Vitamin B12 (cobalamin) is an essential vitamin your body cannot make, and its best-proven benefit is preventing and correcting deficiency. Human trials of B12 for heart disease, thinking and memory, and child growth have mostly found little or no effect, and cancer findings are mixed. If your levels are normal, the studies below give no solid reason to expect extra B12 to do more.
Quick facts about vitamin B12
| Item | What the research shows |
|---|---|
| Forms | Cyanocobalamin (most common in supplements), methylcobalamin, adenosylcobalamin, hydroxycobalamin. Cyanocobalamin and hydroxycobalamin are also given as prescription injections; a prescription nasal gel exists. |
| Everyday intake level | The adult Recommended Dietary Allowance is 2.4 mcg a day (2.6 in pregnancy, 2.8 while breastfeeding), per the NIH Office of Dietary Supplements fact sheet. |
| Doses used in studies | From 0.9 mcg a day in children to 1,000–2,000 mcg in oral-versus-injection trials (details below). |
| Typical study length | Weeks to months for deficiency and children’s trials; 2 to 7.3 years in cancer and heart trials. |
| Main studied uses | Correcting deficiency, vegan and vegetarian status, child growth and development, heart disease, cognition, cancer risk, neural tube defects in pregnancy. |
| Where it works best | People who are actually deficient or at high risk of deficiency. |
What the human evidence shows
The uses below run from the best-supported to the weakest. Terms like “low-certainty” and “meta-analysis” are explained in our guides to meta-analyses and systematic reviews and to evaluating evidence quality.
1. Correcting and preventing deficiency
B12 is required for nerve function, red blood cell formation and DNA synthesis, according to the NIH fact sheet. Deficiency can cause megaloblastic anemia, fatigue, numbness and tingling in the hands and feet, a sore tongue, palpitations and memory problems. Because the body stores about 1 to 5 mg, symptoms can take years to appear.
Tablets versus injections in adults. A 2018 Cochrane review, summarized in the NIH fact sheet, pooled three randomized trials with 153 participants. They compared very high oral doses (1,000–2,000 mcg) with intramuscular B12 for deficiency. The evidence was low quality, but oral doses normalized blood B12 about as well as injections. The same fact sheet says injections are typically used because they bypass absorption barriers, especially in pernicious anemia, where the stomach does not make intrinsic factor. Harvard Health notes that in pernicious anemia, pills may not work and injections may be needed. Which route suits you is a clinician’s decision based on the cause of the deficiency.
Children with deficiency anemia. A 2025 systematic review in Nutrition Reviews found only one eligible randomized trial, rated high risk of bias. After 3 months, B12 levels were higher with injections than with oral therapy (median 653 vs 506 pg/mL), and hemoglobin rose more (2.7 vs 0.5 g/dL). The review gave no safety data and urged caution, so one weak trial cannot settle the question. Read the review.
Vegans. A 2024 systematic review and meta-analysis (19 studies in the review, 17 in the meta-analysis) found vegan adults had lower serum B12 and higher homocysteine than omnivores. Vegans who used B12 supplements had better results on every marker than unsupplemented vegans. Read the review. These are blood markers rather than symptoms (see our guide to surrogate outcomes).
2. Child growth, development and thinking
A 2026 Cochrane review (no dedicated funding) pooled 16 randomized trials with 4,083 children under 12. Compared with placebo:
- B12 reduced deficiency (1.8% vs 7.6%; 3 trials, 642 children; high-certainty evidence) and probably raised blood B12 by about 68.5 pmol/L.
- It may make little or no difference to height (2 trials, 143 children), attention and thinking (2 trials, 438 children), motor development or anemia, all low-certainty.
- Adding B12 to other micronutrients also made little or no difference to growth, cognition or development.
No placebo-controlled trial reported adverse effects, so safety in children is not well described. Read the Cochrane review.
3. Heart disease and stroke
B12 helps lower homocysteine, which is linked to heart disease, so researchers tested whether supplements cut cardiovascular risk. The NIH fact sheet reports they lower homocysteine but not cardiovascular events. A Cochrane review of 15 studies (71,422 participants) concluded B12 alone or with other B vitamins does not prevent heart attacks or reduce deaths. A follow-up of the B-PROOF trial (1,298 people; 400 mcg folic acid plus 500 mcg B12 daily; median 54 months) found no effect on cardiovascular risk. Two cohort meta-analyses, covering about 370,000 and 390,000 people, found no link between dietary B12 and coronary heart disease or stroke.
4. Memory, dementia and mood
Observational studies link low B12 to poorer cognition, but some find no link. The NIH fact sheet states randomized trials showed no cognitive improvement in older adults, with or without dementia, from B12 alone or combined with folic acid or B6. Some observational studies and a meta-analysis link low B12 intake to depression. That shows an association only, not that supplements treat depression.
5. Cancer risk
The evidence is mixed in both directions, per the NIH fact sheet:
- Higher B12 and higher risk: In a UK cohort of 757,185 people, those with blood B12 above 813 pg/mL had a 1.74 to 4.72 times higher 1-year cancer risk. In the VITAL cohort (77,118 adults aged 50–76), using at least 55 mcg/day for about 10 years was linked to 40% higher lung cancer risk in men, but not in women.
- Trials: In B-PROOF (2,524 people, 500 mcg B12 plus 400 mcg folic acid for 2–3 years), colorectal cancer was 3.4% vs 2% with placebo. Folic acid may explain this, and overall cancer risk did not differ. A meta-analysis of 18 trials (74,498 people, 20–2,000 mcg/day B12, 2–7.3 years) found little or no effect on cancer incidence, cancer deaths or total mortality.
- Lower B12 and higher risk: Male smokers with low B12 had 5.8 times the gastric cancer risk in one cohort.
A May 2026 ScienceDaily/The Conversation article adds that high blood B12 in cancer patients is often thought to be a marker of disease rather than a cause. It cautions against long-term high-dose supplements without medical advice. Observational studies cannot prove cause either way.
6. Neural tube defects in pregnancy
A 2025 meta-analysis of 38 studies (2,316 pregnancies with neural tube defects, 4,298 controls) found lower maternal blood B12 in affected pregnancies (standardized mean difference -0.23). The difference persisted when folate levels were similar. These are comparisons of blood levels, not trials of B12 supplements. The authors suggested monitoring B12 and considering B12 with folic acid; discuss pregnancy planning with a clinician. Read the review.
7. Fibromyalgia
A 2025 meta-analysis of 25 studies (three measured B12) found no difference in B12 levels between people with fibromyalgia and healthy people. It does not show B12 treats fibromyalgia. Read the review.
Energy, hair, testosterone and “men’s” benefits
Fatigue is a deficiency symptom, so fixing a real deficiency can matter for tiredness. For people with normal B12, this page cites no controlled human study showing extra B12 raises energy, improves hair or raises testosterone. Treat such claims as unproven. Men over 50 are more likely to be low because stomach acid tends to fall with age, and Harvard Health reports about 3.2% of adults 50 and older have very low levels and up to 20% borderline. A blood test answers the question better than a “tonic”.
Is it good to take B12 daily?
The RDA is a daily amount for healthy people, and most people in the U.S. already meet it from food. NIH data show mean intake from food of 5.84 mcg a day for men and 3.69 mcg for women, and about 24% of men and 29% of women use a B12-containing supplement. Whether a supplement helps you depends on whether you are low or at risk. Harvard Health’s expert says it is warranted for people who eat no animal products and sensible for older adults.
Doses used in studies
These are amounts researchers used, not recommendations.
| Study setting | Dose used |
|---|---|
| Oral vs injection for deficiency (3 trials, adults) | 1,000–2,000 mcg oral |
| Crohn’s disease deficiency (study cited by NIH) | 1,000 mcg/day oral cyanocobalamin |
| After gastric bypass (study cited by NIH) | 1,000 mcg/day oral methylcobalamin vs hydroxycobalamin injections |
| Children’s trials (Cochrane) | 0.9–150 mcg/day, 28 days to 31 months |
| B-PROOF trial | 500 mcg/day plus 400 mcg folic acid, 2–3 years |
| Cancer trials meta-analysis | 20–2,000 mcg/day, 2–7.3 years |
| VITAL observational cohort | at least 55 mcg/day, about 10 years |
Absorption falls as the dose rises. The NIH fact sheet says about 50% is absorbed at doses under 1–2 mcg, but only about 2% at 500 mcg and 1.3% at 1,000 mcg. It also says no evidence shows absorption differs by form, and that oral and sublingual forms appear equally effective.
Safety, side effects and medicine interactions
- Medicines that can lower B12: The NIH fact sheet names long-term metformin and proton pump inhibitors. Cleveland Clinic also lists H2 blockers and oral birth control pills. Do not stop or change any of these on your own; ask the prescriber about B12 testing.
- Form and kidneys: A Harvard Health expert prefers methylcobalamin over cyanocobalamin because studies suggest cyanocobalamin may impair kidney function in people with borderline kidney problems. The NIH fact sheet says absorption does not differ by form. If you have kidney disease, ask your clinician which form is suitable.
- High doses over years: See the cancer findings above. The evidence is observational or confounded, and not settled.
- Injections: Cleveland Clinic and NIH describe them as a prescription treatment. The reviews above gave little safety data for injections or for children.
Who should talk to a clinician first
- Anyone with numbness, tingling, balance problems, memory changes or marked tiredness. These can have other causes and should be tested, not guessed.
- Adults 65 and older, since one Harvard-affiliated geriatrician reports checking from that age.
- Vegans and vegetarians, and infants’ mothers on vegan diets, because breastfed infants can develop deficiency early.
- People with pernicious anemia, celiac or Crohn’s disease, atrophic gastritis, or stomach or weight-loss surgery.
- People taking metformin, acid-reducing medicines or birth control pills.
- People with kidney problems, a cancer history, or a persistently high B12 result without supplement use.
- Anyone pregnant, planning pregnancy or breastfeeding.
How deficiency is checked
The NIH fact sheet says most labs call serum B12 below 200 or 250 pg/mL subnormal. If the result is 150–399 pg/mL, experts suggest a methylmalonic acid (MMA) test to confirm, since MMA is the most sensitive marker. MMA also rises with kidney problems and age, and homocysteine is less specific. About 3.6% of U.S. adults are deficient by the under-200 cutoff, and about 12.5% fall below 300 pg/mL.
Foods, depletion and quick fixes
B12 occurs naturally only in animal foods. Fortified cereals and nutritional yeast also supply it. Amounts per serving, from the NIH fact sheet (using USDA data):
| Food | B12 (mcg) |
|---|---|
| Beef liver, 3 oz | 70.7 |
| Nutritional yeast, fortified, about ¼ cup | 8.3–24 (check label) |
| Oysters, 3 oz | 14.9 |
| Salmon, 3 oz | 2.6 |
| Tuna, canned light, 3 oz | 2.5 |
| Ground beef, 3 oz | 2.4 |
| Milk, 1 cup | 1.3 |
| Plain fat-free yogurt, 6 oz | 1.0 |
| Fortified cereal, 1 serving | 0.6 |
| Cheddar, 1½ oz | 0.5 |
| Turkey breast, 3 oz | 0.3 |
Are 2 eggs enough? Eggs count as a source, but check the egg amount in USDA’s FoodData Central against the 2.4 mcg adult RDA instead of relying on a rule of thumb. Bioavailability also differs: about three times higher from dairy than from meat, fish or poultry, and supplements are absorbed about 50% better than food.
What depletes B12? Reduced stomach acid with age, acid-suppressing drugs, metformin, weight-loss or stomach surgery, pernicious anemia, Crohn’s or celiac disease, vegan diets without fortified foods or supplements, and heavy alcohol use (per NIH, Harvard Health and Cleveland Clinic).
What are the first signs of low B12? Harvard Health says early signs can be so subtle they go unnoticed, such as muscle weakness. Later signs include extreme fatigue, numbness or tingling, balance problems, confusion, depression and anemia. Cleveland Clinic lists reduced appetite and weight loss among the physical symptoms.
What raises B12 quickly? The studies above show blood levels rise with injections, high oral doses and sublingual forms. The only pediatric randomized trial found a bigger rise with injections. Which route you need depends on the cause, so confirm it with a clinician.
What is still unknown
- Whether long-term high doses raise cancer risk, and whether high blood levels cause or merely signal disease.
- Whether extra B12 helps energy, mood, hair or hormone levels in people with normal B12.
- Whether injections beat high-dose tablets in children and in most adults, since the trials were few and low quality.
- Safety of B12 in children and with injections, because trials reported few safety data.
How to read a B12 label
- Amount: Look for micrograms (mcg) of B12 per serving, and the % Daily Value. The FDA’s Daily Value is 2.4 mcg for adults, per the NIH fact sheet. Typical NIH-reported ranges are 5–25 mcg in multivitamins, 50–500 mcg in B-complex products and 500–1,000 mcg in B12-only products.
- Form: Cyanocobalamin, methylcobalamin, adenosylcobalamin or hydroxycobalamin. NIH says absorption does not appear to differ by form, so a different name does not mean more B12.
- Delivery: Sublingual tablets and lozenges appeared no more effective than ordinary tablets.
- Combinations: Folic acid is often paired with B12. In the B-PROOF colorectal cancer finding, folic acid may be the cause.
Before you order
- Ask a clinician whether a blood test (B12, and MMA if borderline) is right for you before buying anything.
- Check your medicine list for metformin, acid reducers or birth control pills and ask about B12.
- Look up the label amount and form.
- Ignore claims of cures for energy, hair or hormones unless a controlled human study backs them.
By ClinicalStudyConnect.com Research Desk
This article is for general information purposes only and does not constitute medical advice. Consult your doctor or qualified healthcare provider before making changes to your health routine.