Cyanocobalamin is a form of vitamin B12 used to treat vitamin B12 deficiency except in the presence of cyanide toxicity. The deficiency may occur in pernicious anemia, following surgical removal of the stomach, with fish tapeworm, or due to bowel cancer. It is less preferred than hydroxocobalamin for treating vitamin B12 deficiency. It is used by mouth, by injection into a muscle, or as a nasal spray.Cyanocobalamin is generally well tolerated. Minor side effects may include diarrhea and itchiness. Serious side effects may include anaphylaxis, low blood potassium, and heart failure. Use is not recommended in those who are allergic to cobalt or have Lebers disease. Vitamin B12 is an essential nutrient meaning that it cannot be made by the body but is required for life.Cyanocobalamin was first manufactured in the 1940s. It is available as a generic medication and over the counter. In 2019, it was the 155th most commonly prescribed medication in the United States, with more than 4 million prescriptions. Medical use
Cyanocobalamin is usually prescribed after surgical removal of part or all of the stomach or intestine to ensure adequate serum levels of vitamin B12. It is also used to treat pernicious anemia, vitamin B12 deficiency (due to low intake from food or inability to absorb due to genetic or other factors), thyrotoxicosis, hemorrhage, malignancy, liver disease and kidney disease. Cyanocobalamin injections are often prescribed to gastric bypass patients who have had part of their small intestine bypassed, making it difficult for B12 to be acquired via food or vitamins.
SAMe is the universal methyl donor and is used for DNA methylation and to make phospholipid membranes, choline, sphingomyelin, acetylcholine, and other neurotransmitters. In mitochondria
The enzymes that use B12 as a built-in cofactor are methylmalonyl-CoA mutase (PDB 4REQ) and methionine synthase (PDB 1Q8J).The metabolism of propionyl-CoA occurs in the mitochondria and requires Vitamin B12 (as adenosylcobalamin) to make succinyl-CoA. When the conversion of propionyl-CoA to succinyl-CoA in the mitochondria fails due to Vitamin B12 deficiency, elevated blood levels of methylmalonic acid (MMA) occur. Thus, elevated blood levels of homocysteine and MMA may both be indicators of vitamin B12 deficiency. Adenosylcobalamin is needed as cofactor in methylmalonyl-CoA mutase—MUT enzyme. Processing of cholesterol and protein gives propionyl-CoA that is converted to methylmalonyl-CoA, which is used by MUT enzyme to make succinyl-CoA. Vitamin B12 is needed to prevent anemia, since making porphyrin and heme in mitochondria for producing hemoglobin in red blood cells depends on succinyl-CoA made by vitamin B12. Absorption and transport
Inadequate absorption of vitamin B12 may be related to coeliac disease. Intestinal absorption of vitamin B12 requires successively three different protein molecules: haptocorrin, intrinsic factor and transcobalamin II. See also
Methylcobalamin
Hydroxocobalamin
Adenosylcobalamin
Cobalamin biosynthesis

References
External links
"Cyanocobalamin". Drug Information Portal. U.S. National Library of Medicine.