Description Injectable form of vitamin B 12 that has been used therapeutically to treat vitamin B 12 deficiency. Mechanism of action Vitamin B12 exists in four major forms referred to collectively as cobalamins- deoxyadenosylcobalamin, methylcobalamin, hydroxocobalamin, and cyanocobalamin. Two of these, methylcobalamin and 5-deoxyadenosyl cobalamin, are primarily used by the body. Methionine synthase needs methylcobalamin as a cofactor. This enzyme is involved in the conversion of the amino acid homocysteine into methionine. Methionine in turn is required for DNA methylation. 5-Deoxyadenosyl cobalamin is a cofactor needed by the enzyme that converts L-methylmalonyl-CoA to succinyl-CoA. This conversion is an important step in the extraction of energy from proteins and fats. Furthermore, succinyl CoA is necessary for the production of hemoglobin, the substances that carries oxygen in red blood cells.