Some patients receiving vitamin B12 show an allergic reaction. The true frequency is unknown since most reported cases involve parenteral vitamin B12 administration. The association of a reaction to oral vitamin B12 might go unnoticed.
Forms of vitamin B12:
(1) hydroxycobalamin
(2) cyanocobalamin
Reactions:
(1) anaphylaxis
(2) anaphylactoid (with direct release of histamine)
(3) skin rash
(4) urticaria and dyspnea
Some patients react with one of the forms of vitamin B12 but can tolerate the other.
Allergy testing may include skin testing and blood RAST testing. However, a person can react even when all testing is negative.
A patient with a serious allergic reaction can undergo desensitization.
If the reaction is an anaphylactoid reaction then administration of low doses may be tolerated.
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