Short answer. Adults need 2.4 mcg of vitamin B12 a day and children aged 1 to 13 need 0.9 to 1.8 mcg, according to the NIH Office of Dietary Supplements. No upper limit has been set. Methylcobalamin, adenosylcobalamin, hydroxocobalamin and cyanocobalamin all raise B12 levels, and the NIH finds no evidence that absorption differs by form.
The four forms of vitamin B12 at a glance
| Form | Where you find it | What to know |
|---|---|---|
| Cyanocobalamin | The most common form in supplements and fortified foods | Synthetic; becomes active after conversion; used in most of the dosing studies below |
| Methylcobalamin | Lozenges, tablets, B-complexes | One of the two forms cells use; the methyl group is removed inside the cell and added back as needed |
| Adenosylcobalamin | Usually paired with hydroxo- or methylcobalamin | The second working form cells use; usually sold in combination with another form |
| Hydroxocobalamin | Prescription injections, some tablets and lozenges | Becomes active after the body converts it to the two working forms |
| All forms | No evidence that absorption from supplements varies by form (NIH ODS) | |
Source: NIH Office of Dietary Supplements; Paul and Brady 2017 (PMID 28223907).
What does vitamin B12 do in the body?
Vitamin B12 is a water-soluble vitamin that contains the mineral cobalt, which is why its forms are called cobalamins. The body needs it for the development and normal function of the central nervous system, for healthy red blood cell formation, and to make DNA. With folate and vitamin B6, it helps keep blood homocysteine in the normal range.
Inside cells, B12 works in two forms that serve two enzymes. Methylcobalamin helps convert homocysteine into the amino acid methionine, and adenosylcobalamin helps the body break down propionate, a short-chain fatty acid. A 2017 review of the research found that every form you swallow, whether from food or a supplement, is first stripped down to a core cobalamin molecule. The cell then builds its own methylcobalamin and adenosylcobalamin, and the ratio between them does not depend on the form you took (Paul and Brady 2017). In other words, taking methylcobalamin does not mean the methyl group ends up where you hoped.
The body stores about 1 to 5 mg of B12, roughly 1,000 to 2,000 times what a person eats in a day. That is why low intake can take several years to show up (NIH Office of Dietary Supplements).
How much vitamin B12 do children and adults need?
Infants have an Adequate Intake based on what healthy breastfed babies get. From age 1 there is a Recommended Dietary Allowance (RDA). The amounts are measured in micrograms (mcg), millionths of a gram.
| Age | Recommended daily amount | Upper limit (UL) |
|---|---|---|
| 0–6 months | 0.4 mcg (Adequate Intake) | Not set |
| 7–12 months | 0.5 mcg (Adequate Intake) | Not set |
| 1–3 years | 0.9 mcg | Not set |
| 4–8 years | 1.2 mcg | Not set |
| 9–13 years | 1.8 mcg | Not set |
| 14–18 years | 2.4 mcg | Not set |
| 19 years and older | 2.4 mcg | Not set |
| Pregnancy | 2.6 mcg | Not set |
| Lactation | 2.8 mcg | Not set |
Source: NIH Office of Dietary Supplements.
The Daily Value on labels is 2.4 mcg for adults and children 4 and older. Supplements are usually far above it: multivitamins typically carry 5 to 25 mcg, B-complexes 50 to 500 mcg, and B12-only products 500 to 1,000 mcg or more. A 2,500 mcg tablet is about 1,000 times the adult RDA. That is common for B12, but a child's dose still belongs with the pediatrician.
Can you get enough vitamin B12 from food?
Most people in the US do. In the 2017 to 2020 national survey, men ate an average of 5.84 mcg a day and women 3.69 mcg, and only 5% of men and 11% of women fell below the average requirement (NIH Office of Dietary Supplements). B12 occurs naturally only in animal foods. Plant foods have none unless they are fortified, as many breakfast cereals and nutritional yeasts are.
| Food and serving | Vitamin B12, mcg |
|---|---|
| Beef liver, pan fried, 3 ounces | 70.7 |
| Clams, cooked, 3 ounces | 17 |
| Nutritional yeast, fortified, about ¼ cup | 8.3 to 24 (check label) |
| Atlantic salmon, cooked, 3 ounces | 2.6 |
| Light tuna, canned in water, 3 ounces | 2.5 |
| Ground beef, 85% lean, 3 ounces | 2.4 |
| Milk, 2%, 1 cup | 1.3 |
| Plain yogurt, 6 ounces | 1.0 |
| Egg, 1 large | 0.5 |
Source: NIH Office of Dietary Supplements.
B12 from dairy appears to be about three times better absorbed than B12 from meat, fish and poultry, and B12 from supplements about 50% better than from food. Families who eat no animal foods rely on fortified foods or a supplement, and the NIH notes that these substantially reduce the risk of running low.
Who is most likely to run low on vitamin B12?
Low B12 is usually a problem of absorption rather than diet. B12 in food is bound to protein. Stomach acid and enzymes release it, and it is then absorbed with the help of intrinsic factor, a protein made by the stomach. B12 in supplements and fortified foods is already free, so it skips the release step. According to the NIH, the groups most at risk are:
- Older adults. Depending on the definition, 3% to 43% of older adults living at home have low B12, often because the stomach makes less acid and intrinsic factor with age.
- People who have had stomach or bowel surgery, including weight-loss surgery, or who have digestive conditions that affect the small intestine.
- Vegans and vegetarians who do not use fortified foods or supplements.
- Breastfed infants of mothers who eat no animal foods. These babies can have very small reserves and can run low early in life, so the mother's own B12 matters during pregnancy and breastfeeding.
- People who take metformin or acid-reducing medicines for a long time (see the interactions section below).
Signs that justify a blood test include tiredness, a pale complexion, a sore or swollen tongue, a racing heartbeat, and numbness or tingling in the hands and feet. The NIH notes that nerve symptoms can appear without anemia, so early testing matters. None of these signs is specific to B12.
How is vitamin B12 tested?
The usual first test is serum B12. Most laboratories call a value below 200 or 250 pg/mL low. Because a single B12 result can mislead, experts suggest also checking methylmalonic acid (MMA) when serum B12 falls between 150 and 399 pg/mL; MMA is the most sensitive marker of B12 status. Homocysteine rises when B12 runs low, but it also rises with low folate and reduced kidney function, so it is less specific (NIH Office of Dietary Supplements). It makes sense to test before starting a high-dose supplement, so there is a baseline to compare against.
One more reason to test rather than guess: large amounts of folate can correct the anemia of B12 deficiency without fixing the nerve damage, which can hide a deficiency until the nerve effects become permanent (NIH Office of Dietary Supplements, folate fact sheet). Anyone taking high-dose folate should know their B12 status.
Is methylcobalamin better than cyanocobalamin?
For raising B12 levels, the evidence does not show a winner. The NIH states that no evidence indicates absorption from supplements varies by form. The 2017 review mentioned above concluded that for most people all forms likely have similar bioavailability and effects, although its authors, one of whom works for a supplement company, still prefer the natural forms over cyanocobalamin (Paul and Brady 2017).
What matters more is the dose. When a person cannot absorb food B12 well, the body relies on passive absorption, which takes in only a small share: about 2% of a 500 mcg dose and 1.3% of a 1,000 mcg dose, or roughly 10 to 13 mcg (NIH Office of Dietary Supplements). Several trials show what this means in practice:
- In a 16-week trial of 120 older people with mild deficiency, daily cyanocobalamin at 2.5, 100, 250, 500 and 1,000 mcg lowered MMA by 16%, 16%, 23%, 33% and 33%. It took 647 to 1,032 mcg a day to reach 80% to 90% of the maximum effect, about 270 times the RDA (Eussen 2005).
- In 38 newly diagnosed adults, 2,000 mcg of oral cyanocobalamin daily raised mean serum B12 to 1,005 pg/mL after 4 months, compared with 325 pg/mL on monthly injections (Kuzminski 1998).
- A Cochrane review of three trials with 153 participants found that 1,000 mcg a day by mouth raised serum B12 about as well as injections, although the evidence was of low quality (Wang 2018).
Families who have been advised to avoid the methyl form can use hydroxocobalamin or adenosylcobalamin instead. That is a reasonable preference, but no form has been shown to work better for raising B12.
Are B12 lozenges better than swallowed tablets?
Probably not by much. In a small trial of adults with low B12, 500 mcg a day either dissolved under the tongue or swallowed raised serum B12 by similar amounts in 4 weeks, with no significant difference between groups (Sharabi 2003). The NIH also finds no difference in efficacy between oral and sublingual forms. Choose the format a person will take every day: a lozenge for someone who cannot swallow pills, a capsule for someone who dislikes the taste.
Can you take too much vitamin B12?
The Food and Nutrition Board did not set an upper limit for B12 because of its low potential for toxicity. The NIH describes it as generally safe even at large doses, because the body does not store the excess. That does not make more better. The NIH notes that B12 supplements do not appear to improve energy or athletic performance in people whose B12 status is already adequate. A high dose is a choice to make with a clinician, not a default.
Does vitamin B12 interact with medicines?
Two common medicine groups lower B12. Anyone who takes them regularly should ask about B12 testing.
- Acid-reducing medicines, including proton pump inhibitors such as omeprazole and lansoprazole and H2 blockers such as cimetidine, make it harder to release B12 from food. In a study of 25,956 people with new B12 deficiency and 184,199 without, a 2-year or longer supply of a proton pump inhibitor was linked to 1.65 times the odds of deficiency, and more than 1.5 pills a day to 1.95 times (Lam 2013).
- Metformin may reduce B12 absorption. In a US study across 27 centers, low or borderline B12 was found in 19.1% of people on metformin versus 9.5% on placebo after 5 years, and in 20.3% versus 15.6% after 13 years. Each year of use raised the odds of deficiency by a factor of 1.13, and the authors suggested routine B12 testing (Aroda 2016).
Which ASDHelp products contain vitamin B12?
Doses are copied from each label. B12 has no upper limit, so none of these doses is above a UL; check combination products for their other nutrients.
- B 12 Adenosyl/Hydroxy: 2,500 mcg as adenosylcobalamin and hydroxocobalamin per quick-dissolve tablet, with no methylcobalamin. Label directions: 1 tablet a day; no children's dose on the label.
- 3-B12-Folate: 3,000 mcg as methylcobalamin, hydroxocobalamin and adenosylcobalamin per lozenge, with 680 mcg DFE of folate as 5-MTHF. Label directions: 1 lozenge a day; no children's dose on the label.
- Spectrum B Complex + Benfotiamine: 400 mcg as methylcobalamin per capsule, with folate, B6 and the other B vitamins. One capsule once or twice a day.
- Spectrum Formula Chewable + Iron: 50 mcg as methylcobalamin per 4 chewable tablets, inside a children's multivitamin with label directions from age 1.
Questions to take to your doctor
- Should we check serum B12, and MMA if the result is borderline, before starting a supplement?
- Do any of our medicines, such as metformin or omeprazole, call for regular B12 testing?
- What daily dose fits our situation, and is there a reason to prefer one form over another?
- How much B12 and folate do all of our current products add up to each day?
Sources
- NIH Office of Dietary Supplements. Vitamin B12 fact sheet for health professionals, updated July 2, 2025. ods.od.nih.gov
- NIH Office of Dietary Supplements. Folate fact sheet for health professionals. ods.od.nih.gov
- National Academies. Dietary Reference Intakes tables (NCBI Bookshelf). ncbi.nlm.nih.gov/books/NBK545442
- Paul C, Brady DM. Comparative Bioavailability and Utilization of Particular Forms of B12 Supplements With Potential to Mitigate B12-related Genetic Polymorphisms. Integr Med 2017. PMID 28223907
- Eussen SJ et al. Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med 2005. PMID 15911731
- Kuzminski AM et al. Effective treatment of cobalamin deficiency with oral cobalamin. Blood 1998. PMID 9694707
- Wang H et al. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev 2018. PMID 29543316
- Sharabi A et al. Replacement therapy for vitamin B12 deficiency: comparison between the sublingual and oral route. Br J Clin Pharmacol 2003. PMID 14616423
- Lam JR et al. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA 2013. PMID 24327038
- Aroda VR et al. Long-term Metformin Use and Vitamin B12 Deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab 2016. PMID 26900641
These statements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure or prevent any disease. This article is educational and is not medical advice.