Type I immediate hypersensitivity reaction to cyanocobalamin but not hydroxycobalamin.

We report a case of a 42-year-old woman with a background of autoimmune polyglandular syndrome, who developed a type I immediate hypersensitivity reaction to intramuscular cyanocobalamin. Intradermal testing showed a positive reaction to cyanocobalamin. The patient was subsequently treated with intramuscular hydroxycobalamin after negative intradermal testing to this alternative B(12) compound. A review of previously described cases of hypersensitivity to either compound provides a rationale for the management of this rare but serious side-effect.