Vitamin B12 is a water-soluble vitamin your body needs to make healthy red blood cells, keep nerves working normally and make DNA. Most adults need 2.4 micrograms (mcg) a day, a little more during pregnancy and breastfeeding, and it comes almost entirely from animal foods, fortified foods and supplements.
Key Takeaways
- Vitamin B12 is needed for red blood cell formation, normal nerve function and DNA synthesis. The adult RDA is 2.4 mcg a day, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding.
- B12 occurs naturally only in animal foods. Fortified cereals and fortified nutritional yeast are the main plant-based options.
- Absorption depends on intrinsic factor, which handles only about 1 to 2 mcg at a time. Only about 1.3% to 2% of a 500 to 1,000 mcg dose is absorbed.
- NIH reports no evidence that cyanocobalamin, methylcobalamin, adenosylcobalamin or hydroxocobalamin differ in absorption from supplements.
- Older adults, vegans and vegetarians, and people who have had stomach or intestinal surgery are more likely to have low B12 status.
- No upper limit has been set. In people with normal B12 status, trials have not shown better memory, mood or athletic performance from extra B12.
Evidence grade: Strong. B12's roles and daily needs are established NIH reference values, and a Cochrane review of randomized trials supports high-dose oral B12 for raising low blood levels; claims about memory, mood, energy, hair and skin in people with normal status are not supported by trials.
What Is Vitamin B12?
Vitamin B12 contains the mineral cobalt, which is why compounds with B12 activity are called cobalamins. It is found naturally in some foods, added to others, and sold as a dietary supplement and a prescription medicine. Adults store about 1 to 5 milligrams, roughly 1,000 to 2,000 times a typical day's intake, so signs of low status can take years to appear.
Most people in the United States get enough. Average daily intake from food is 5.84 mcg for men and 3.69 mcg for women, and only about 5% of men and 11% of women fall below the Estimated Average Requirement of 2 mcg.
How Vitamin B12 Works in the Body
B12 is a helper (cofactor) for two enzymes. Methionine synthase turns homocysteine into methionine, which the body uses to make S-adenosylmethionine, a methyl donor for DNA, RNA, proteins and lipids. Methylmalonyl-CoA mutase helps break down propionate, a short-chain fatty acid; when B12 runs low, methylmalonic acid (MMA) builds up, which is why MMA is used as a lab marker.
Through these enzymes, B12 is required for the development, myelination and function of the central nervous system, healthy red blood cell formation and DNA synthesis. For more on how the B vitamins work together in nerve tissue, see our article on the impact of B vitamins on the nervous system.
How Much Vitamin B12 Do You Need?
The Food and Nutrition Board based the adult RDAs mainly on the amount needed to keep blood cells healthy and serum B12 normal.
| Life stage | Daily amount |
|---|---|
| Birth to 6 months | 0.4 mcg (Adequate Intake) |
| 7 to 12 months | 0.5 mcg (Adequate Intake) |
| 1 to 3 years | 0.9 mcg |
| 4 to 8 years | 1.2 mcg |
| 9 to 13 years | 1.8 mcg |
| 14 years and older | 2.4 mcg |
| Pregnancy | 2.6 mcg |
| Breastfeeding | 2.8 mcg |
Food Sources of Vitamin B12
Plants do not naturally contain B12, but fortified breakfast cereals and nutritional yeasts are well-absorbed sources. Values below are from the NIH fact sheet, based on USDA FoodData Central. The Daily Value (DV) is 2.4 mcg.
| Food | B12 per serving | % DV |
|---|---|---|
| Beef liver, pan fried, 3 oz | 70.7 mcg | 2,944% |
| Clams, cooked, 3 oz | 17 mcg | 708% |
| Nutritional yeast, fortified, about 1/4 cup (check label) | 8.3 to 24 mcg | 346% to 1,000% |
| Salmon, Atlantic, cooked, 3 oz | 2.6 mcg | 108% |
| Ground beef, 85% lean, 3 oz | 2.4 mcg | 100% |
| Milk, 2%, 1 cup | 1.3 mcg | 54% |
| Fortified breakfast cereal (25% DV), 1 serving | 0.6 mcg | 25% |
| Egg, whole, cooked, 1 large | 0.5 mcg | 19% |
| Spinach, kidney beans, bananas, whole wheat bread | 0 mcg | 0% |
B12 from dairy appears to be about three times more available than B12 from meat, fish and poultry, and B12 from supplements about 50% more available than B12 from food.
Forms of Vitamin B12 Compared
Methylcobalamin and adenosylcobalamin are the forms your cells use. Cyanocobalamin and hydroxocobalamin become active after the body converts them.
| Form | What it is | Where you see it | Evidence notes |
|---|---|---|---|
| Cyanocobalamin | Stable synthetic form; the body removes the cyanide group and converts the core | Most common form in supplements and fortified foods; prescription injection | Tested in the oral dose-finding trial in older adults described below |
| Methylcobalamin | Active coenzyme form | Supplements marketed as "active" B12 | 1,000 mcg a day matched hydroxocobalamin injections for normalizing B12 values after gastric bypass |
| Adenosylcobalamin | Active coenzyme form used in mitochondria | Some supplements, often with methylcobalamin | Not studied on its own in the trials cited here |
| Hydroxocobalamin | Natural form found in animal foods | Mainly prescription injections | One of the two injectable forms NIH lists |
Is methylcobalamin better than cyanocobalamin?
No one has shown that it is. NIH states that no evidence indicates absorption from supplements varies by form. A 2015 review explained why: inside cells, every form is stripped to the same core molecule and rebuilt into methylcobalamin or adenosylcobalamin, so the coenzyme forms are unlikely to be advantageous over cyanocobalamin. A 2017 review preferred the natural forms but still concluded that for most people all forms likely work similarly; one of its authors was an executive at a supplement company. Choosing methylcobalamin because cyanocobalamin is synthetic is a preference, not a proven advantage.
Sublingual tablets appear to work no differently from tablets you swallow. A prescription nasal gel also raises blood B12, with about 2% bioavailability from a 1,000 mcg dose in one small study, similar to oral.
How Vitamin B12 Is Absorbed
B12 in food is bound to protein. Stomach acid and enzymes release it, it rides on a saliva protein called haptocorrin, and in the small intestine it binds to intrinsic factor, a protein made by the stomach lining. That pair is absorbed at the end of the small intestine. B12 in supplements and fortified foods is already free, so it skips the acid step.
Intrinsic factor handles about 1 to 2 mcg at a time. At small doses, about 50% is absorbed; at 500 mcg it is about 2%, and at 1,000 mcg about 1.3%. A small share still gets through at high doses, which is why studies of people with absorption problems use 1,000 mcg or more. For practical tips, read how to increase vitamin B12 absorption and how to improve B12 absorption naturally.
Who Is More Likely to Run Low on B12
- Older adults. Depending on the definition, 3% to 43% of older adults living in the community have low serum B12, especially those with atrophic gastritis, which affects 8% to 9% of adults 65 and older and reduces stomach acid and intrinsic factor.
- People with pernicious anemia, an autoimmune condition in which the stomach stops making intrinsic factor. It is the most common cause of clinically evident low B12 worldwide and needs a clinician's care.
- People with stomach or intestinal conditions, such as celiac disease, or who have had gastrointestinal surgery, including weight-loss surgery.
- Vegans and vegetarians, because natural sources are limited to animal foods. Fortified foods and supplements substantially reduce this risk.
- Breastfed infants of mothers who eat a vegan diet, who may have very limited reserves.
Long-term use of certain medicines can also lower B12; see Safety and Interactions. When B12 deficiency develops, NIH describes possible signs including tiredness, pale skin, a sore tongue, palpitations, and numbness or tingling in the hands and feet, sometimes before blood counts change. These signs are not specific, so only testing can tell.
Testing Your B12 Status
- Serum B12: most labs call values below 200 or 250 pg/mL low; cutoffs vary.
- Methylmalonic acid (MMA): the most sensitive marker; above 0.271 micromol/L suggests low status. It also rises with reduced kidney function and age.
- Homocysteine: rises as B12 falls but is also affected by folate and kidney function.
- Holotranscobalamin: the bioactive fraction of B12, recommended by researchers alongside the other markers.
Experts suggest checking MMA when serum B12 is between 150 and 399 pg/mL. Marginal status (200 to 300 pg/mL) without symptoms affects up to 40% of people in Western populations.
Oral B12 vs B12 Injections
B12 injections (cyanocobalamin or hydroxocobalamin) are prescription medicines that bypass the gut, so clinicians use them when absorption is the problem. MedlinePlus describes daily injections for the first week, tapering to once a month, which may continue for life.
High-dose oral B12 is an alternative route many people do not know about. A 2018 Cochrane review of three randomized trials (153 people with low B12) found that 1,000 mcg a day by mouth raised blood B12 about as well as injections, and one trial of 2,000 mcg a day found higher levels with oral dosing. The evidence was low quality because the trials were small; oral dosing cost less. A 2005 trial in 120 older adults with mildly low B12 found that about 647 to 1,032 mcg a day of oral cyanocobalamin was needed for most of the possible drop in MMA, more than 200 times the RDA. Which route fits you is a decision for your healthcare provider.
B12 shot side effects
MedlinePlus lists diarrhea and a feeling of whole-body swelling as side effects to report if severe or lasting. Uncommon but serious reactions include hives, rash, itching, trouble breathing or swallowing, swelling of the arms or legs, fast heartbeat, muscle cramps and dizziness, which need prompt medical attention. A dermatology review notes injection-site allergic reactions, anaphylaxis, acne-like breakouts and rosacea flares with B12 therapy, and reactions in people sensitive to cobalt.
B12 is also sold in clinic "energy" or "skinny" shots. NIH notes that B12 does not appear to improve energy or performance in people with adequate status, and weight loss is not one of its established roles.
What Human Research Has Measured
| Outcome | What trials and reviews found | Certainty |
|---|---|---|
| Raising low blood B12 by mouth | 1,000 to 2,000 mcg a day matched injections in three small RCTs (153 people) | Low to moderate |
| Dose for mildly low status, older adults | About 647 to 1,032 mcg a day for most of the MMA drop (120 people) | Moderate (one RCT) |
| Memory and thinking, normal status | No effect in a 2021 meta-analysis of 16 RCTs (6,276 people); little to no effect in a Cochrane review of 14 studies (27,882 people) | Moderate to high: no benefit |
| Mood scores, normal status | No overall effect on depressive-symptom measures (same 2021 meta-analysis) | Moderate: no benefit |
| Energy and athletic performance | No benefit without a shortfall, per NIH | Moderate: no benefit |
| Homocysteine and heart outcomes | B vitamins lowered homocysteine but not heart attacks or deaths in large trials | High: no effect on events |
| Hair growth | No controlled trials in people with normal status | Very low |
| Skin | Reviews and case reports only | Very low |
Memory and mood
Observational studies often link low B12 with poorer cognitive scores, but trials disagree. In a two-year trial of 2,919 adults 65 and older, 500 mcg of B12 plus 400 mcg of folic acid lowered homocysteine without changing cognitive scores. The 2021 meta-analysis also found no overall effect on depressive-symptom scores. B12 matters for the nervous system when you are low; extra B12 has not been shown to sharpen memory or lift mood when you are not.
Hair and skin
Cells at the base of hair follicles divide rapidly, and B12 is needed for DNA synthesis. A 2019 review called the role of micronutrients in hair growth important but not entirely clear and asked for large placebo-controlled trials. A 2015 dermatology review lists skin darkening, hair and nail changes and mouth changes as signs of low B12 status, and acne-like and allergic reactions as possible effects of B12 therapy. If you notice a rash after starting B12, see our article on B12 skin rash.
Doses Used in Studies
| Setting | Daily dose | Absorbed or result |
|---|---|---|
| Adult RDA | 2.4 mcg | About 50% absorbed at 1 to 2 mcg |
| Typical multivitamin | 5 to 25 mcg | Above the RDA for people who absorb normally |
| B-complex supplements | 50 to 500 mcg | Percentage absorbed falls as dose rises |
| B12-only supplements | 500 to 1,000 mcg | About 10 mcg (2%) and 13 mcg (1.3%) absorbed |
| Older adults, mildly low status (2005 RCT) | 647 to 1,032 mcg | 80% to 90% of the maximum MMA drop |
| Oral vs injection trials (Cochrane) | 1,000 to 2,000 mcg | Serum B12 similar to injections |
| After gastric bypass | 1,000 mcg methylcobalamin | As effective as hydroxocobalamin injections |
Pregnancy and Breastfeeding
Needs rise to 2.6 mcg a day in pregnancy and 2.8 mcg while breastfeeding. Serum B12 tends to fall during pregnancy and usually returns to normal after delivery. NIH notes that low B12 status in pregnant and breastfeeding women might affect a baby's development, and that breastfed infants of mothers who eat no animal foods can have very limited reserves. If you are vegan or vegetarian and pregnant or planning to be, ask your prenatal provider how to cover B12.
Timing and How to Take It
Supplemental B12 is already free from food protein, so it does not depend on stomach acid the way food B12 does. No best time of day has been established, so take it when you will remember it. Sublingual and swallowed forms appear to work the same. B12 is measured in micrograms: a 1,000 mcg tablet is 1 mg.
Stacking Notes: Folate and Other B Vitamins
B12 and folate work in the same methylation cycle. NIH notes that large amounts of folic acid can correct the blood-cell changes of low B12 status but not the nerve effects, raising concern that high folic acid could hide a B12 shortfall. B-complex products often carry 50 to 500 mcg of B12, so add up all your sources. For B-complex amounts used in nerve research, see our guide to vitamin B complex dosage; if your B-complex includes benfotiamine, a form of vitamin B1, see benfotiamine side effects.
Where an OPA Product Fits
OPA Men's Daily Multivitamin includes a B-vitamin complex with thiamine, riboflavin, niacin, vitamin B6, folate, vitamin B12 and pantothenic acid to support energy metabolism and daily wellness, in a two-capsule serving. Its listing does not state the per-serving B12 amount, so check the Supplement Facts panel. NIH says multivitamins typically contain 5 to 25 mcg of B12: more than the 2.4 mcg RDA, but far below the 1,000 to 2,000 mcg used in studies of people with low levels. A multivitamin is not a substitute for testing or for a dose your healthcare provider recommends.
Safety and Interactions
Upper limit. No Tolerable Upper Intake Level has been set because B12 has low potential for toxicity, and NIH considers it generally safe even at large doses. "No upper limit" does not mean more is better: some observational studies linked very high blood B12, or 10 years of 55 mcg a day or more from supplements, with higher risk of certain cancers, including lung cancer in men in one cohort, while a meta-analysis of 18 trials found little or no effect. NIH calls the evidence mixed. Injections carry the side effects described above.
Medications. If you take any of these regularly, talk to your doctor or pharmacist about your B12 status and whether testing makes sense:
- Metformin may reduce B12 absorption. In a long-term U.S. study of 2,155 adults, low B12 occurred in 4.3% of the metformin group vs 2.3% on placebo at 5 years, and low or borderline-low B12 in 19.1% vs 9.5%. Each year of use was linked to about 13% higher odds of low B12, and the authors suggested routine B12 testing be considered.
- Acid-reducing medicines, including proton pump inhibitors such as omeprazole and lansoprazole and H2 blockers such as cimetidine, can reduce absorption of B12 from food. In a study of 25,956 people with low B12 and 184,199 comparison patients, two or more years of use was linked to 65% higher odds for proton pump inhibitors and 25% higher odds for H2 blockers.
Do not change a prescribed medicine on your own. Also talk to a clinician first if you have had stomach or intestinal surgery, are pregnant or breastfeeding (especially on a vegan diet), have reduced kidney function, are sensitive to cobalt or have reacted to B12 before, take high-dose folic acid, or have Leber's hereditary optic neuropathy, an inherited eye condition MedlinePlus lists as a precaution for B12 injections.
Frequently Asked Questions
What is the best form of vitamin B12 to take?
No form is proven best. NIH reports no evidence that absorption differs between cyanocobalamin, methylcobalamin, adenosylcobalamin and hydroxocobalamin, and a 2015 review concluded the coenzyme forms are unlikely to be superior to cyanocobalamin, which is the most common form in supplements. Pick the form and format you will take consistently.
How much vitamin B12 should an adult take a day?
The adult RDA is 2.4 mcg, 2.6 mcg in pregnancy and 2.8 mcg while breastfeeding, and most people who eat animal foods meet it from diet. People who rely on supplements often take more because only a small share of a large dose is absorbed. If your levels are low, your healthcare provider will set the dose.
Can you take too much vitamin B12?
No upper limit has been set, and trials used 1,000 to 2,000 mcg a day; a Cochrane review found oral B12 appeared as safe as injections, though that evidence was very low quality. Injections can cause side effects and rare allergic reactions, and observational links between very high B12 and some cancers remain unresolved.
Does vitamin B12 give you energy or help with mood or weight?
Not if your levels are already adequate. NIH says B12 does not appear to improve energy or performance without a shortfall, a 2021 meta-analysis of 16 trials found no effect on cognitive or depressive-symptom scores in people without low B12, and weight loss is not an established role of B12, including in "skinny shots."
Why do some people feel worse after a B12 shot?
MedlinePlus lists diarrhea and a swollen feeling, plus uncommon reactions such as headache, dizziness, muscle cramps, fast heartbeat, facial redness, rash or itching. Trouble breathing or swallowing, hives or leg swelling needs prompt medical attention. People sensitive to cobalt can react to any B12 form.
Does vitamin B12 help hair growth?
Low B12 status is associated with hair and nail changes, but no controlled trials show that extra B12 grows hair in people with normal levels. If you are shedding more than usual, ask your healthcare provider about testing B12, iron and other nutrients.
Sources
- NIH Office of Dietary Supplements. Vitamin B12: Fact Sheet for Health Professionals. Updated July 2, 2025.
- NIH Office of Dietary Supplements. Folate: Fact Sheet for Health Professionals. Updated November 30, 2022.
- MedlinePlus Drug Information. Cyanocobalamin Injection. U.S. National Library of Medicine.
- Wang H, Li L, Qin LL, Song Y, Vidal-Alaball J, Liu TH. Oral vitamin B12 versus intramuscular vitamin B12 for vitamin B12 deficiency. Cochrane Database Syst Rev. 2018.
- Eussen SJ, de Groot LC, Clarke R, et al. Oral cyanocobalamin supplementation in older people with vitamin B12 deficiency: a dose-finding trial. Arch Intern Med. 2005.
- Obeid R, Fedosov SN, Nexo E. Cobalamin coenzyme forms are not likely to be superior to cyano- and hydroxyl-cobalamin in prevention or treatment of cobalamin deficiency. Mol Nutr Food Res. 2015.
- Paul C, Brady DM. Comparative bioavailability and utilization of particular forms of B12 supplements with potential to mitigate B12-related genetic polymorphisms. Integr Med (Encinitas). 2017.
- Markun S, Gravestock I, Jager L, et al. Effects of vitamin B12 supplementation on cognitive function, depressive symptoms, and fatigue: a systematic review, meta-analysis, and meta-regression. Nutrients. 2021.
- Aroda VR, Edelstein SL, Goldberg RB, et al. Long-term metformin use and vitamin B12 deficiency in the Diabetes Prevention Program Outcomes Study. J Clin Endocrinol Metab. 2016.
- Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013.
- Hannibal L, Lysne V, Bjorke-Monsen AL, et al. Biomarkers and algorithms for the diagnosis of vitamin B12 deficiency. Front Mol Biosci. 2016.
- Almohanna HM, Ahmed AA, Tsatalis JP, Tosti A. The role of vitamins and minerals in hair loss: a review. Dermatol Ther (Heidelb). 2019.
- Brescoll J, Daveluy S. A review of vitamin B12 in dermatology. Am J Clin Dermatol. 2015.
Disclosure: OPA Nutrition sells supplements, including products mentioned in this article. This content is for education only and is not medical advice. Statements about dietary supplements have not been evaluated by the Food and Drug Administration. These products are not intended to diagnose, treat, cure, or prevent any disease. Talk to your healthcare provider before using any supplement, especially if you take medication or have a medical condition.
Written and reviewed by Michael Gonzales, Founder, OPA Nutrition · Last reviewed: · How OPA researches and writes
