Cyanocobalamin is a form of vitamin used to treat vitamin 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 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 Leber's disease. Vitamin 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 2020, it was the 105th most commonly prescribed medication in the United States, with more than 6million 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 . It is also used to treat pernicious anemia, vitamin 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 to be acquired via food or vitamins. Cyanocobalamin is also used to perform the Schilling test to check ability to absorb vitamin . Cyanocobalamin is also produced in the body (and then excreted via urine) after intravenous hydroxycobalamin is used to treat cyanide poisoning. Side effects Possible side effects of cyanocobalamin injection include allergic reactions such as hives, difficult breathing; redness of the face; swelling of the arms, hands, feet, ankles or lower legs; extreme thirst; and diarrhea. Less-serious side effects may include headache, dizziness, leg pain, itching, or rash. Treatment of megaloblastic anemia with concurrent vitamin deficiency using vitamers (including cyanocobalamin), creates the possibility of hypokalemia due to increased erythropoiesis (red blood cell production) and consequent cellular uptake of potassium upon anemia resolution. When treated with cyanocobalamin, patients with Leber's disease may develop serious optic atrophy, possibly leading to blindness. Chemistry Vitamin is the "generic descriptor" name for any vitamers of vitamin . Animals, including humans, can