Vitamin B12 (Cobalamin)
B12 is found almost exclusively in animal products, so deficiency risk is well documented and high in vegans/vegetarians and older adults with reduced absorption. Correcting deficiency has very strong evidence.
1. Overview
What it is
A water-soluble vitamin required for DNA synthesis, red blood cell formation, and maintenance of the myelin sheath around nerves.
How it works
Functions as a cofactor for methionine synthase and methylmalonyl-CoA mutase, enzymes essential for one-carbon metabolism and fatty acid breakdown.
Who should take it
Vegans and vegetarians, adults over 50 (reduced stomach acid impairs absorption), people on metformin or long-term acid-reducing medication, and those with pernicious anemia.
Who should avoid it
No well-established groups need to avoid B12 — it has an excellent safety profile even at high doses because it is water-soluble.
Evidence summary
Extremely well-established nutrient with strong evidence from decades of clinical and epidemiological research; deficiency causes measurable, reversible hematologic and neurological disease.
Scientific mechanism
B12 must bind intrinsic factor (produced in the stomach) for absorption in the terminal ileum; it is then used as a cofactor in two key enzymatic reactions in the body.
2. Scientific Evidence Rating
Very Strong Evidence — based on high-quality human evidence including systematic reviews, meta-analyses, randomized controlled trials, and major clinical guidance where available.
Ratings reflect the overall quality and consistency of human evidence, not marketing claims. Where evidence is mixed, this is stated explicitly rather than rounded up.
3. Safety Rating
4. Is It Worth Taking?
B12 is found almost exclusively in animal products, so deficiency risk is well documented and high in vegans/vegetarians and older adults with reduced absorption. Correcting deficiency has very strong evidence.
5. Benefits
Each benefit is tagged with its own evidence strength — not every claim about a supplement is equally well supported.
- ★★★★★ Prevents and reverses megaloblastic anemia caused by deficiency
- ★★★★☆ Prevents and can partially reverse neurological symptoms of deficiency
- ★★★★★ Essential for vegans/vegetarians who have no reliable dietary source
- ★☆☆☆☆ No evidence of benefit for boosting energy in people who are already B12-replete
6. Risks
Side effects
Very well tolerated; no established toxicity from oral B12 even at high doses, due to limited absorption capacity.
Drug interactions
Metformin and long-term proton pump inhibitor/H2-blocker use reduce absorption over time.
Who should avoid it / use caution
7. Recommended Dosage
8. Best Time To Take
- Any time of day
- Sublingual or with food; large doses are absorbed passively regardless of stomach acid
9. Food Sources
- Meat, poultry, fish
- Eggs and dairy
- Fortified nutritional yeast and plant milks (for vegans)
10. Deficiency Symptoms
Supplementation is most clearly evidence-based when it corrects an actual shortfall like the ones below — not as a routine addition for people who are already sufficient.
- Fatigue and weakness
- Megaloblastic anemia
- Numbness/tingling in hands and feet
- Memory problems and cognitive changes
- Glossitis (sore tongue)
FAQ
Can B12 deficiency be reversed?
Hematologic changes typically reverse fully with treatment; neurological symptoms may only partially reverse if deficiency was prolonged, which is why early correction matters.
Is a B12 injection better than oral supplements?
For most people with simple dietary deficiency, high-dose oral B12 is as effective as injections. Injections are typically reserved for malabsorption conditions like pernicious anemia.
11. Scientific References
Ratings and content on this page are based on the consensus of the following types of high-quality sources. This is educational information, not individual medical advice.
Key Research Papers
- Scoping review: vitamin B12 supplementation in vegans (PMC)
- Review: metformin therapy and B12 deficiency risk (PMC)
General Reference Organizations
⚕️ This page does not diagnose, treat, cure, or prevent any disease. Vitamin B12 (Cobalamin) is not a substitute for a varied diet or professional medical care. Always consult a qualified healthcare provider before starting any new supplement, especially if pregnant, breastfeeding, managing a health condition, or taking medication.