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Vitamin B12

Evidence Level
Very Strong
10 Clinical Trials
7 Documented Benefits
5/5 Evidence Score

Vitamin B12, also known as cobalamin, is a water-soluble vitamin commonly supplemented as cyanocobalamin or methylcobalamin, essential for red blood cell production, neurological function, and DNA synthesis. It supports energy metabolism by aiding in the conversion of food into energy and prevents megaloblastic anemia by facilitating red blood cell formation. B12 is crucial for maintaining nerve health, as it supports myelin sheath production, potentially reducing the risk of neuropathy. It may also improve mood and cognitive function by regulating homocysteine levels, which are linked to neurological disorders. Supplements typically provide 500–1000 mcg/day, with higher doses used for deficiency correction, especially in vegans or those with absorption issues (e.g., pernicious anemia). Side effects are rare but may include mild diarrhea or allergic reactions. Consult a healthcare provider for appropriate dosing, especially if on medications like metformin or with conditions affecting B12 absorption.

Studied Dose 2.4 mcg/day (RDA); deficiency treatment: 1,000–2,000 mcg/day oral (sublingual preferred); methylcobalamin or adenosylcobalamin preferred over cyanocobalamin
Active Compound Methylcobalamin / Cyanocobalamin
Deficiency information View details

Vitamin B12 deficiency affects roughly 6% of US adults under 60 and ~20% over 60. It's especially common in vegans, the elderly, and long-term users of metformin or proton pump inhibitors. Untreated deficiency can cause irreversible neurological damage, so early detection matters.

Common symptoms

  • Fatigue, weakness, and shortness of breath (from megaloblastic anemia)
  • Numbness or tingling in the hands and feet (peripheral neuropathy)
  • Cognitive issues — brain fog, memory problems, difficulty concentrating
  • Mood changes — depression, irritability
  • Sore or swollen tongue (glossitis), mouth ulcers
  • Pale or jaundiced skin
  • Balance problems or unsteady gait (in advanced cases)

At-risk groups

  • Vegans and strict vegetarians (B12 is found almost exclusively in animal foods)
  • Adults aged 60+ (atrophic gastritis reduces B12 absorption)
  • Long-term metformin users (>4 years) — metformin reduces B12 absorption by up to 30%
  • Long-term proton pump inhibitor users (omeprazole, pantoprazole, esomeprazole >1 year)
  • People with pernicious anemia (autoimmune destruction of intrinsic factor)
  • Post gastric bypass or ileal resection surgery patients
  • People with Crohn's disease or celiac disease
When to see a doctor: If you fall into an at-risk group AND experience persistent fatigue plus neurological symptoms (numbness, tingling, balance issues, or memory problems), ask your doctor for a serum B12 test. Methylmalonic acid (MMA) and homocysteine are more sensitive markers when B12 is borderline.
Explore related deficiencies →
Sources: NIH ODS — Vitamin B12 Health Professional Fact Sheet · PMID 28768707 — Vitamin B12 deficiency review (NEJM)

Benefits

Red Blood Cell Formation and Anemia Prevention

B12 is critical for red blood cell (RBC) production and DNA synthesis. Supplementation corrects megaloblastic anemia in B12-deficient individuals by supporting proper RBC maturation, reducing symptoms like fatigue, weakness, and pallor. Studies show that doses of 1,000 µg/day (oral or injectable) rapidly improve hematological parameters in pernicious anemia or dietary deficiency.

Supported by Trial 1, Trial 3

Neurological Health

B12 maintains myelin sheath integrity and supports nerve function. Supplementation can reverse neurological symptoms (e.g., numbness, tingling, cognitive issues) in deficient individuals and may reduce homocysteine levels, a risk factor for neurodegenerative diseases. A 2024 trial in Brazil suggested B12 reduces CCL11 expression in long COVID patients with visuoconstructive deficits, indicating potential neuroprotective effects.

Supported by Trial 2, Trial 1

Cardiovascular Health

B12 lowers homocysteine levels, which are linked to cardiovascular disease (CVD). Trials (e.g., a 2010 study in Turkey) show B12 supplementation improves coronary flow reserve in deficient elderly individuals, potentially reducing CVD risk. However, benefits are less clear in non-deficient populations.

Supported by Trial 8, Trial 1

Energy Metabolism

B12 supports energy production by aiding in the metabolism of fats and carbohydrates. Supplementation may alleviate fatigue in deficient individuals, though evidence for energy enhancement in healthy people is limited.

Supported by Trial 6, Trial 9

Pregnancy and Fetal Development

Adequate B12 supports fetal brain and nervous system development. Trials in Nepal and India (2020–2024) indicate supplementation in deficient pregnant women may reduce preterm birth risk and improve infant metabolic markers, though neurodevelopmental benefits are inconsistent.

Supported by Trial 3, Trial 5

Mental Health and Cognitive Function

B12 deficiency is linked to depression and cognitive decline. Supplementation may improve mood and cognitive symptoms in deficient individuals, but RCTs (e.g., Hong trial) found no significant cognitive benefit in non-deficient older adults with mild deficiency.

Supported by Trial 2, Trial 6

Bone Health

Some studies suggest B12, by lowering homocysteine, may support bone health indirectly. However, the B-PROOF trial (2015) found no significant impact on physical performance or fracture risk in older adults.

Supported by Trial 5

Mechanism of action

1

Red Blood Cell Formation and DNA Synthesis

B12 acts as a cofactor for methionine synthase, an enzyme in the folate cycle that converts 5-methyltetrahydrofolate to tetrahydrofolate, enabling DNA and RNA synthesis. This is essential for erythropoiesis (red blood cell production). In B12 deficiency, folate is trapped as 5-methyltetrahydrofolate, impairing DNA synthesis and leading to megaloblastic anemia (large, immature red blood cells). Supplementation (e.g., 1,000 µg/day oral or injectable) restores methionine synthase activity, normalizing cell division and correcting anemia.

2

Neurological Function and Myelin Maintenance

B12 is a cofactor for methylmalonyl-CoA mutase, which converts methylmalonyl-CoA to succinyl-CoA in the mitochondria, a step in fatty acid and energy metabolism. This supports myelin sheath formation, crucial for nerve insulation and signal transmission. Deficiency leads to methylmalonic acid (MMA) accumulation, disrupting myelin synthesis and causing neurological symptoms (e.g., neuropathy, cognitive impairment). Supplementation reduces MMA and homocysteine levels, restoring nerve function and potentially preventing demyelination. A 2024 trial showed B12 reduces CCL11 expression in long COVID via methyl-dependent epigenetic mechanisms, aiding visuoconstructive function.

3

Homocysteine Metabolism and Cardiovascular Health

B12 facilitates the conversion of homocysteine to methionine via methionine synthase, reducing homocysteine levels, a risk factor for cardiovascular disease (CVD). Elevated homocysteine damages vascular endothelium and promotes thrombosis. Supplementation (e.g., 500 µg/day with folate) lowers homocysteine, improving coronary flow reserve (as seen in a 2010 Turkish trial) and potentially reducing CVD risk in deficient individuals.

4

Energy Metabolism

Through its role in methylmalonyl-CoA mutase, B12 supports the citric acid cycle by producing succinyl-CoA, a key intermediate for ATP production. This enhances energy metabolism in cells, alleviating fatigue in deficient individuals by ensuring efficient fat and carbohydrate breakdown.

5

Fetal Development

B12 supports DNA synthesis and methylation reactions critical for fetal growth and neural tube development. Supplementation in deficient pregnant women ensures adequate methyl group availability for epigenetic regulation, potentially reducing preterm birth risk (as seen in 2024 Nepal and India trials).

Clinical trials

1
Vitamin B12 Supplementation in Older Adults — Clinical Trial

Double-blind, placebo-controlled clinical trial by London School of Hygiene & Tropical Medicine in 201 older adults examining B12 supplementation effects on cognition.

201 older adults.

Modest improvements on certain cognitive measures in B12-deficient subgroup. Note: B12 deficiency is common in elderly (~10-20%) due to atrophic gastritis reducing intrinsic factor; proton pump inhibitors and metformin both impair B12 absorption — long-term users at risk.

2
B12 Supplementation in Diabetic Older Adults — Clinical Trial

27-month clinical trial in Hong Kong in 271 diabetic outpatients aged ≥70 with mild B12 deficiency receiving B12 vs placebo.

271 elderly diabetic patients.

B12 supplementation improved cognitive function and peripheral neuropathy markers vs placebo. Note: metformin is a major risk factor for B12 deficiency — chronic users should have annual B12 status checked. ADA guidelines now recognize this.

3
B12 for Neurodevelopment in Nepalese Infants — Clinical Trial

Population-based, double-blind clinical trial in Nepal in 600 infants at risk of B12 deficiency. (PLoS Med)

600 infants in Nepal.

B12 supplementation improved neurodevelopmental scores in B12-deficient infants. Note: developing world B12 deficiency contributes to neurodevelopmental delays; addressing maternal/infant B12 status is critical public health intervention.

4
Sucrosomial® vs Cyanocobalamin Bioavailability — Clinical Trial

Clinical trial in three centers in Pakistan (April-) in healthy adults with B12 deficiency comparing oral Sucrosomial® B12 vs intramuscular cyanocobalamin.

Adults with B12 deficiency.

Oral Sucrosomial® B12 produced comparable plasma B12 elevation to IM cyanocobalamin in the studied population. Industry-funded but important: high-dose oral B12 (1,000-2,000 µg/day) can effectively treat most cases of B12 deficiency without injections — well-established but underrecognized clinically.

5
B-PROOF: B12 + Folate for Bone Health — Clinical Trial (negative)

Double-blind clinical trial (B-PROOF) in Netherlands in 2,919 adults aged ≥65 with elevated homocysteine receiving 2 years of B12 + folate supplementation.

2,919 elderly with elevated homocysteine.

Primary endpoint negative: B12 + folate did not reduce fracture risk vs placebo. Important rigorous negative trial — homocysteine reduction does not translate to fracture prevention. Pattern of homocysteine reduction trials being negative for hard outcomes (similar VISP and HOPE-2 negative for stroke).

6
B12 in Elderly with Low Serum B12 — Older Trial

Placebo-controlled trial in 39 elderly subjects identified through community screening with low serum B12 receiving supplementation.

39 elderly with low B12.

B12 supplementation modestly improved cognitive measures in the deficient population. Older trial; modest sample.

7
B12 as Epidrug in Long-COVID — Cohort Study

Longitudinal cohort study in Brazil in patients with persistent visuoconstructive deficit (VCD) 10-16 months post-COVID. (2024)

Long-COVID patients with VCD.

B12 supplementation modulated peripheral blood biomarkers. Critical caveat: observational/cohort design, not clinical trial. Long-COVID is heterogeneous; supplement claims for long-COVID should be tempered.

8
Folate + B12 for Coronary Flow Reserve — Trial

Clinical trial in Turkey in elderly subjects with low serum B12 receiving folic acid + B12 supplementation. Outcomes: coronary flow reserve.

Elderly with low B12.

Modest improvements in coronary flow reserve. Note: HOPE-2 (large clinical trial) showed homocysteine reduction with B-vitamins did not reduce CV events overall.

9
Methyl-B12 for Pediatric Autism — Clinical Trial

2016 clinical trial in children with ASD testing methylcobalamin (B12) injections (75 µg/kg every 3 days).

Children with ASD.

Modest improvements in ASD behavioral measures. Note: ASD treatment landscape primarily uses behavioral interventions (ABA, speech therapy, OT); methyl-B12 has emerging but limited evidence; individual MTHFR variants may modulate response.

10
Methyl-B12 for Pediatric ASD Behavior — Pilot Clinical Trial

2010 pilot clinical trial in 30 children with autism testing methylcobalamin injections (64.5 µg/kg every 3 days for 8 weeks).

30 children with ASD (small).

Modest improvements in behavioral and biomarker measures in subset of responders. Pilot only.

Side effects and drug interactions

Common Potential side effects

Gastrointestinal Issues: Nausea, diarrhea, or upset stomach with high-dose oral supplements (e.g., 1,000–2,000 µg/day). High doses may irritate the gastrointestinal tract, particularly in sensitive individuals or with certain formulations (e.g., cyanocobalamin tablets).
Allergic Reactions: Hypersensitivity reactions, including rash, itching, or anaphylaxis, typically linked to additives or preservatives in injectable or oral formulations. Immune response to B12 or excipients (e.g., benzyl alcohol in injections).
Skin Reactions: Acne-like rashes or itching, particularly with high-dose methylcobalamin. Possible alteration in skin microbiota or metabolic pathways, though poorly understood.
Hypokalemia (Low Potassium): Seen in severe B12 deficiency treatment, especially with high-dose injections during rapid correction of megaloblastic anemia. Increased red blood cell production consumes potassium, potentially lowering serum levels.
Headache or Dizziness: Mild headaches or dizziness reported anecdotally, possibly due to rapid metabolic changes during deficiency correction. Potentially related to neurological adjustments or formulation sensitivity.

Important Drug interactions

Metformin — significantly reduces vitamin B12 absorption (up to 30% reduction); regular B12 monitoring recommended for long-term metformin users
Proton pump inhibitors (omeprazole, lansoprazole) and H2 blockers — reduce gastric acid needed for B12 release from food; oral B12 supplements compensate by bypassing this step
Colchicine — impairs B12 absorption; monitor B12 status in long-term colchicine users
Chloramphenicol — may blunt B12-stimulated red blood cell maturation in some patients

Featured in these supplement guides

Brain Fog→Energy & Fatigue→Healthy Brain Aging→Liver Health→Low Iron & Anemia Support→Low Mood & Emotional Wellbeing→Nerve Pain & Numbness→Restless Legs Syndrome→Vertigo & Dizziness→

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Frequently asked questions about Vitamin B12

How much vitamin B12 should I take?

The RDA is only 2.4 mcg per day, but supplements use 500 to 1,000 mcg because only a small fraction is absorbed from a pill. For correcting a deficiency, 1,000 mcg daily by mouth is common. Higher doses are not harmful, since excess water-soluble B12 is simply excreted.

Which form of B12 is best, methylcobalamin or cyanocobalamin?

Both raise B12 levels effectively for most people. Cyanocobalamin is inexpensive and very stable; methylcobalamin and adenosylcobalamin are the active coenzyme forms some prefer. For general supplementation the practical difference is small.

Who is most at risk of low B12?

Vegans and vegetarians (B12 comes from animal foods), older adults (absorption declines with age), people on long-term metformin or acid-reducing medication, and those with pernicious anemia or gut conditions. These groups benefit most from supplementing or getting tested.

Can you take too much vitamin B12?

B12 has no established upper limit and is considered very safe, since the body excretes what it does not use. That said, an unexplained high B12 level on a blood test when you are not supplementing is worth discussing with a doctor.

What is Vitamin B12?

Vitamin B12, also known as cobalamin, is a water-soluble vitamin commonly supplemented as cyanocobalamin or methylcobalamin, essential for red blood cell production, neurological function, and DNA synthesis.

What is Vitamin B12 used for?

Vitamin B12 is researched primarily for Cognitive, Energy, and Mood & Mental Health. B12 is critical for red blood cell (RBC) production and DNA synthesis. Supplementation corrects megaloblastic anemia in B12-deficient individuals by supporting proper RBC maturation, reducing symptoms like fatigue, weakness, and pallor.

What are the signs of Vitamin B12 deficiency?

Vitamin B12 deficiency affects roughly 6% of US adults under 60 and ~20% over 60. It's especially common in vegans, the elderly, and long-term users of metformin or proton pump inhibitors. Untreated deficiency can cause irreversible neurological damage, so early detection matters.

What is the recommended dosage of Vitamin B12?

The clinically studied dose is 2.4 mcg/day (RDA); deficiency treatment: 1,000–2,000 mcg/day oral (sublingual preferred); methylcobalamin or adenosylcobalamin preferred over cyanocobalamin Always follow the product label and check with a healthcare provider for personal advice.

Is Vitamin B12 safe, and does it have side effects?

For most healthy adults, Vitamin B12 is well tolerated at studied doses. Reported effects can include: Gastrointestinal Issues: Nausea, diarrhea, or upset stomach with high-dose oral supplements (e.g., 1,000–2,000 µg/day). High doses may irritate the gastrointestinal tract, particularly in sensitive individuals or with certain formulations (e.g., cyanocobalamin tablets). It may also interact with some medications. Vitamin B12 is not right for everyone, so check with a healthcare provider first if you are pregnant or breastfeeding, have a medical condition, or take prescription medication.

Does Vitamin B12 interact with any medications?

Possible interactions include: Metformin — significantly reduces vitamin B12 absorption (up to 30% reduction); regular B12 monitoring recommended for long-term metformin users Proton pump inhibitors (omeprazole, lansoprazole) and H2 blockers — reduce gastric acid needed for B12 release from food; oral B12 supp… If you take prescription medication, check with a pharmacist or doctor before using it.

How strong is the scientific evidence for Vitamin B12?

NutraSmarts rates the evidence for Vitamin B12 as Very Strong (5 out of 5). It is backed by 10 clinical trials and 8 cited references summarized on this page. A higher rating reflects more, larger, and better-designed human studies.

References(8 citations)

Evidence ratings on NutraSmarts are based on the totality of human clinical research, with emphasis on randomized controlled trials, meta-analyses, and systematic reviews. The references below directly support claims made throughout this page.

  1. Eussen SJ, de Groot LC, Joosten LW, Bloo RJ, Clarke R, Ueland PM, Schneede J, Blom HJ, Hoefnagels WH, van Staveren WA. Effect of oral vitamin B-12 with or without folic acid on cognitive function in older people with mild vitamin B-12 deficiency: a randomized, placebo-controlled trial. Am J Clin Nutr. 2006;84(2):361-70. doi: 10.1093/ajcn/84.1.361.PubMedUsed to support: Oral B12 (with or without folic acid) for 24 weeks in older adults with mild B12 deficiency — modest cognitive findings; backs the page's framing that cognitive improvement occurs primarily in deficient subgroups.
  2. van Wijngaarden JP, Swart KM, Enneman AW, Dhonukshe-Rutten RA, van Dijk SC, Ham AC, Brouwer-Brolsma EM, van der Zwaluw NL, Sohl E, van Meurs JB, Zillikens MC, van Schoor NM, van der Velde N, Brug J, Uitterlinden AG, Lips P, de Groot LC. Effect of daily vitamin B-12 and folic acid supplementation on fracture incidence in elderly individuals with an elevated plasma homocysteine concentration: B-PROOF, a randomized controlled trial. Am J Clin Nutr. 2014;100(6):1578-86. doi: 10.3945/ajcn.114.090043.PubMedUsed to support: B-PROOF trial (n=2,919 elderly with elevated homocysteine): 2 years of B12 + folic acid did not reduce osteoporotic fracture incidence — the key negative result that homocysteine reduction does not translate to fracture prevention.
  3. Lonn E, Yusuf S, Arnold MJ, Sheridan P, Pogue J, Micks M, McQueen MJ, Probstfield J, Fodor G, Held C, Genest J Jr. Homocysteine lowering with folic acid and B vitamins in vascular disease. N Engl J Med. 2006;354(15):1567-77. doi: 10.1056/NEJMoa060900.PubMedUsed to support: HOPE-2 trial (n=5,522, 5-year follow-up): folic acid + B6 + B12 reduced homocysteine ≈25% but did not reduce major cardiovascular events (death, MI, stroke). Backs the page's framing that homocysteine reduction with B-vitamins does not translate to CV event reduction overall.
  4. Strand TA, Ulak M, Hysing M, Ranjitkar S, Kvestad I, Shrestha M, Ueland PM, McCann A, Shrestha PS, Shrestha LS, Chandyo RK. Effects of vitamin B12 supplementation on neurodevelopment and growth in Nepalese Infants: A randomized controlled trial. PLoS Med. 2020;17(12):e1003430. doi: 10.1371/journal.pmed.1003430.PubMedUsed to support: RCT in 600 marginally stunted Nepalese infants (6–11 months): 12 months of B12 improved metabolic markers but did not improve neurodevelopment, growth, or hemoglobin. Authors conclude findings do not justify broad B12 supplementation in this population.
  5. Hendren RL, James SJ, Widjaja F, Lawton B, Rosenblatt A, Bent S. Randomized, Placebo-Controlled Trial of Methyl B12 for Children with Autism. J Child Adolesc Psychopharmacol. 2016;26(9):774-783. doi: 10.1089/cap.2015.0159.PubMedUsed to support: RCT in 57 children with ASD (ages 3–7): methyl-B12 75 μg/kg every 3 days for 8 weeks improved clinician-rated symptoms (CGI-I) and methylation capacity (SAM/SAH ratio); parent-reported behavioral measures did not improve.
  6. Didangelos T, Karlafti E, Kotzakioulafi E, Margariti E, Giannoulaki P, Batanis G, Tesfaye S, Kantartzis K. Vitamin B12 Supplementation in Diabetic Neuropathy: A 1-Year, Randomized, Double-Blind, Placebo-Controlled Trial. Nutrients. 2021;13(2):395. doi: 10.3390/nu13020395.PubMedUsed to support: RCT in 90 metformin-treated T2DM patients with diabetic neuropathy: 1 year of oral methylcobalamin 1000 μg/day improved sural nerve conduction velocity, amplitude, vibration perception threshold, pain scores, and quality of life vs placebo. Supports that B12 correction benefits metformin-induced B12-deficient neuropathy.
  7. Memon NM, Conti G, Brilli E, Tarantino G, Chaudhry MNA, Baloch A, Shafiq A, Mumtaz SU, Qaisar W, Iqtadar S, Abrar S, Kanwal A, Akhtar MH, Latif H, Rabbani F, Ujjan ID, Turroni S, Khan A. Comparative bioavailability study of supplemental oral Sucrosomial® vs. oral conventional vitamin B12 in enhancing circulatory B12 levels in healthy deficient adults: a multicentre, double-blind randomized clinical trial. Front Nutr. 2024;11:1493593. doi: 10.3389/fnut.2024.1493593.PubMedUsed to support: Multicentre RCT across 3 Pakistani centers: oral Sucrosomial® B12 1000 μg/day raised serum B12 levels significantly faster and higher than conventional oral B12 formulations, crossing the deficiency-borderline threshold within 24 hours. Supports that high-dose oral B12 is a viable alternative to injection.
  8. Bertoglio K, Jill James S, Deprey L, Brule N, Hendren RL. Pilot study of the effect of methyl B12 treatment on behavioral and biomarker measures in children with autism. J Altern Complement Med. 2010;16(5):555-60. doi: 10.1089/acm.2009.0177.PubMedUsed to support: Pilot study in 30 children with ASD: subcutaneous methyl-B12 every 3 days for 12 weeks improved CGI scores and increased glutathione redox ratio in 9 of 30 responders. Small pilot only — preceded and motivated the Hendren 2016 RCT cited above.