Vitamin B12 is a popular supplement. Commonly touted for energy and weight management, research is more supportive of its potential link in preventing dementia, reducing pain, and improving mood among other benefits. To better understand vitamin B12, it's worth reviewing its effects on the body.

The Function of Vitamin B12

The action of vitamin B12 in the body is somewhat complicated. As a vitamin, B12 has two main functions. First, it is involved in the breakdown of certain fats and amino acids. Second, it acts to regenerate methionine. Methionine is an amino acid required for the formation of S-adenosylmethionine (SAMe). SAMe is a “methyl donor” critical for the production of numerous compounds, including deoxynucleic acid (DNA), neurotransmitters (like serotonin), and myelin (the protective sheath around nerve cells). SAMe is also sold as a supplement for helping with pain and for improving mood.

Different Forms of Vitamin B12

Vitamin B12 comes in several different forms that function somewhat differently. The cheapest form, cyanocobalamin, is a precursor to vitamin B12 that can be transformed by the body into the active version of the vitamin. Hydroxycobalamin is another precursor form that is also readily converted into active vitamin B12.

The two active forms of the vitamin are adenosylcobalamin and methylcobalamin. Adenosylcobalamin is utilized for the breakdown of fat and amino acids, whereas methylcobalamin is used as a methyl donor. Methylcobalamin is the active form of the vitamin most commonly available in nutritional supplements.

Vitamin B12 Deficiency

A deficiency of vitamin B12 is surprisingly common. The vitamin is challenging for the body to absorb since its molecule is quite large. It also requires a strong digestive process as both stomach acid and intrinsic factors, secreted by stomach cells, are needed for it to be properly digested and absorbed from food. As such, deficiency often occurs as people get older. Some estimates suggest that as many as one in five older individuals are deficient in vitamin B12. However, even younger individuals can be deficient in the vitamin and standard blood tests do not always accurately indicate when levels are low. To detect low levels, it can be necessary to run additional blood testing that includes functional markers of vitamin B12 deficiency, including homocysteine and methylmalonic acid levels.

Symptoms of B12 deficiency can include both physical and mental symptoms. Fatigue, anemia, peripheral neuropathy (nerve pain in the extremities), dementia, depression, mania, delirium, and psychosis are all possible depending on severity.

A number of conditions can increase the risks for vitamin B12 deficiency. Autoimmune conditions, gastrointestinal diseases, and weight-loss surgery can all decrease absorption of the vitamin. Some common medications can also reduce vitamin B12 assimilation, including medications for heartburn, birth control pills, and the diabetes medication metformin.

Daily Vitamin B12 Requirements

The daily required intake for vitamin B12 in different age groups are listed on the National Institutes of Health’s website:

  • Birth to 6 months: 0.4 mcg
  • 7–12 months: 0.5 mcg
  • 1–3 years: 0.9 mcg
  • 4–8 years: 1.2 mcg
  • 9–13 years: 1.8 mcg
  • 14–18 years: 2.4 mcg
  • 19+ years: 2.4 mcg

These are the bare minimum levels required for most people to avoid a deficiency. Unlike other nutrients, vitamin B12 does not have an established upper limit of safety. In other words, there is no toxic dose officially recognized for vitamin B12, a testament to its potential safety.

Food Sources of B12

Common food sources of vitamin B12 include meat, dairy, eggs, and seafood. Vitamin B12 is primarily found in animal products making it difficult for vegetarians to get enough of the vitamin. However, a recent study suggests that nori, a type of seaweed, may contain enough vitamin B12 to be useful as a vegetarian source of the nutrient.

Health Benefits of B12

As for health benefits, vitamin B12 appears to have a key role in prevention or protection from several conditions, including low energy, dementia, osteoporosis, and heart disease. In addition, research suggests that the nutrient plays a role in mood and may have a place in helping to treat nerve pain.

Low Energy and Anemia

Both vitamin B12 and folate are needed to produce DNA during cellular reproduction. In cases of B12 deficiency, blood cells do not form properly, becoming enlarged and reduced in number. This can lead to anemia, where the number of red blood cells is decreased and oxygen delivery to tissues becomes compromised. Without enough oxygen, energy production can falter leading to weakness and fatigue. Consuming adequate vitamin B12 ensures a ready supply so that blood cell formation is not affected.


Low levels of vitamin B12, along with folate and vitamin B6, can also lead to increased levels of homocysteine. Homocysteine is a toxic amino acid known to damage both the heart and the brain. Both low vitamin B12 and elevated homocysteine are recognized risk factors for developing dementia. Studies have found that individuals with dementia often have nutrient deficiencies, with vitamin B12, folate, and vitamin D being common.

There are also numerous case reports of reversible cognitive decline and dementia from vitamin B12 deficiency—and not just in the elderly. Cognitive decline can also be a sign of B12 deficiency in younger individuals. A published case of rapidly reversible dementia in a 29-year-old woman is a poignant example. Due to how common B12 deficiency is found in older individuals, and its relationship to dementia, maintaining adequate vitamin B12 is critical for maintaining brain function as we age.


Data suggests that high homocysteine and low vitamin B12 correlate both to increased risks of osteoporosis and bone fractures. As bone fractures are a leading cause of disability and death in the elderly, preventing them is of critical importance. Many nutrients throughout the lifespan are crucial for bone health, including Vitamin B12 along with calciummagnesiumpotassiumzincvitamin Dvitamin K, and others.

Heart Health

High homocysteine is generally correlated with an increased risk for both heart disease and stroke. However, trials using B-vitamins to reduce this risk have sometimes failed to improve outcomes, including heart attacks and death from heart disease. Due to the conflicting data, there is a need for more research to better understand the overall relationship between vitamin B12 and heart health.

A recent review highlighted the discrepancies but still recommended lowering homocysteine with folate as a reasonable approach. As vitamin B12 also reduces homocysteine and is recommended with folate supplementation, it would make sense to include vitamin B12 for individuals with high homocysteine. Hopefully, future research can better help us to understand the complex relationship between B-vitamins, homocysteine, and heart disease.

Depression and Low Mood

Interest in the effects of vitamin B12 and mood have been around for decades. Due to the effects of vitamin B12 on neurotransmitter production, it’s understandable. A number of nutrients are key when it comes to the synthesis of serotonin, dopamine, and other neurotransmitters, including vitamin B12.

A clinical trial combining B-vitamins with standard antidepressants found the combination more effective at maintaining mood improvements with medication treatment. An earlier study also found that vitamin B12 levels correlated with better outcomes in depressed patients.

While the evidence is somewhat mixed, a recent review concluded that vitamin B12 supplementation can “delay the onset of depression and improve the effectiveness of antidepressants.” Supplementing with vitamin B12 appears to be a fairly straightforward approach for trying to improve mood. A separate review of the evidence concluded that B12 is likely most helpful when supplemented over the long term.

Nerve and Chronic Pain

Since its discovery, vitamin B12 has been used to treat a number of different pain conditions. While not successful for all types of pain, the evidence does suggest benefits for nerve pain and chronic back pain.

The most well-studied pain condition treated with vitamin B12 is diabetic neuropathy. Diabetes causes elevated blood sugar, which, over time, can cause significant nerve damage. Usually starting in the feet, diabetic neuropathy can become disabling. A number of clinical trials using vitamin B12 for diabetic neuropathy have documented benefits, including a recent trial in 2021. It’s likely that B12 supplementation helps to repair the myelin sheath surrounding the nerves.

While not as well studied, a couple of clinical trials using injectable B12 for chronic back pain also found impressive results. Considering the extent of disability from low back pain, injectable vitamin B12 could make a welcome addition to this challenging-to-treat condition.

The Use of Vitamin B12 for Weight Management

Of interest, vitamin B12 injections are often administered as a tool for enhancing weight management. While a transient increase of energy is not an uncommon side effect after a B12 injection, there is no published clinical evidence that suggests vitamin B12 is effective for reducing weight. Clinical trials are needed to better understand if vitamin B12 has a place in weight loss management.


Vitamin B12 is a critical nutrient for maintaining health. Involved in the processing of fat and protein, along with methylation reactions, it plays a key role in neurotransmitter production, homocysteine levels, DNA production, myelin synthesis, and other systems throughout the body. Getting adequate B12, especially as we age, is crucial for maintaining health, and may help prevent a number of chronic health conditions. The vitamin also appears to be important for maintaining mood and may have a place in pain treatment.


  1. Almeida OP, Ford AH, Flicker L. Systematic review and meta-analysis of randomized placebo-controlled trials of folate and vitamin B12 for depression. Int Psychogeriatr. 2015 May;27(5):727-37. doi: 10.1017/S1041610215000046. Epub 2015 Feb 3. PMID: 25644193.
  2. Almeida OP, Ford AH, Hirani V, Singh V, vanBockxmeer FM, McCaul K, Flicker L. B vitamins to enhance treatment response to antidepressants in middle-aged and older adults: results from the B-VITAGE randomised, double-blind, placebo-controlled trial. Br J Psychiatry. 2014 Dec;205(6):450-7. doi: 10.1192/bjp.bp.114.145177. Epub 2014 Sep 25. PMID: 25257064.
  3. Andrès E, Loukili NH, Noel E, et al. Vitamin B12 (cobalamin) deficiency in elderly patients. CMAJ. 2004;171(3):251-259. doi:10.1503/cmaj.1031155
  4. Buesing S, Costa M, Schilling JM, Moeller-Bertram T. Vitamin B12 as a Treatment for Pain. Pain Physician. 2019 Jan;22(1):E45-E52. PMID: 30700078.
  5. Chiu CK, Low TH, Tey YS, Singh VA, Shong HK. The efficacy and safety of intramuscular injections of methylcobalamin in patients with chronic nonspecific low back pain: a randomised controlled trial. Singapore Med J. 2011 Dec;52(12):868-73. PMID: 22159928.
  6. Chrysant SG, Chrysant GS. The current status of homocysteine as a risk factor for cardiovascular disease: a mini review. Expert Rev Cardiovasc Ther. 2018 Aug;16(8):559-565. doi: 10.1080/14779072.2018.1497974. Epub 2018 Jul 17. PMID: 29979619.
  7. 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 Jan 27;13(2):395. doi: 10.3390/nu13020395. PMID: 33513879; PMCID: PMC7912007.
  8. Herrmann W, Obeid R. Causes and early diagnosis of vitamin B12 deficiency. Dtsch Arztebl Int. 2008;105(40):680-685. doi:10.3238/arztebl.2008.0680
  9. Hintikka J, Tolmunen T, Tanskanen A, Viinamäki H. High vitamin B12 level and good treatment outcome may be associated in major depressive disorder. BMC Psychiatry. 2003 Dec 2;3:17. doi: 10.1186/1471-244X-3-17. PMID: 14641930; PMCID: PMC317314.
  10. Huddar A, Seshagiri DV, Ramakrishnan S, Kenchaiah R. Pearls & Oy-sters: Rapidly Reversible Dementia: Vitamin B12 Deficiency in a 29-Year-Old Woman. Neurology. 2021 Aug 10;97(6):e643-e646. doi: 10.1212/WNL.0000000000012102. Epub 2021 Apr 30. PMID: 33931528.
  11. Ishida M, Taguchi R, Sakaguchi H, Itami K, Yoshioka A, Sato I, Uchida N, Onishi H. Reversible dementia due to vitamin B12 deficiency in a lung cancer patient: Relevance of preoperative evaluation. Palliat Support Care. 2021 Jun;19(3):377-379. doi: 10.1017/S1478951521000481. PMID: 33947505.
  12. Lam JR, Schneider JL, Zhao W, Corley DA. Proton pump inhibitor and histamine 2 receptor antagonist use and vitamin B12 deficiency. JAMA. 2013 Dec 11;310(22):2435-42. doi: 10.1001/jama.2013.280490. PMID: 24327038.
  13. Mauro GL, Martorana U, Cataldo P, Brancato G, Letizia G. Vitamin B12 in low back pain: a randomised, double-blind, placebo-controlled study. Eur Rev Med Pharmacol Sci. 2000 May-Jun;4(3):53-8. PMID: 11558625.
  14. McNulty H, Ward M, Hoey L, Hughes CF, Pentieva K. Addressing optimal folate and related B-vitamin status through the lifecycle: health impacts and challenges. Proc Nutr Soc. 2019 Aug;78(3):449-462. doi: 10.1017/S0029665119000661. Epub 2019 Jun 3. PMID: 31155015.
  15. National Institutes of Health. Vitamin B12. Updated April 6, 2021. Accessed February 11, 2022.
  16. Sangle P, Sandhu O, Aftab Z, Anthony AT, Khan S. Vitamin B12 Supplementation: Preventing Onset and Improving Prognosis of Depression. Cureus. 2020 Oct 26;12(10):e11169. doi: 10.7759/cureus.11169. PMID: 33251075; PMCID: PMC7688056.
  17. Sashindran VK, Aggarwal V, Khera A. Prevalence of Vitamin B12 deficiency in elderly population (>60 years) presenting with dementia to outpatient department. Med J Armed Forces India. 2022 Jan;78(1):94-98. doi: 10.1016/j.mjafi.2020.11.003. Epub 2021 Feb 23. PMID: 35035050; PMCID: PMC8737092.
  18. Shipton MJ, Thachil J. Vitamin B12 deficiency - A 21st century perspective. Clin Med (Lond). 2015;15(2):145-150. doi:10.7861/clinmedicine.15-2-145
  19. Shipton MJ, Thachil J. Vitamin B12 deficiency - A 21st century perspective . Clin Med (Lond). 2015;15(2):145-150. doi:10.7861/clinmedicine.15-2-145
  20. Shrestha L, Shrestha B, Gautam K, Khadka S, Mahara Rawal N. Plasma Vitamin B-12 Levels and Risk of Alzheimer's Disease: A Case-Control Study. Gerontol Geriatr Med. 2022 Feb 7;8:23337214211057715. doi: 10.1177/23337214211057715. PMID: 35155719; PMCID: PMC8832580.
  21. Shrestha L, Shrestha B, Gautam K, Khadka S, Mahara Rawal N. Plasma Vitamin B-12 Levels and Risk of Alzheimer's Disease: A Case-Control Study. Gerontol Geriatr Med. 2022 Feb 7;8:23337214211057715. doi: 10.1177/23337214211057715. PMID: 35155719; PMCID: PMC8832580.
  22. Smith AD, Refsum H, Bottiglieri T, Fenech M, Hooshmand B, McCaddon A, Miller JW, Rosenberg IH, Obeid R. Homocysteine and Dementia: An International Consensus Statement. J Alzheimers Dis. 2018;62(2):561-570. doi: 10.3233/JAD-171042. PMID: 29480200; PMCID: PMC5836397.
  23. Watanabe F, Bito T. Vitamin B12 sources and microbial interaction. Exp Biol Med (Maywood). 2018 Jan;243(2):148-158. doi: 10.1177/1535370217746612. Epub 2017 Dec 7. PMID: 29216732; PMCID: PMC5788147.
  24. Watanabe F, Yabuta Y, Bito T, Teng F. Vitamin B₁₂-containing plant food sources for vegetarians. Nutrients. 2014 May 5;6(5):1861-73. doi: 10.3390/nu6051861. PMID: 24803097; PMCID: PMC4042564.
  25. Wolffenbuttel BHR, Wouters HJCM, Heiner-Fokkema MR, van der Klauw MM. The Many Faces of Cobalamin (Vitamin B12) Deficiency. Mayo Clin Proc Innov Qual Outcomes. 2019;3(2):200-214. Published 2019 May 27. doi:10.1016/j.mayocpiqo.2019.03.002