Vitamin B12 is one of the 8 B vitamins and is water soluble. It is found bound to protein, where it is used in several metabolic processes, and is important for one-carbon metabolism. One-carbon metabolism is a series of metabolic processes that are crucial for cellular function.^[3][4] Vitamin B12 acts as a cofactor in these metabolic processes throughout the body, including the production of methionine, one of the amino acids that is integral for the production of the purines that make up DNA.^[4]

Vitamin B12 is often used as an over-the-counter treatment for general fatigue and also might benefit people with myalgic encephalomyelitis (chronic fatigue syndrome) (ME/CFS) as a potential treatment. A B12 deficiency may even be a potential cause of the condition. A 2022 review found that some people with ME/CFS had a low B12 level in cerebrospinal fluid (CSF) samples but a normal level in blood samples. This could imply a low B12 supply in the brain, despite a normal level in the blood. Another study found that intranasal vitamin B12 improved symptoms of fatigue in two thirds of people with ME/CFS.^[5] For general fatigue symptoms in otherwise healthy people, very little research has been done on this, so we are not sure if it is effective.^[6]

Vitamin B12 may be beneficial for various types of neuropathy (nerve damage). Postherpetic neuralgia is a complication after the reactivation of a herpes virus (e.g., chicken pox) causes damage to the involved nerves, and vitamin B12 may improve the outcome of treatment when added to conventional treatment for this condition.^[7] Similarly, a 2020 randomized control trial^[8] found that supplementing with 1000 μg of vitamin B12 improved the symptoms of diabetic neuropathy. This could be because around 20% of people with diabetes are also deficient in B12.^[9]

Vitamin B12 is crucial in the first trimester of pregnancy. A deficiency, as with a deficiency of folate (vitamin B9), can lead to neural tube defects. The neural tube develops in the third week of pregnancy, around day 20-28. It is ideal to start supplementation prior to pregnancy to ensure sufficient levels are present from day 1.^[10]

Vitamin B12 supplements are sometimes used to improve cognitive symptoms in people with neurodegenerative disease. However, meta-analyses have not supported this use so far.^[6][11][12]

A recent review suggests that vitamin B12 could be useful in viral infections, including SARS-CoV-2 (COVID-19). It seems that many people with viral infections may have low B12 levels, or levels on the low end of the normal range that could still result in symptoms. In these cases, providing a supplement could improve outcomes. However, this still needs to be studied further, and we can’t be sure it is helpful just yet.^[1]

Vitamin B12 is considered to be a safe supplement. At the recommended dosages, toxicity is highly unlikely. Since it is a water soluble vitamin, the body will excrete any excess through the urine. As such, no upper limit of intake has been established yet.^[13]

It seems that a high vitamin B12 level could increase the risk of certain cancers and decrease the risk of others. One study^[14] found an association between elevated B12 levels and prostate cancer; however, it did not establish a causal relationship. A cohort study in 2017 found an increased lung cancer risk for men who smoked and took relatively high doses of vitamin B12. While this association doesn’t necessarily mean B12 caused the increased rate of lung cancer, it is certainly worth considering before supplementing in this population group.^[15]

Conversely, another study found that a higher vitamin B12 intake was associated with a lower risk of colorectal cancer.^[16]

In cardiovascular disease, the data also remains uncertain. A 2024 meta-analysis found that increased B12 levels were associated with an increase in cardiovascular disease mortality. However, this study looked at endogenous vitamin B12, not vitamin B12 intake, and this might indicate that B12 levels rise in the presence of cardiovascular disease.^[17] Another study found that vitamin B12 had little impact on cardiovascular disease outcomes.^[18] More studies will need to be done to assess supplemental vitamin B12 as a risk factor for cardiovascular disease.

Broadly speaking, vitamin B12 is important because it’s a necessary part of many chemical reactions in the body.

Vitamin B12 and folate work as cofactors in the synthesis of the amino acid methionine via the enzymes methionine synthase and L-malonyl-CoA mutase. A deficiency of vitamin B12 or of folate disrupts this process and leads to the production of abnormally large red blood cells, resulting in megaloblastic anemia.^[19]

Vitamin B12 is also used by methyl-malonyl-CoA mutase to produce succinyl CoA. Interrupting this reaction leads to the neurological symptoms of B12 deficiency.^[4]

B12 is also used in the production of the myelin sheaths that surround nerves and help with nerve signal transmission. If the sheath is damaged, the signals can become disrupted or abnormal, causing pain or loss of sensation. In vitamin B12 deficiency, peripheral neuropathy can occur, which is the result of damage to the myelin sheaths in the nerves of the hands and the feet.^[19]

Vitamin B12 is part of one-carbon metabolism, the metabolic process that produces DNA in the body. It helps enzymes work in the production of the building blocks of DNA called purines and pyrimidines.^[4]