B12 Forms Compared: Cyanocobalamin vs Methylcobalamin, and What the Evidence Says

Most B12 supplements contain 500 to 1000 mcg. The RDA is 2.4 mcg. That's a gap of roughly 200 to 400 times the daily requirement, which looks odd until you understand how B12 absorption works.

The form of B12 also matters, particularly if you have an MTHFR variant, take certain medications, or have a condition that affects absorption. This is what the evidence actually says.

Why B12 supplement doses are so high

B12 from food is absorbed through a protein called intrinsic factor, produced by the stomach. This system is efficient but has a ceiling: it can absorb roughly 1.5 to 2 mcg per meal, regardless of how much B12 is present.

At high supplement doses (500 mcg and above), a second absorption pathway activates: passive diffusion, which does not require intrinsic factor. This pathway absorbs roughly 1% of the dose. So 500 mcg via passive diffusion yields about 5 mcg absorbed, well above the RDA.

This is why high doses make sense, particularly for people with stomach acid issues, pernicious anaemia, or other conditions that reduce intrinsic factor activity. They are not relying on the standard pathway.

The four forms

Cyanocobalamin

The most common and most studied form. It is synthetic and stable, which makes it inexpensive to produce. Most fortified foods use cyanocobalamin.

The body converts cyanocobalamin to methylcobalamin or adenosylcobalamin before using it. This conversion works well in healthy individuals. The 'cyano' prefix refers to a cyanide group removed during conversion. At normal supplement doses, the amount of cyanide released is negligible and within safe limits. It is worth noting for people with significantly impaired kidney function, where clearance is slower.

Methylcobalamin

The form found primarily in blood. No conversion needed. It is the form most often preferred in functional medicine. It is immediately available to cells, with no conversion step. It is the relevant form for the methylation cycle. Sublingual methylcobalamin may also absorb particularly well for people with reduced stomach acid.

Methylcobalamin is more expensive than cyanocobalamin and less stable. Light degrades it, so quality storage matters. Look for opaque packaging.

Hydroxocobalamin

Used in NHS B12 injections and the standard medical treatment for pernicious anaemia. It has a longer half-life than other forms and is released slowly into circulation. Less commonly found in oral supplements. If you receive B12 injections, this is likely the form you're getting.

Adenosylcobalamin

The form used inside mitochondria for energy metabolism. Often combined with methylcobalamin in products marketed as 'active B12.' The evidence for oral adenosylcobalamin is thinner than for methyl or hydroxo forms, but it is a reasonable addition in a combined product where the methylcobalamin dose is the primary component.

Which form should you choose

For most people with no specific genetic or absorption considerations, either cyanocobalamin or methylcobalamin will work. The practical difference at standard supplement doses is small.

Methylcobalamin is the better default if: you have an MTHFR variant, you have reduced stomach acid (common over 50, and in long-term PPI users), or you prefer not to rely on the conversion step.

Cyanocobalamin is a reasonable choice if cost matters and you have no specific reason to avoid it. Its track record in clinical research is longer than any other form.

The MTHFR question

MTHFR variants affect how your body processes folate, not B12 directly. But B12 and folate work together in the methylation cycle: B12 is required to regenerate active folate from homocysteine. When people with MTHFR variants take methylated B vitamins, the goal is to support this cycle without relying on a conversion step that works less efficiently in them.

If you have a confirmed MTHFR variant (C677T or A1298C, available from 23andMe or other genetic testing), methylcobalamin paired with methylfolate is the commonly recommended approach. For most people without a known MTHFR variant, the practical difference is small.

Testing: serum B12 vs active B12

Standard serum B12 tests measure total B12 in the blood, including B12 that is bound to proteins and unavailable to cells. A normal serum B12 does not rule out functional deficiency.

Active B12 (holotranscobalamin) measures only the fraction of B12 actually being transported to cells. It is a more sensitive marker for early deficiency. Private labs including Medichecks and Forth offer it. Most NHS tests measure serum B12 only.

If you have symptoms consistent with B12 deficiency (fatigue, brain fog, tingling or numbness, mood changes) and a normal serum B12, asking specifically for holotranscobalamin is worth doing.

Who actually needs to think about B12

Most people eating a varied diet with animal products have adequate B12. Those who need to pay attention:

• Vegans and vegetarians. B12 occurs almost exclusively in animal products. If you eat no animal products and don't supplement, you will become deficient. This is not a debate in the evidence.
• Adults over 50. Stomach acid production drops with age, which reduces B12 absorption from food. Even people with adequate dietary intake can become deficient. Sublingual supplements or higher oral doses help bypass the stomach acid dependency.
• People on metformin. Long-term metformin use depletes B12. If you have been on metformin for more than a year, ask your GP to check your levels.
• People on PPIs or H2 blockers. Proton pump inhibitors reduce stomach acid, which impairs B12 absorption from food. Long-term use is associated with depletion.
• People with pernicious anaemia. Autoimmune destruction of intrinsic factor causes severe B12 deficiency that oral supplements can only partially address. If you've been diagnosed with pernicious anaemia, you likely need injections.

Timing

B12 is water-soluble, so excess is excreted rather than stored to a dangerous level. Take it in the morning. B vitamins, particularly B6 and B12, can interfere with sleep if taken in the evening, possibly because of their role in melatonin synthesis. Sublingual forms can be taken any time, but morning is a practical default.

How Stack Almanac helps

B12 is one of the markers the Almanac Advisor tracks in your bloodwork log. If you've recorded a serum B12 result, it uses that alongside your dietary notes to give a clearer picture of whether your current supplement is likely doing the job. It also flags if you're on metformin or a PPI, where B12 monitoring is worth keeping up with.

Stack Almanac defaults to methylated B12 in both the vegan and women's protocols, and as part of the methylated B-complex in the energy protocol, for exactly the absorption reasons above.