Health Reference LibraryWhich form of vitamin B12 should you buy?
This entry is part of the Nutri Tailor Health Reference Library — cited research on supplements, nutrients and adjacent areas of health.
Summary
Vitamin B12 supplements come in four forms: cyanocobalamin, methylcobalamin, adenosylcobalamin and hydroxocobalamin. All four are converted by the body into the same two active coenzymes, so form matters less than dose and consistency for most people. Cyanocobalamin has the largest evidence base and the best stability; hydroxocobalamin is the UK first-line injectable under NICE NG239; methylcobalamin and adenosylcobalamin are already-active forms some people prefer. Test before supplementing where deficiency is suspected, because confirmed deficiency with neurological symptoms needs GP-led care rather than a shop-bought supplement.
How it works
Whatever form is on the label, absorbed cobalamin is processed to the same two coenzymes inside cells, methylcobalamin in the cytosol and adenosylcobalamin in mitochondria. The labelled form is stripped and rebuilt during cellular processing, which is why the NIH Office of Dietary Supplements states that no evidence indicates absorption of supplemental B12 varies by form.
Effective dose
Active absorption uses intrinsic factor, which saturates at roughly 1.5 to 2mcg per dose. Beyond that, about 1% of a large oral dose is absorbed passively, which is why deficiency-correction protocols use 1000mcg or more daily by mouth while everyday maintenance needs only the UK reference intake of 1.5mcg. NICE NG239 allows high-dose oral cyanocobalamin in selected cases where malabsorption is not suspected.
Forms compared
Cyanocobalamin is synthetic, the most studied and the most stable in liquids and capsules. Hydroxocobalamin is the form used in UK injections and shows longer tissue retention. Methylcobalamin and adenosylcobalamin are the two coenzyme forms, popular with people who prefer a body-identical form. Delivery route matters less than marketing suggests, since sublingual dosing performed no better than swallowed dosing at the same dose in a randomised comparison (Sharabi 2003).
Timing
Any time of day, with or without food. Supplemental B12 is free cobalamin and does not depend on stomach acid to release it from protein the way food-bound B12 does. Consistency matters more than clock time.
Safety profile
No UK or EU upper intake level is set for vitamin B12 and excess is excreted in urine. High oral doses in studies carry a benign safety record. Very high doses occasionally produce acne-like skin eruptions that settle on stopping.
Special populations
Adults over 65, people on metformin or long-term acid suppressants, vegans and vegetarians, and anyone after stomach or ileal surgery carry higher deficiency risk and benefit most from testing first. Confirmed deficiency with neurological symptoms, and pernicious anaemia, belong with a GP under NICE NG239 rather than self-supplementation.
Interactions
Long-term metformin use lowers B12 absorption. Proton pump inhibitors and H2 blockers reduce absorption of food-bound B12 specifically. Both groups are flagged for periodic B12 testing in the NIH ODS professional fact sheet and NICE NG239.
Guideline positions
NICE NG239 (2024): intramuscular hydroxocobalamin is UK first-line for confirmed deficiency, with high-dose oral considered where malabsorption is not suspected. NIH ODS: intrinsic factor saturates at roughly 1.5 to 2mcg and passive absorption of about 1% underpins high-dose oral protocols; no evidence that supplement absorption varies by form. UK reference nutrient intake: 1.5mcg per day for adults.
Practical framework
Decide in three steps. First, establish whether you need correction or maintenance: symptoms or risk factors mean a serum B12 test with your GP or a home panel comes before any purchase. Second, for maintenance on a plant-based or low-animal-food diet, any of the four forms at a modest dose does the job, and price and format can decide. Third, if you want a single coenzyme form at a measurable per-drop dose, methylcobalamin liquid is the worked example on our shelf at nutritailor.co.uk/products/vitamin-b12-methylcobalamin-liquid. Confirmed deficiency, especially with neurological symptoms, goes to your GP first.
Common misconceptions
That methylcobalamin is better absorbed than cyanocobalamin: comparative human evidence does not establish superiority, and cellular processing converts either into both coenzymes. That drops or sublingual tablets bypass the gut: measured outcomes matched swallowed tablets at the same dose (Sharabi 2003). That a normal serum B12 always settles the question: functional markers can disagree, covered in the related entries.
Sources
- National Institute for Health and Care Excellence (NICE, UK government) 2024. Vitamin B12 deficiency in over 16s: diagnosis and management (NG239). National Institute for Health and Care Excellence (NICE).
- NIH Office of Dietary Supplements 2025. Vitamin B12: Fact Sheet for Health Professionals. National Institutes of Health, Office of Dietary Supplements.
- Sharabi A, Cohen E, Sulkes J, Garty M 2003. Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route. British Journal of Clinical Pharmacology 56(6):635-8, DOI 10.1046/j.1365-2125.2003.01907.x, PMID 14616423.