Vitamin B12: Forms and Which One to Choose
Vitamin B12 supplements come in a confusing array of forms, each promoted as superior to the others. Methylcobalamin is often marketed as the "active" choice, while cyanocobalamin is dismissed as synthetic. Our editorial team examined the science behind each form, how B12 is absorbed, and what UK guidance says about choosing a supplement.
Why vitamin B12 matters
Vitamin B12, or cobalamin, is needed for producing red blood cells, maintaining the nervous system and synthesising DNA. It works together with folate in a pathway that recycles homocysteine, and it supports energy metabolism within mitochondria.
Deficiency can cause megaloblastic anaemia, with fatigue, breathlessness and pale skin, as well as neurological symptoms such as pins and needles, numbness, problems with balance and memory difficulties. Neurological damage may become permanent if deficiency is left untreated for too long.
The UK reference nutrient intake for adults is 1.5 micrograms per day. B12 is found naturally almost exclusively in animal foods: meat, fish, eggs and dairy. Vegans and many vegetarians therefore rely on fortified foods or supplements.
For athletes, B12 is not a performance enhancer in people with adequate status. Its importance lies in preventing deficiency, which can impair oxygen transport and training capacity.
How B12 is absorbed
Absorption of B12 is unusually complex. In the stomach, acid and enzymes release it from food proteins. It then binds to intrinsic factor, a protein made by the stomach lining, and the complex is absorbed in the final part of the small intestine, the terminal ileum.
This active system is easily saturated and can absorb only around 1.5–2 micrograms from a single meal. However, a small fraction of any dose, roughly 1 per cent, is absorbed by passive diffusion without intrinsic factor. That is why high oral doses can work even when the normal pathway fails.
Several conditions impair absorption: pernicious anaemia, an autoimmune disease that destroys intrinsic factor production; gastric surgery; coeliac or Crohn's disease; and long-term use of metformin or acid-suppressing medicines. A randomised trial by de Jager and colleagues in the BMJ in 2010 showed that metformin lowered B12 levels over four years.
Older adults are also at higher risk because stomach acid production tends to decline with age, reducing release of B12 from food.
The four main forms compared
All forms of B12 share the same core cobalamin structure but differ in the group attached to the central cobalt atom. In the body, they are all converted into two active coenzymes: methylcobalamin and adenosylcobalamin.
| Form | Characteristics | Typical use in the UK |
|---|---|---|
| Cyanocobalamin | Very stable, inexpensive, well studied; trace cyanide group is harmless at supplement doses | Most tablets and fortified foods |
| Hydroxocobalamin | Binds strongly to proteins, stays in the body longer | Standard NHS injection for deficiency |
| Methylcobalamin | One of the active coenzymes; less stable to light | Supplements marketed as "active B12" |
| Adenosylcobalamin | The mitochondrial coenzyme; limited research as a supplement | Combination products |
Cyanocobalamin is the form used in most clinical trials of oral B12, including the landmark study by Kuzminski and colleagues in Blood in 1998, which showed that high-dose oral cyanocobalamin corrected deficiency as effectively as injections. A Cochrane-style systematic review by Butler and colleagues in 2006 reached a similar conclusion.
Methylcobalamin is often claimed to be better absorbed or retained, but direct head-to-head human studies are few. A review by Paul and Brady in 2017 discussed theoretical advantages for people with certain genetic variants, but robust clinical evidence of superiority is lacking.
Tablets, sprays and injections
B12 supplements are available as swallowed tablets, sublingual lozenges, oral sprays, and injections. Sublingual and spray products are marketed as bypassing the gut, but studies suggest that most of the B12 is swallowed and absorbed in the usual way. Their effectiveness appears broadly similar to swallowed tablets at comparable doses.
Injections of hydroxocobalamin are the standard NHS treatment for pernicious anaemia and for deficiency with neurological symptoms. They are prescription-only and given by a healthcare professional.
The NICE guideline on vitamin B12 deficiency in people over 16, published in 2024, supports oral cyanocobalamin for many people with deficiency not caused by an absorption problem, while recommending injections for those with pernicious anaemia, neurological symptoms or severe malabsorption.
For maintenance in vegans and others with low intake, guidance from bodies such as the Vegan Society suggests either regular small doses throughout the day via fortified foods, a daily supplement of at least 10 micrograms, or a weekly supplement of around 2,000 micrograms.
How to choose the right product
For most healthy people simply wishing to prevent deficiency, cyanocobalamin is effective, stable and affordable. Methylcobalamin is a reasonable alternative if preferred, but there is no strong evidence it works better. Some points to keep in mind:
- If you suspect deficiency, get a blood test before starting supplements, as they can mask the diagnosis.
- If you take metformin or acid-suppressing medicines long term, ask your GP whether your B12 should be checked.
- Vegans should include a reliable B12 source every day or week, not occasionally.
- Store methylcobalamin away from light, as it degrades more easily.
- Very high doses are generally well tolerated, but there is no benefit to megadosing without deficiency.
Some observational studies have linked very high blood B12 levels with other health problems, but this probably reflects underlying disease rather than harm from supplements. Persistently high levels without supplementation should still be discussed with a doctor.
Conclusions of the editorial team
All forms of vitamin B12 are converted into the same active coenzymes in the body. Cyanocobalamin is the best-studied oral form and remains a sound choice; methylcobalamin is acceptable but not proven superior.
The bigger questions are whether you need B12 at all, whether you can absorb it normally, and whether an injection is required. People with pernicious anaemia, neurological symptoms or malabsorption need medical treatment rather than self-supplementation.
You may also wish to read our articles on nutrient gaps in plant-based diets, on iron and anaemia in athletes, and on folate and homocysteine.
References
- Stabler SP. Vitamin B12 deficiency. N Engl J Med. 2013;368(2):149–160.
- Kuzminski AM, Del Giacco EJ, Allen RH, Stabler SP, Lindenbaum J. Effective treatment of cobalamin deficiency with oral cobalamin. Blood. 1998;92(4):1191–1198.
- Butler CC, Vidal-Alaball J, Cannings-John R, et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency: a systematic review of randomized controlled trials. Fam Pract. 2006;23(3):279–285.
- de Jager J, Kooy A, Lehert P, et al. Long term treatment with metformin in patients with type 2 diabetes and risk of vitamin B-12 deficiency: randomised placebo controlled trial. BMJ. 2010;340:c2181.
- Paul C, Brady DM. Comparative bioavailability and utilization of particular forms of B12 supplements with potential to mitigate B12-related genetic polymorphisms. Integr Med (Encinitas). 2017;16(1):42–49.
- National Institute for Health and Care Excellence. Vitamin B12 deficiency in over 16s: diagnosis and management. NICE guideline NG239. London: NICE; 2024.
