Vitamin B12 Supplements: Deficiency, Dosage and Who Needs It
Vitamin B12 is needed for red blood cells, nerves and DNA. Most adults need just 2.4 mcg a day. A supplement is important for vegans, many adults over 50, and people taking metformin or long-term acid-reducing medicines, who are more likely to run low. B12 boosts energy only if you're actually deficient.
Evidence at a glance
| Use | Grade | What the research shows |
|---|---|---|
| Treating B12 deficiency | A | Oral or injected B12 reliably corrects deficiency. |
| Preventing deficiency in vegans | A | Essential because plant foods don't provide reliable B12. |
| Energy in people without deficiency | D | No evidence it boosts energy if your levels are normal. |
| Memory and cognition in healthy older adults | D | Trials in people without deficiency show no clear benefit. |
A: strong, consistent human evidence · B: moderate · C: limited or mixed · D: good evidence of no benefit.
Vitamin B12 is found naturally only in animal foods such as meat, fish, eggs and dairy, plus fortified foods. Your body stores several years' worth in the liver, so deficiency develops slowly, and can be advanced before it's noticed.
Who is most at risk
| Group | Why |
|---|---|
| Vegans and many vegetarians | Little or no B12 in the diet |
| Adults over 50 | Less stomach acid to release B12 from food |
| Metformin users | Long-term use lowers B12 absorption |
| Long-term PPIs or H2 blockers | Reduced stomach acid |
| Pernicious anemia | The body can't make intrinsic factor needed to absorb B12 |
| Gut surgery or Crohn's disease | Reduced absorption |
The NIH advises adults over 50 to get most of their B12 from supplements or fortified foods, which are absorbed more easily.
Symptoms of deficiency
- Tiredness, weakness and shortness of breath (from anemia)
- Numbness or tingling in hands and feet
- Problems with balance
- Memory problems, confusion or low mood
- Sore, smooth tongue
Nerve damage from long-term deficiency can become permanent, so don't ignore these symptoms. A blood test confirms it.
Forms and doses
- Cyanocobalamin: stable, inexpensive and well studied.
- Methylcobalamin: an active form; works well but has no proven advantage for most people.
Because only a small portion of a large oral dose is absorbed, supplements often contain 25–1,000 mcg. High oral doses can correct deficiency even in many people with absorption problems. Injections are used for severe deficiency or pernicious anemia.
Safety
No upper limit has been set; excess is excreted. B12 can interact with some medicines: metformin, PPIs, H2 blockers and chloramphenicol may lower levels. Our dose calculator shows the daily target for your age.
Sources
This page is for education and is not a substitute for advice from your doctor. Read our medical disclaimer.