To the Editor: Cyanocobalamin and hydroxycobalamin are synthetically derived preparations of vitamin B12. Allergy to vitamin B12 injection is infrequent, but may be serious. We describe a patient with allergy to hydroxycobalamin, without cross-reaction to cyanocobalamin.

Our patient was a 45-year-old woman with vitamin B12 deficiency. She had positive antiparietal cell antibodies and normal results of Schilling's test after addition of intrinsic factor. Otherwise she was in good health, with no other evidence of autoimmune disease.

Her allergy commenced after an intramuscular injection of hydroxycobalamin, with onset of mild generalised pruritus. Subsequent monthly 1 mg injections of hydroxycobalamin were followed by incrementally worsening pruritus, and then frank urticaria. The last of nine injections was followed by urticaria, bronchospasm and oropharyngeal angioedema, which responded to administration of adrenalin.

The patient underwent skinprick and intradermal testing with hydroxycobalamin and cyanocobalamin. Wheal-and-flare reactions occurred with injection of dilutions of hydroxycobalamin, suggesting an IgE-mediated response. No reactions were evident with dilutions of cyanocobalamin (Box). Subsequently, the patient had no reaction to a challenge of subcutaneously administered cyanocobalamin 0.1 mL (100 μg), and then intramuscularly administered cyanocobalamin 0.5 mL (500 μg). Her macrocytic anaemia resolved with ongoing monthly injections.

After one year of treatment, the patient described an episode of delayed urticaria after a routine cyanocobalamin injection. The skinprick and intradermal tests were repeated, with negative reactions to cyanocobalamin, and wheal-and-flare reactions to hydroxycobalamin. She has since tolerated monthly intramuscular cyanocobalamin for over 12 months.

Vitamin B12 allergy is rare, but has been reported.1-3 Positive results of basophil histamine release assay and skin testing suggest an IgE-mediated mechanism.2 Desensitisation is therefore theoretically possible; however, anaphylaxis during desensitisation has occurred.3

In Australia, both hydroxycobalamin (Neo-cytamen, David Bull) and cyanocobalamin (Cytamen, David Bull) are available. The excipients of each preparation are identical — sodium chloride, glacial acetic acid, and sterile water.

Although the risk of severe allergy is low, adequate facilities for resuscitation should be available when parenteral vitamin B12 is administered. One approach to dealing with vitamin B12 allergy is to use the alternative compound after skin testing to exclude cross-reactivity. If cross-reactivity occurs, then desensitisation may be considered. Alternatively, oral administration of vitamin B12 may also be used.4