Side Effects of Intramuscular Cyanocobalamin 2000 mcg Daily for One Week
Intramuscular cyanocobalamin 2000 mcg daily for one week is generally safe with minimal side effects, though serious adverse events including anaphylactic shock, severe hypokalemia, and sudden death have been reported, particularly in patients with severe megaloblastic anemia. 1
Serious Adverse Events (Rare but Critical)
Anaphylactic shock and death have been reported after parenteral vitamin B12 administration, warranting consideration of an intradermal test dose in patients suspected of being sensitive to this drug 1
Severe hypokalemia and sudden death may occur when severe megaloblastic anemia is treated intensely with parenteral B12, requiring close monitoring of serum potassium during the first 48 hours of treatment 1
Severe and swift optic atrophy has occurred in patients with early Leber's disease (hereditary optic nerve atrophy) who were treated with cyanocobalamin 1
Special Population Warnings
Benzyl alcohol toxicity: This product contains benzyl alcohol, which has been associated with a fatal "Gasping Syndrome" in premature infants 1
Aluminum toxicity: The product contains aluminum that may reach toxic levels with prolonged parenteral administration if kidney function is impaired, particularly in premature neonates who are at risk for central nervous system and bone toxicity at levels greater than 4-5 mcg/kg/day 1
Common Side Effects and Monitoring Requirements
Rapid biochemical changes occur within 24 hours of intramuscular administration, including significant increases in serum vitamin B12 and plasma total cysteine, with decreases in serum folate, plasma total homocysteine, and serum methylmalonic acid 2
Hematological response typically begins within 5-7 days, with reticulocyte counts increasing and mean cell volume decreasing by day 14 of therapy 1, 2
Local injection site reactions are possible with any intramuscular injection, though not specifically quantified in the literature for this dose 3
Critical Monitoring Protocol
Serum potassium must be observed closely during the first 48 hours of treatment and replaced if necessary, particularly in patients with pernicious anemia or severe megaloblastic anemia 1
Hematocrit and reticulocyte counts should be repeated daily from the fifth to seventh days of therapy and then frequently until the hematocrit is normal 1
Folate and iron levels should be monitored, as complicating deficiencies may inhibit marrow response 1
Important Clinical Caveats
The 2000 mcg daily dose for one week exceeds standard guideline recommendations, which typically suggest 1000 mcg on alternate days or three times weekly for initial treatment 3, 4
Cyanocobalamin should be avoided in patients with renal dysfunction due to potential accumulation of the cyanide moiety and increased cardiovascular risk; methylcobalamin or hydroxocobalamin are preferred alternatives in this population 5
Folic acid should never be administered before treating vitamin B12 deficiency, as it may mask anemia while allowing progression of irreversible neurological damage (subacute combined degeneration of the spinal cord) 1