Cyanocobalamin is contraindicated in those with hypersensitivity to cobalt moiety or cobalamin molecule due to the risk of anaphylaxis.
The use of cyanocobalamin is warned in patients with early Leber’s disease as there have been reports of severe and swift optic atrophy with its administration. Appropriate caution should be exercised while treating severe megaloblastic anemia with cyanocobalamin as intense treatment may lead to hypokalemia and sudden death. Cautious use of parenteral cyanocobalamin is also recommended in patients with renal impairment, including premature neonates, because of the possibility of greater aluminum accumulation, which may cause central nervous system and bone toxicity. Formulations of cyanocobalamin injection containing benzyl alcohol as a preservative should also be avoided in premature neonates and those with hypersensitivity due to its association with ‘gasping syndrome.
A history of the patient’s allergies/hypersensitivity should be obtained before administering cyanocobalamin injection. If the patient is suspected to be sensitive to cobalt or other components of cobalamin, an intradermal test dose is recommended.
Several laboratory tests should be performed prior to treatment with cyanocobalamin, including serum vitamin B12, folate, iron, hematocrit, and reticulocyte count. All these parameters need to be normal before initiating the treatment. Serum levels of vitamin B12 and peripheral blood counts should be monitored in one month. For hematocrit and reticulocyte counts, recommendations are to repeat these tests daily from the 5th to 7th days of treatment and then frequently until the hematocrit returns to a normal range.
Both serum potassium concentrations and the platelet count need to be monitored carefully after parenteral administration of cyanocobalamin. This is because hypokalemia and thrombocytosis could occur due to the increase in erythrocyte metabolism following vitamin B12 therapy. Potassium replacement therapy should be administered if necessary.
Patients with pernicious anemia are three times more likely to have gastric carcinoma compared to general population; thus, appropriate tests need to be carried out to rule out this condition if suspected.
Therapeutic response to cyanocobalamin may decrease due to elderly age, infection, renal insufficiency, diabetes mellitus, marrow suppressants use (e.g. chloramphenicol), and concurrent iron or folic acid deficiency. Therefore, these factors should be taken into consideration and regular monitoring should be performed in these conditions while treating vitamin B12 deficiency with cyanocobalamin.