B12 is needed for red blood cells and nerves. It comes almost only from animal foods, so vegans, vegetarians and many people over 50 rely on supplements or fortified food.
Calculator Official intake Study doses Best form Works with Safety Sources
AI (adequate intake)
| Men | 4 µg |
|---|---|
| Women | 4 µg |
| Pregnancy | 4.5 µg |
| Breastfeeding | 5 µg |
Safe upper limit: not set
No adverse effects defined, so no safe limit.
RDA (recommended dietary allowance)
| Men | 2.4 µg |
|---|---|
| Women | 2.4 µg |
| Pregnancy | 2.6 µg |
| Breastfeeding | 2.8 µg |
Safe upper limit: not set
No safe limit set.
Over 50, NIH advises getting B12 mainly from fortified food or a supplement (absorption from food drops).
Source: National Academies (NASEM) 2019
These are doses tested in research, not personal advice. Some exceed the upper limit because they were short courses or given under medical care.
1,000-2,000 µg a day
A Cochrane review found 1000 µg a day by mouth raised blood B12 about as well as injections; one trial used 2000 µg a day. Evidence quality was low (3 small trials).
No clear winner
Cyanocobalamin (a good choice among equals)
Why: Cyanocobalamin is the most common form in supplements. No evidence shows that any form of B12 is absorbed better than another.
Alternatives: Methylcobalamin, adenosylcobalamin and hydroxocobalamin work too. Absorption falls at high doses (about 2% at 500 µg), which is why high-dose pills still work.
Large amounts of folic acid can correct the anaemia of B12 deficiency but not the nerve damage, so a B12 problem can go unnoticed. If you take folic acid, make sure your B12 intake is adequate too.
Meat, fish, eggs, dairy, fortified plant milks and nutritional yeast.
Metformin and acid-lowering drugs (PPIs) can lower B12 absorption. High folic acid can hide the anaemia of B12 deficiency.
No upper limit is set; only a small share of a large oral dose is absorbed.
Pregnant, breastfeeding, on medication, or managing a health condition? Check with a doctor or pharmacist before starting a supplement.
Values checked: November 12, 2026.
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