Vitamin B12 is commonly used over-the-counter to treat general fatigue and may also be beneficial for patients with myalgic encephalomyelitis (chronic fatigue syndrome) (ME/CFS), potentially serving as a treatment option. Vitamin B12 deficiency may even be a potential cause of the disease. A 2022 review found that some ME/CFS patients had low levels of vitamin B12 in their cerebrospinal fluid (CSF) samples, but normal levels in their blood samples. This may mean that although blood vitamin B12 levels are normal, the brain may be under-supplied with vitamin B12. Another study found that intranasal vitamin B12 supplementation improved fatigue symptoms in two-thirds of ME/CFS patients. Research on its efficacy in treating general fatigue symptoms in healthy individuals is very limited, so its effectiveness remains uncertain.

Vitamin B12 may be beneficial for various types of neuropathy (nerve damage). Postherpetic neuralgia, a complication caused by a recurrence of herpesvirus (such as varicella-zoster virus), can lead to damage to the affected nerves. Adding vitamin B12 to standard treatment may improve the treatment outcome for this condition. Similarly, a 2020 randomized controlled trial found that supplementation with 1000 micrograms of vitamin B12 improved symptoms of diabetic neuropathy. This may be because approximately 20% of diabetic patients also have a vitamin B12 deficiency.

Vitamin B12 is crucial in early pregnancy. A deficiency in vitamin B12, like a deficiency in folic acid (vitamin B9), can lead to neural tube defects. The neural tube develops during the third week of pregnancy, approximately days 20-28. Ideally, vitamin B12 supplementation should begin before conception to ensure sufficient levels in the body from the first day of pregnancy.

Vitamin B12 supplements are sometimes used to improve cognitive symptoms in patients with neurodegenerative diseases. However, meta-analyses to date have not supported this use.

A recent review suggests that vitamin B12 may be beneficial in viral infections, including SARS-CoV-2 (COVID-19) infection. It appears that many people with viral infections have low levels of vitamin B12, or levels at the lower end of the normal range, yet they may still experience symptoms. In these cases, vitamin B12 supplementation may improve prognosis. However, further research is needed, and we cannot currently determine its effectiveness.

Vitamin B12 is considered a safe supplement. The risk of toxicity is extremely low at recommended doses. Because it is a water-soluble vitamin, the body excretes excess vitamin B12 through urine. Therefore, there is currently no established upper limit for vitamin B12 intake.

High levels of vitamin B12 appear to increase the risk of certain cancers while decreasing the risk of others. One study found an association between elevated vitamin B12 levels and prostate cancer; however, the study did not confirm a causal relationship. A 2017 cohort study found that men who smoked and took higher doses of vitamin B12 had an increased risk of lung cancer. While this association does not necessarily mean that vitamin B12 causes an increased incidence of lung cancer, it is certainly worth considering this factor before supplementing this population with vitamin B12.

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

In the field of cardiovascular disease, relevant data remain uncertain. A meta-analysis in 2024 found that elevated vitamin B12 levels were associated with increased cardiovascular disease mortality. However, this study examined endogenous vitamin B12, rather than vitamin B12 intake, which may indicate that elevated vitamin B12 levels occurred primarily in patients with cardiovascular disease. Another study found that vitamin B12 had little effect on the prognosis of cardiovascular disease. More research is needed to evaluate the role of vitamin B12 supplementation as a risk factor for cardiovascular disease.

In general, vitamin B12 is important because it is an essential component of many chemical reactions in the body.

Vitamin B12 and folic acid act as cofactors in the synthesis of the amino acid methionine via methionine synthase and L-malondioxanone mutase. A deficiency in either vitamin B12 or folic acid disrupts this process, leading to the production of abnormally large red blood cells, which in turn causes megaloblastic anemia.

Vitamin B12 can also be used by methylmalondiamide-CoA mutase to produce succinyl-CoA. Interruption of this reaction can lead to neurological symptoms of vitamin B12 deficiency.

Vitamin B12 also participates in the formation of myelin sheaths, which surround nerves and facilitate the transmission of nerve signals. If the myelin sheath is damaged, signals will be interrupted or abnormal, leading to pain or loss of sensation. Vitamin B12 deficiency can cause peripheral neuropathy, which is due to damage to the myelin sheaths of the nerves in the hands and feet.

Vitamin B12 is part of single-carbon metabolism, the metabolic process by which DNA is produced in the body. It helps enzymes function in the synthesis of purines and pyrimidines (building blocks of DNA).