For Patients & Caregivers
Tell your healthcare providers about any dietary supplements you’re taking, such as herbs, vitamins, minerals, and natural or home remedies. This will help them manage your care and keep you safe.
How It Works
Vitamin B12 is found in many foods. For those who may not get enough, a healthcare provider can identify and appropriately treat deficiencies.
Vitamin B12 is involved in the synthesis of phospholipids, neurotransmitters, DNA, and the metabolism of fatty acids and amino acids. It is found in meat, fish, dairy products, and fortified cereals.
A number of studies have evaluated B12, folic acid, and B6 as part of homocysteine-lowering therapy. This is because elevated homocysteine levels can be a marker for various conditions including heart disease, depression, cognitive disorders, and risk of fracture. However, findings are mixed on whether long-term supplementation with this combination improves fracture risk or medical conditions even when it lowers homocysteine levels. Data on whether this supplement combination can lower cancer risk are also mixed.
Since vitamin B12 is found in many foods, deficiency is more common in the elderly, strict vegetarians, or those who take oral contraceptives or medications for gastric disorders or insulin resistance. Also, high folic acid intake may mask a B12 deficiency, so it’s important for patients to discuss any supplement use with their physicians.
Diagnosed deficiencies can be effectively treated with B12 therapy if dietary changes alone are not enough.
The injectable form of B12 is used as a prescription drug to treat pernicious anemia.
When combined with folate and B6, vitamin B12 can reduce homocysteine levels, which may offer benefit for cardiovascular disorders, although results from studies are mixed.
Studies are mixed on whether vitamin B12 in combination with folate and B6 can reduce cancer risk, and it may increase risk of lung or colorectal cancers. Additional studies are needed to determine how vitamin B12 levels may affect cancer risk.
In those without a vitamin B12 deficiency, there is little indication in the medical literature that B12 supplementation can improve fatigue symptoms.
Do Not Take If
For Healthcare Professionals
Vitamin B12, part of the vitamin B-complex family, refers to the group of compounds that consists of cyanocobalamin, hydroxycobalamin, and related substances (1). It is essential for normal nerve function, DNA synthesis, hematopoiesis, fatty acid metabolism, and amino acid synthesis in the mitochondria. It also plays an important role in homocysteine metabolism.
Vitamin B12 is abundant in diets that include meat and dairy products, although deficiency can occur in those with malabsorption syndromes (2) or those who take medications for gastric disorders (3) (4), for conditions associated with insulin resistance (5), or in those who follow a strict vegetarian diet (6). B12 deficiency is also more common among the elderly (7).
Lack of B12 can lead to a wide variety of hematologic, neurologic, and psychiatric disorders, may increase the risk of cardiovascular disease (8), and cause pseudo-thrombotic microangiopathy following gastrectomy (55). Vitamin B12 is often included in vitamin products used to reduce homocysteine, which is associated with cardiovascular diseases.
High dietary intakes of B vitamins including B12 have been associated with a decreased risk for cataracts (36). However, combined supplementation with B12, B6, and folic acid may increase the risk of cataract extraction (37).
Although low serum B12 levels have been associated with increased bone turnover and fracture risk (9), long-term B12 and folic acid supplementation did not improve bone mineral density (38), or reduce osteoporotic fracture incidence in elderly individuals with elevated plasma homocysteine levels (39).
Repletion of vitamin B12 may improve response in patients resistant to antidepressants (10), and in older adults with major depression, long-term supplementation with a combination of B12, B6, and folic acid enhanced antidepressant treatment efficacy (40). However, such combined supplementation did not reduce depression risk among middle-age and older women despite significant reductions in homocysteine levels (41).
Homocysteine-lowering therapy may have some cardiovascular benefits following coronary interventions (11), but does not appear to lower risk of further cardiovascular events including stroke (12) (13) (14). In addition, increased risk of in-stent restenosis with such treatment has been reported in patients after coronary stenting (15). Long-term B12 and folic acid supplementation also had no effect on arterial stiffness and atherosclerosis in elderly patients with hyperhomocysteinemia (42), and a meta-analysis suggests that evidence for cardiovascular preventive benefits with B-vitamins is lacking (61).
In other studies, B12 did not improve cognitive function (16) (17) (18) (19) (45) (47) (56), but may positively affect gait and physical performance in the elderly (48), or reduce analgesic use in patients with postherpetic neuralgia (59).
Preliminary data suggest B12 supplementation may help reduce pain and improve quality of life in patients with aromatase inhibitor-associated musculoskeletal symptoms (62). However, individual B12 or iron supplement use during chemotherapy may be significantly associated with poorer outcomes (63). Additional studies are needed to elucidate these findings.
Mechanism of Action
Vitamin B12 is involved in the transfer of methyl groups, and in methylation reactions essential for the synthesis of phospholipids and neurotransmitters in the central nervous system. It is also required for synthesis of nucleic acid, notably DNA, the metabolism of fatty acids and amino acids in the mitochondria, and in delivering SAMe, the universal methyl donor (16) (30). In humans, two enzymatic reactions dependent on B12 are produced and activated in separate cellular compartments: methylcobalamin in the cytosol and adenosylcobalamin in the mitochondria (1). As a coenzyme in methyl transfer reactions, B12 converts homocysteine to methionine, and participates in L-methylmalonyl-coenzyme A (CoA) conversion to succinyl-CoA (8). In the first enzyme reaction, methylcobalamin is used to recycle the folate cofactor 5-methyltetrahydrofolate to tetrahydrofolate thereby allowing the folate cofactor to participate in a cycle involving the biosynthesis of purines and pyrimidines. During this reaction, homocysteine is converted to methionine yielding the methyl groups required for methylation that is essential in biosynthesis (31).
When used with pemetrexed, B12/folic acid therapy further increases sub-G1 populations in human adenocarcinoma and large-cell carcinoma cell lines, independent of p53 status (28).
Proton pump inhibitors or histamine type 2 (H2)-receptor antagonists: Medications to treat gastric disorders that interfere with or suppress gastric acid and intrinsic factor production can lead to decreased vitamin B12 absorption (3) (4).
Biguanides (metformin): Medications used to treat type 2 diabetes, metabolic syndrome, nonalcoholic fatty liver disease, and polycystic ovary syndrome can lead to decreased vitamin B12 absorption (5).
Folic acid: Ingesting amounts greater than the Tolerable Upper Intake Level may mask vitamin B12 deficiency symptoms (33).
Oral contraceptives: May lower serum vitamin B12 concentrations (34).