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Vitamin B12

Vitamin B12
Systematic (IUPAC) name
CAS number 68-19-9
ATC code B03BA01
PubChem 5479203
DrugBank APRD00326
Chemical data
Formula C63H88CoN14O14P 
Mol. mass 1355.37 g/mol
Pharmacokinetic data
Bioavailability readily absorbed in lower half ileum
Protein binding Very high to specific transcobalamins plasma proteins
Binding of hydroxocobalamin is slightly higher than cyanocobalamin.
Metabolism hepatic
Half life Approximately 6 days
(400 days in the liver)
Excretion renal
Therapeutic considerations
Pregnancy cat.


Legal status


Routes oral, iv

Vitamin B-12, known also as vitamin B12 (or commonly B12 or B-12 for short), is a collection of cobalt and corrin-ring molecules which are defined by their particular vitamin function in the body. All of the substrate cobalt-corrin molecules from which B-12 is made must be synthesized by bacteria. However, after this synthesis is complete, the body has a limited power to convert any form of B-12 to another, by means of enzymatically removing certain prosthetic chemical groups from the cobalt atom.

Cyanocobalamin is one such compound that is a vitamin in this B complex, because it can be metabolized in the body to an active co-enzyme form. However, the cyanocobalamin form of B-12 does not occur in nature normally, but is a byproduct of the fact that other forms of B-12 are avid binders of cyanide (-CN) which they pick up in the process of activated charcoal purification of the vitamin after it is made by bacteria in the commercial process. Since the cyanocobalamin form of B-12 is deeply red colored, easy to crystallize, and is not sensitive to air-oxidation, it is typically used as a form of B-12 for food additives and in many common multivitamins. However, this form is not perfectly synonymous with B-12, inasmuch as many subtances have B-12 vitamin activity and can properly be labeled vitamin B-12, but cyanocobalamin is just one of them. (Thus, all cyanocobalamin is vitamin B-12, but not all vitamin B-12 is cyanocobalamin). [1]

Vitamin B-12 is important for the normal functioning of the brain and nervous system and for the formation of blood. It is normally involved in the metabolism of every cell of the body, especially affecting the DNA synthesis and regulation but also fatty acid synthesis and energy production. However, many (though not all) of the effects of functions of B-12 can be replaced by sufficient quantities of folic acid (another B vitamin), since B-12 is used to regenerate folate in the body. Most "B-12 deficient symptoms" are actually folate deficient symptoms, since they include all the effects of pernicious anemia and megaloblastosis, which are due to poor synthesis of DNA when the body does not have a proper supply of folic acid for the production of thymine. When sufficient folic acid is available, all known B-12 functions normalize, save those connected with the enzyme Methylmalonyl Coenzyme A mutase (MUT), and its products (S-adenosylmethionine, SAMe) and substrates (methylmalonic acid, MMA).



The name vitamin B-12 generally refers to all forms of the vitamin. Some medical practitioners have suggested that its use be split into two different categories, however.

  • In a broad sense B-12 still refers to a group of cobalt-containing vitamer compounds known as cobalamins: these include cyanocobalamin (an artifact formed as a result of the use of cyanide in the purification procedures), hydroxocobalamin (another medicinal form), and finally, the two naturally occurring cofactor forms of B-12: methylcobalamin (MeB-12), the cofactor of Methylmalonyl Coenzyme A mutase (MUT), and 5-deoxyadenosylcobalamin (adenosylcobalamin—AdoB-12), the cofactor of 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR).
  • The term B-12 may be properly used to refer to cyanocobalamin, the principal B-12 form used for foods and in nutritional supplements. This ordinarily creates no problem, except perhaps in rare cases of eye nerve damage, where the body is only marginally able to use this form due to high cyanide levels in the blood due to cigarette smoking, and thus requires cessation of smoking, or else B-12 given in another form, for the optic symptoms to abate. [2] However, tobacco amblyopia is a rare enough condition that debate continues about whether or not it represents a peculiar B-12 deficiency which is resistant to treatment with cyanocobalamin.

Finally, so-called Pseudo-B-12 refers to B-12-like substances which are found in certain organisms, including spirulina (a cyanobacterium) and some algae. These substances are active in tests of B-12 activity by highly sensitive antibody-binding serum assay tests, which measure levels of B-12 and B-12 like compounds in blood. However, these substances do not have B-12 biological activity for humans, a fact which may pose a theoretical danger to vegans and others on limited diets who do not ingest B-12 producing bacteria, but who nevertheless may show normal "B-12" levels in the standard immunoassay which has become the normal medical method for testing for B-12 deficiency. [3]


B-12 is the most chemically complex of all the vitamins. The structure of B-12 is based on a corrin ring, which is similar to the porphyrin ring found in heme, chlorophyll, and cytochrome. The central metal ion is Co (cobalt). Four of the six coordination sites are provided by the corrin ring, and a fifth by a dimethylbenzimidazole group. The sixth coordination site, the center of reactivity, is variable, being a cyano group (-CN), a hydroxyl group (-OH), a methyl group (-CH3) or a 5'-deoxyadenosyl group (here the C5' atom of the deoxyribose forms the covalent bond with Co), respectively, to yield the four B-12 forms mentioned above. The covalent C-Co bond is one of first examples of carbon-metal bonds in biology. The hydrogenases and, by necessity, enzymes associated with cobalt utilization, involve metal-carbon bonds.[4]


Vitamin B-12 cannot be made by plants or animals[5], as the only type of organism that have the enzymes required for the synthesis of B-12 is bacteria. The total synthesis of B-12 was reported by Robert Burns Woodward[6][7] and Albert Eschenmoser[8][9], and remains one of the classic feats of total synthesis.

Species from the following genera are known to synthesize B-12: Aerobacter, Agrobacterium, Alcaligenes, Azotobacter, Bacillus, Clostridium, Corynebacterium, Flavobacterium, Micromonospora, Mycobacterium, Nocardia, Propionibacterium, Protaminobacter, Proteus, Pseudomonas, Rhizobium, Salmonella, Serratia, Streptomyces, Streptococcus and Xanthomonas. Industrial production of B-12 is through fermentation of selected microorganisms.[10] The most used species are Pseudomonas denitrificans and Propionibacterium shermanii, often genetically engineered and grown under special conditions to enhance yield.


Coenzyme B-12's reactive C-Co bond participates in two types of enzyme-catalyzed reactions. [11]

  1. Rearrangements in which a hydrogen atom is directly transferred between two adjacent atoms with concomitant exchange of the second substituent, X, which may be a carbon atom with substituents, an oxygen atom of an alcohol, or an amine.
  2. Methyl (-CH3) group transfers between two molecules.

In humans, only two coenzyme B-12-dependent enzymes are known:

  1. Methylmalonyl Coenzyme A mutase (MUT) which uses the AdoB-12 form and reaction type 1 to catalyze a carbon skeleton rearrangement (the X group is -COSCoA). MUT's reaction converts MMl-CoA to Su-CoA, an important step in the extraction of energy from proteins and fats (for more see MUT's reaction mechanism). This functionality is always lost in vitamin B-12 deficiency, and can be measured clinically as an increased methylmalonic acid (MMA) level. Unfortunately, an elevated MMA, though sensitive to B-12 deficiency, is probably overly sensitive, and not all who have it actually have B-12 deficiency. For example, MMA is elevated in 90-98% of patients with B-12 deficiency; however 25-20% of patients over the age of 70 have elevated levels of MMA, yet 25-33% of them do not have B-12 deficiency. For this reason, MMA is not routinely recommended in the elderly. [12] The "gold standard" test for B-12 deficiency continues to be low blood levels of the vitamin.

    The MUT function, however, remains the one B-12 function which cannot be restored with folate supplementation, and which is necessary for myelin synthesis (see mechanism below) and certain other functions of the central nervous system. Other functions of B-12 related to DNA, as well as elevated homocysteine levels (see below) can often be corrected with supplementation with the vitamin folic acid.

    In a vitamin B12-dependent MUT reaction, methionine is subsequently converted to S-adenosyl-methionine. S-adenosyl-methionine is necessary for methylation of myelin sheath phospholipids. In a second reaction dependent on MUT, B-12 is used to convert methylmalonyl coenzyme A into succinyl coenzyme A. Failure of this second reaction to occur results in elevated levels of methylmalonic acid. Excessive methylmalonic acid will prevent normal fatty acid synthesis, or it will be incorporated into fatty acid itself rather than normal malonic acid. If this abnormal fatty acid subsequently is incorporated into myelin or if the methylation of the myelin sheath phospholipids fails to occur, the resulting myelin will be too fragile, and demyelination will occur. The result is subacute combinded degeneration of central nervous system and spinal cord. [13]

  2. 5-methyltetrahydrofolate-homocysteine methyltransferase (MTR), a methyl transfer enzyme, which uses the MeB-12 and reaction type 2 to catalyze the conversion of the amino acid Hcy into Met (for more see MTR's reaction mechanism).[14] This functionality is lost in vitamin B-12 deficiency, and can be measured clinically as an increased homocysteine level in vitro. Increased homocysteine can also be caused by a folic acid deficiency, since B-12 helps to regenerate folic acid, and thus decrease the need for it in the diet. There is some controversy over whether it is the reduced availability of methionine, or the reduced availability of THF (produced in the conversion of homocysteine to methionine) that is responsible for the reduced availability of 5,10-methylene-THF. In any case, 5,10-methylene-THF is involved in the synthesis of thymine, and hence reduced availability of 5,10-methylene-THF results in problems with DNA synthesis, and ultimately in ineffective production of blood cells, and also in intestinal wall cells which are responsible for absorption, in the once-dreaded and fatal disease, pernicious anemia. All of these effects, including the anemia of pernicious anemia, resolve if sufficient folate is at hand, making this best known function of B-12 (the one involved with DNA synthesis and restoration of cell-division and anemia) actually a facultative function which is mediated by folate. [15].

On the other hand, the absolutely B-12 dependent MUT reaction (mentioned first) is now the one with the most important secondary effects, now that folic acid is being added to fortify flour in many countries (so that folate deficiency is now more rare), and now that folate-sensitive tests for anemia and erythrocyte size are routinely done in even simple medical test clinics (so these biochemical effects are more often directly detected). In the MUT reaction, the transmethylating agent S-adenosylmethionine (SAMe) is produced, which is involved in the synthesis of myelin, necessary for normal nerve function. Methylmalonic acid (MMA), produced when B-12 is deficient and MUT is inactive, is also a myelin destabalizer, as noted. These reasons explain why B-12 deficiency causes neuropathies, even if folic acid is present in good supply and anemia is not present. In addition, SAMe is involved in the manufacture of certain neurotransmitters, catecholamines and in brain metabolism. These neurotransmitters are important for maintaining mood, possibly explaining why depression is associated with B-12 deficiency.

Human absorption and distribution

The human physiology of vitamin B-12 is complex, and therefore is prone to mishaps leading to vitamin B-12 deficiency. The vitamin as it occurs in foods enters the digestive tract bound to proteins, known as salivary R-binders. Stomach proteolysis of these proteins requires an acid pH, and also requires proper pancreatic release of proteolytic enzymes. (Even small amounts of B-12 taken in supplements bypasses these steps and thus any need for gastric acid, which may be blocked by antacid drugs).

The free B-12 then attaches to gastric intrinsic factor, which is generated by the gastric parietal cells. If this step fails due to gastric parietal cell atrophy (the problem in pernicious anemia), sufficient B-12 is not absorbed later on, unless administered orally in relatively massive doses (500 to 1000 mcg/day).

The conjugated vitamin B-12-intrinsic factor complex (IF/B-12) is then normally absorbed by the terminal ileum of the small bowel. Absorption of food vitamin B-12 therefore requires an intact and functioning stomach, exocrine pancreas, intrinsic factor, and small bowel. Problems with any one of these organs makes a vitamin B-12 deficiency possible.

Once the IF/B-12 complex is recognized by specialized ileal receptors, it is transported into the portal circulation. The vitamin is then transferred to transcobalamin II (TC-II/B12), which serves as the plasma transporter of the vitamin. Genetic deficiencies of this protein are known, also leading to functional B-12 deficiency.

For the vitamin to serve inside cells, the TC-II/B-12 complex must bind to a cell receptor, and be endocytosed. The transcobalamin-II is degraded within a lysozyme, and the B-12 is finaly released into the cytoplasm, where it may be transformed into the proper coenzyme, by certain cellular enzymes (see above).

Hereditary defects in production of the transcobalamins and their receptors may produce functional deficiencies in B-12 and infantile megaloblastic anemia, and abnormal B-12 related biochemistry, even in some cases with normal blood B-12 levels. [16].

History as a treatment for anemia

B-12 deficiency is the cause of pernicious anemia, a usually-fatal disease of unknown etiology when it was first described in medicine. The cure was discovered by accident. William Murphy had been inducing anemia in dogs by bleeding them, and then conducting experiments in which he fed them various foods to observe which diets allowed them fastest recovery from the anemia. In the process, he discovered that ingesting large amounts of liver seemed to most-rapidly cure the anemia of traumatic blood loss, and suggested that therefore liver extracts be tried for pernicious anemia, an anemic disease of the time with no known cause or cure. The treatment of pernicious anemia with raw liver juice proved a success, but when George Minot and George Whipple set about to chemically isolate the curative substance in liver which cured anemia in dogs, they found it was iron. They found further that the partly isolated water-soluble liver-substance which cured pernicious anemia in humans, was something else entirely-- and which had no effect at all on dogs under the conditions used. The cure for pernicious anemia had been found by coincidence. Minot and Whipple ultimately were able to isolate previously unsuspected vitamin B-12 from the extract of the liver which cured pernicious anemia. For this, the three men shared the 1934 Nobel Prize in Medicine.

The chemical structure of the molecule was determined by Dorothy Crowfoot Hodgkin and her team in 1956, based on crystallographic data.

Symptoms and damage from deficiency

Vitamin B-12 deficiency can potentially cause severe and irreversible damage, especially to the brain and nervous system. At levels only slightly lower than normal, a range of symptoms such as fatigue, depression, and poor memory may be felt.[17] However, these symptoms by themselves are too nonspecific to diagnose deficiency of the vitamin.

B-12 can be supplemented in healthy subjects by oral pill; sublingual pill, liquid, or strip; by injection; or by nasal spray. B-12 is available singly or in combination with other supplements.

The Dietary Reference Intake for an adult ranges from 2 to 3 µg (micrograms). The recommended optimal daily intake (ODI) is 10 to 15 µg.


Vitamin B-12 is naturally found in foods of animal origin including meat (especially liver and shellfish) and milk products. Eggs are often mentioned as a good source, but they also contain a factor that blocks absorption.[18] Interestingly, certain insects such as termites have been found to contain B-12.[19] An NIH Fact Sheet lists a variety of food sources of vitamin B-12.

Plants only supply B-12 when the soil containing B-12 producing microorganisms has not been washed from them. However, it is rare to eat unwashed vegetables in the West, so vegans need to take special care to supplement their diets accordingly. According to the U.K. [Vegan Society], the only reliable vegans sources of B-12 are foods fortified with B-12 (including some plant milks, some soy products and some breakfast cereals) and B-12 supplements.[20] Fortified breakfast cereals are a particularly valuable source of vitamin B-12 for vegetarians and vegans.

While lacto-ovo vegetarians usually get enough B-12 through dairy products, it may be found lacking in those practicing vegan diets who do not use multivitamin supplements or eat B-12 fortified foods, such as fortified breakfast cereals, fortified soy-based products, and fortified energy bars. Claimed sources of B-12 that have been shown through direct studies[20] of vegans to be inadequate or unreliable include, nori (a seaweed), barley grass, and human gut bacteria. People on a vegan raw food diet are also susceptible to B-12 deficiency if no supplementation is used[21]. However, the more alkaline intestines of vegans[citation needed] are better able to metabolize hydroxocobalamin, a more efficient cobalamin than cyanocobalamin.[citation needed]

The Vegan Society, the Vegetarian Resource Group and the Physicians Committee for Responsible Medicine, among others, recommend that vegans either consistently eat foods fortified with B-12 or take a daily or weekly B-12 supplement.[22][23][24]

Cyanocobalamin is converted to its active forms, first hydroxocobalamin and then methylcobalamin and adenosylcobalamin in the liver. The sublingual route, in which B-12 is supposed to be absorbed more directly under the tongue, has not proven to be necessary or helpful. A 2003 study found no significant difference in absorption for serum levels from oral vs. sublingual delivery of 500 micrograms of cobalamin.[25]

Injection is sometimes used in cases where digestive absorption is impaired, but there is some evidence that it may not necessary with modern high potency oral supplements (such as 500 to 1000 mcg or more). [26][27] These supplements carry such large doses of the vitamin that the many different components of the B-12 absorption system are not required, and enough of the vitamin (only a few mcg a day) is obtained simply by mass-action transport across the gut. Even pernicious anemia can be treated entirely by the oral route.

For the much lower amounts of B-12 found in food sources, however, oral absorption is complex and requires stomach acid and also specific intestinal transport proteins (intrinsic factor) produced in the stomach. Lack of function in these systems is the causes of much of the increased risk in many elderly persons who develop B-12 deficiency later in life. However, it can be treated with a simple high dose oral B-12 supplement. Cyanocobalamin is also sometimes added to beverages including Diet Coke Plus and many energy drinks, in some cases with over 80 times the recommended intake.[citation needed] It is not known if these amounts are enough to correct malabsorbtion of B-12 due to lack of gastric acid or intrinsic factor.


Vitamin B-12 supplements in theory should be avoided in people sensitive or allergic to cobalamin, cobalt, or any other product ingredients. However, direct allergy to a vitamin or nutrient is extremely rare, and if reported, other causes should be sought.

Side effects, contraindications, and warnings

  • Dermatologic: Itching, rash, transitory exanthema, and urticaria have been reported. Vitamin B-12 (20 micrograms/day) and pyridoxine (80mg/day) has been associated with cases of rosacea fulminans, characterized by intense erythema with nodules, papules, and pustules. Symptoms may persist for up to 4 months after the supplement is stopped, and may require treatment with systemic corticosteroids and topical therapy.
  • Gastrointestinal: Diarrhea has been reported.
  • Hematologic: Peripheral vascular thrombosis has been reported. Treatment of vitamin B-12 deficiency can unmask polycythemia vera, which is characterized by an increase in blood volume and the number of red blood cells. The correction of megaloblastic anemia with vitamin B-12 can result in fatal hypokalemia and gout in susceptible individuals, and it can obscure folate deficiency in megaloblastic anemia. Caution is warranted.
  • Leber's disease: Vitamin B-12 in the form of cyanocobalamin is contraindicated in early Leber's disease, which is hereditary optic nerve atrophy. Cyanocobalamin can cause severe and swift optic atrophy, but other forms of vitamin B-12 are available.[citation needed]

Pregnancy and breastfeeding

Vitamin B-12 is believed to be safe when used orally in amounts that do not exceed the recommended dietary allowance (RDA). The RDA for vitamin B-12 in pregnant women is 2.6mcg per day and 2.8mcg during lactation periods.

There is insufficient reliable information available about the safety of consuming greater amounts of Vitamin B-12 during pregnancy.

Other medical uses

Hydroxycobalamin, or hydoxocobalamin, also known as Vitamin B-12a, is used in Europe both for vitamin B-12 deficiency and as a treatment for cyanide poisoning, sometimes with a large amount (5-10 g) given intravenously, and sometimes in combination with sodium thiosulfate[28]. The mechanism of action is straightforward: the hydroxycobalamin hydroxide ligand is displaced by the toxic cyanide ion, and the resulting harmless B-12 complex is excreted in urine. In the United States, the FDA has approved in 2006 the use of hydroxocobalamin for acute treatment of cyanide poisoning.


Interactions with drugs

  • Alcohol (ethanol): Excessive alcohol intake lasting longer than two weeks can decrease vitamin B-12 absorption from the gastrointestinal tract.
  • Aminosalicylic acid (para-aminosalicylic acid, PAS, Paser): Aminosalicylic acid can reduce oral vitamin B-12 absorption, possibly by as much as 55%, as part of a general malabsorption syndrome. Megaloblastic changes, and occasional cases of symptomatic anemia have occurred, usually after doses of 8 to 12 grams/day for several months. Vitamin B-12 levels should be monitored in people taking aminosalicylic acid for more than one month.
  • Antibiotics: An increased bacterial load can bind significant amounts of vitamin B-12 in the gut, preventing its absorption. In people with bacterial overgrowth of the small bowel, antibiotics such as metronidazole (Flagyl®) can actually improve vitamin B-12 status. The effects of most antibiotics on gastrointestinal bacteria are unlikely to have clinically significant effects on vitamin B-12 levels.
  • Hormonal contraception: The data regarding the effects of oral contraceptives on vitamin B-12 serum levels are conflicting. Some studies have found reduced serum levels in oral contraceptive users, but others have found no effect despite use of oral contraceptives for up to 6 months. When oral contraceptive use is stopped, normalization of vitamin B-12 levels usually occurs. Lower vitamin B-12 serum levels seen with oral contraceptives probably are not clinically significant.
  • Chloramphenicol (Chloromycetin®): Limited case reports suggest that chloramphenicol can delay or interrupt the reticulocyte response to supplemental vitamin B-12 in some patients. Blood counts should be monitored closely if this combination cannot be avoided.
  • Cobalt irradiation: Cobalt irradiation of the small bowel can decrease gastrointestinal (GI) absorption of vitamin B-12.
  • Colchicine: Colchicine in doses of 1.9 to 3.9mg/day can disrupt normal intestinal mucosal function, leading to malabsorption of several nutrients, including vitamin B-12. Lower doses do not seem to have a significant effect on vitamin B-12 absorption after 3 years of colchicine therapy. The significance of this interaction is unclear. Vitamin B-12 levels should be monitored in people taking large doses of colchicine for prolonged periods.
  • Colestipol (Colestid®), Cholestyramine (Questran®): These resins used for sequestering bile acids in order to decrease cholesterol, can decrease gastrointestinal (GI) absorption of vitamin B-12. It is unlikely that this interaction will deplete body stores of vitamin B-12 unless there are other factors contributing to deficiency. In a group of children treated with cholestyramine for up to 2.5 years there was not any change in serum vitamin B-12 levels. Routine supplements are not necessary.
  • H2-receptor antagonists: include cimetidine (Tagamet®), famotidine (Pepcid®), nizatidine (Axid®), and ranitidine (Zantac®). Reduced secretion of gastric acid and pepsin produced by H2 blockers can reduce absorption of protein-bound (dietary) vitamin B-12, but not of supplemental vitamin B-12. Gastric acid is needed to release vitamin B-12 from protein for absorption. Clinically significant vitamin B-12 deficiency and megaloblastic anemia are unlikely, unless H2 blocker therapy is prolonged (2 years or more), or the person's diet is poor. It is also more likely if the person is rendered achlorhydric (with complete absence of gastric acid secretion), which occurs more frequently with proton pump inhibitors than H2 blockers. Vitamin B-12 levels should be monitored in people taking high doses of H2 blockers for prolonged periods.
  • Metformin (Glucophage®): Metformin may reduce serum folic acid and vitamin B-12 levels. These changes can lead to hyperhomocysteinemia, adding to the risk of cardiovascular disease in people with diabetes.[citation needed] There are also rare reports of megaloblastic anemia in people who have taken metformin for 5 years or more. Reduced serum levels of vitamin B-12 occur in up to 30% of people taking metformin chronically.[29][30] However, clinically significant deficiency is not likely to develop if dietary intake of vitamin B-12 is adequate. Deficiency can be corrected with vitamin B-12 supplements even if metformin is continued. The metformin-induced malabsorption of vitamin B-12 is reversible by oral calcium supplementation.[31] The general clinical significance of metformin upon B-12 levels is as yet unknown.[32]
  • Neomycin: Absorption of vitamin B-12 can be reduced by neomycin, but prolonged use of large doses is needed to induce pernicious anemia. Supplements are not usually needed with normal doses.
  • Nicotine: Nicotine can reduce serum vitamin B-12 levels. The need for vitamin B-12 supplementation has not been adequately studied.
  • Nitrous oxide: Nitrous oxide inactivates the cobalamin form of vitamin B-12 by oxidation. Symptoms of vitamin B-12 deficiency, including sensory neuropathy, myelopathy, and encephalopathy, can occur within days or weeks of exposure to nitrous oxide anesthesia in people with subclinical vitamin B-12 deficiency. Symptoms are treated with high doses of vitamin B-12, but recovery can be slow and incomplete. People with normal vitamin B-12 levels have sufficient vitamin B-12 stores to make the effects of nitrous oxide insignificant, unless exposure is repeated and prolonged (such as recreational use). Vitamin B-12 levels should be checked in people with risk factors for vitamin B-12 deficiency prior to using nitrous oxide anesthesia. Chronic nitrous oxide B-12 poisoning (usually from use of nitrous oxide as a recreational drug), however, may result in B-12 functional deficiency even with normal measured blood levels of B-12 [33]
  • Phenytoin (Dilantin®), phenobarbital, primidone (Mysoline®): These anticonvulsants have been associated with reduced vitamin B-12 absorption, and reduced serum and cerebrospinal fluid levels in some patients. This may contribute to the megaloblastic anemia, primarily caused by folate deficiency, associated with these drugs. It's also suggested that reduced vitamin B-12 levels may contribute to the neuropsychiatric side effects of these drugs. Patients should be encouraged to maintain adequate dietary vitamin B-12 intake. Folate and vitamin B-12 status should be checked if symptoms of anemia develop.
  • Proton pump inhibitors (PPIs): The PPIs include omeprazole (Prilosec®, Losec®), lansoprazole (Prevacid®), rabeprazole (Aciphex®), pantoprazole (Protonix®, Pantoloc®), and esomeprazole (Nexium®). The reduced secretion of gastric acid and pepsin produced by PPIs can reduce absorption of protein-bound (dietary) vitamin B-12, but not supplemental vitamin B-12. Gastric acid is needed to release vitamin B-12 from protein for absorption. Reduced vitamin B-12 levels may be more common with PPIs than with H2-blockers, because they are more likely to produce achlorhydria (complete absence of gastric acid secretion). However, clinically significant vitamin B-12 deficiency is unlikely, unless PPI therapy is prolonged (2 years or more) or dietary vitamin intake is low. Vitamin B-12 levels should be monitored in people taking high doses of PPIs for prolonged periods.
  • Zidovudine (AZT, Combivir®, Retrovir®): Reduced serum vitamin B-12 levels may occur when zidovudine therapy is started. This adds to other factors that cause low vitamin B-12 levels in people with HIV, and might contribute to the hematological toxicity associated with zidovudine. However, data suggests vitamin B-12 supplements are not helpful for people taking zidovudine.

Interactions with herbs and dietary supplements

  • Folic acid: Folic acid, particularly in large doses, can mask vitamin B-12 deficiency by completely correcting hematological abnormalities. In vitamin B-12 deficiency, folic acid can produce complete resolution of megaloblastic anemia, while allowing potentially irreversible neurological damage (from continued inactivity of methylmalonyl mutase) to progress. Vitamin B-12 status should be determined before folic acid is given as monotherapy.
  • Potassium: Potassium supplements can reduce absorption of vitamin B-12 in some people. This effect has been reported with potassium chloride and, to a lesser extent, with potassium citrate. Potassium might contribute to vitamin B-12 deficiency in some people with other risk factors, but routine supplements are not necessary.[34]


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  2. ^ Accessed Dec. 3, 2007
  3. ^ Accessed Dec. 3, 2007
  4. ^ Bioorganometallics: Biomolecules, Labeling, Medicine; Jaouen, G., Ed. Wiley-VCH: Weinheim, 2006.3-527-30990-X.
  5. ^ {{cite book| author=G. Loeffler| title= Basiswissen Biochemie| pages= 606| isdn= 3-540-23885-9| year=2005
  6. ^ {{cite journal | author= Woodward,RB | title= The Total Synthesis of Vitamin {{ssub|12| journal= Pure Appl Chem | year= 1973 | volume= 33(1) | pages= 145-77
  7. ^ {{cite journal | author= Khan,AG and Easwaran,SV| title= Woodward's Synthesis of Vitamin B-12| journal= Resonance| year= 2003| volume= 8| pages= 8-16| url=|
  8. ^ {{cite journal | author= Eschenmoser, A. and Wintner, C. | title= Natural Product Synthesis and Vitamin B-12| journal= Science | year= 1976 | volume= 196 | pages= 1410-20
  9. ^ {{cite journal | author= Riether, D. and Mulzer, J. | title= Total Synthesis of Cobyric Acid: Historical Development and Recent Synthetic Innovations| journal= Eur. J. Org. Chem.| year= 2003| issue= 1| pages= 30-45|
  10. ^ {{cite journal | author= J.H. Martens, H. Barg, M.J. Warren and D. Jahn | title= Microbial production of vitamin B-12 | journal= Applied Microbiology and Biotechnology | year= 2002 | volume= 58 | pages= 275-285
  11. ^ {{cite book | author=Donald and Judith Voet| year= 1995| title= Biochemistry| edition= 2nd| pages= 675| publisher= John Wiley & Sons Ltd.| id= ISBN 0-471-58651-X
  12. ^
  13. ^
  14. ^ {{cite journal |author=Banerjee RV, Matthews RG |title=Cobalamin-dependent methionine synthase |journal=FASEB J. |volume=4 |issue=5 |pages=1450–9 |year=1990 |pmid=2407589 |url=
  15. ^ {{cite journal | author=Wickramasinghe SN | title=Morphology, biology and biochemistry of cobalamin- and folate-deficient bone marrow cells | journal=Baillieres Clin Haematol | year=1995 | volume=8 | pages=441-459 | id= PMID 8534956
  16. ^ Accessed Dec. 21, 2007
  17. ^
  18. ^ {{cite journal | author = Doscherholmen A et al. (1975) | title = Proc Soc Exp Biol Med, Sep;149(4):987-90;
  19. ^ {{cite journal | author= Wakayama EJ, Dillwith JW, Howard RW, Blomquist GJ| title= Vitamin B-12 levels in selected insects| journal= Insect Biochemistry| year= 1984| volume= 14| pages= 175-179
  20. ^ a b {{cite web | title=Vegan Society B-12 factsheet | url= | last=Walsh | first=Stephen, RD | publisher=Vegan Society | accessdate=2007-03-12
  21. ^ {{cite journal |title=Metabolic vitamin B-12 status on a mostly raw vegan diet with follow-up using tablets, nutritional yeast, or probiotic supplements | last=Donaldson | first=MS |publisher=Ann Nutr Metab. 2000;44:229-234
  22. ^ {{cite web |url= |title=What every vegan should know about vitamin B-12 |accessdate=2007-12-03 |author=Stephen Walsh |publisher=Vegan Society
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This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Vitamin_B12". A list of authors is available in Wikipedia.
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