivial in healthy people) is metabolized more slowly. Clinicians often prefer hydroxocobalamin or methylcobalamin. Not an issue at supplemental doses for healthy people, but worth mentioning.
• Smokers: Heavy smokers already metabolize significant cyanide from tobacco. Methylcobalamin or hydroxocobalamin may be preferable. The difference at supplemental doses is small; the bigger health move is quitting.
• Inherited cobalamin metabolism disorders: Rare (MTHFR variants etc. do not count — that myth is separate). Patients with genuine methylmalonic acidemia or homocystinuria need specific active forms under clinical supervision.
• Active clinical deficiency: Clinicians usually administer hydroxocobalamin by injection. Tissue retention and half-life are superior to oral forms.

For healthy adults without kidney disease, not heavy smokers, with no rare metabolic disorder, cyanocobalamin is the cleanest choice.

Cost, stability, and availability

• Cyanocobalamin: cheapest, most stable, most widely available. Ships to anywhere. Tablet lifetime 2+ years at room temperature.
• Methylcobalamin: more expensive (often 2–3×), less stable (light- and heat-sensitive), still widely available. Sublingual forms popular.
• Hydroxocobalamin: rarely sold over the counter; usually obtained by prescription injection. Not a realistic oral choice for most consumers.
• Adenosylcobalamin: rare as a standalone oral supplement; often blended with methylcobalamin in “active B12” formulations.

A word on “MTHFR”

A widespread internet claim says that people with MTHFR gene variants “can’t process” cyanocobalamin and “must take” methylcobalamin. This is not supported by evidence.

MTHFR polymorphisms affect folate metabolism, not B12 metabolism. There is no clinical reason for people with MTHFR variants to prefer methylcobalamin over cyanocobalamin specifically. This myth has generated a great deal of supplement sales and a great deal of confusion. It is not true.

So what should I buy?

For prevention of deficiency in a healthy adult vegan:

• Cyanocobalamin, 1,000–2,000 µg, twice a week. Or 25–100 µg daily.
• Any reputable brand. Store in a cupboard, not a bright windowsill.
• See B12 dosage for adults for regimen details.

If you have peace of mind with methylcobalamin and the extra cost doesn’t bother you, it works equally well. The only honest statement about “superior bioavailability” is that differences are small and clinically insignificant in healthy people.

Common misconceptions

• “Methylcobalamin is the active form so my body can use it directly.” True about the molecule; overstated about the practical benefit. Your body converts forms readily.
• “Cyanocobalamin contains cyanide — that’s bad.” The cyanide group is a tiny fraction of the molecule and is detoxified along normal metabolic pathways. Not a concern at supplemental doses.
• “My MTHFR genetics require methyl B12.” Not correct. MTHFR is a folate-cycle enzyme.

The punchline

For almost everyone, cyanocobalamin is the right answer. It is cheap, stable, well-studied, and equally effective. Methylcobalamin and hydroxocobalamin have narrow indications that most people do not meet. The supplement marketplace makes this decision feel harder than it is.

For the full B12 picture, see Vitamin B12.