Sulfur allergy

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Editor, – I refer to the article 'Sulfur allergy' label is misleading' (Aust Prescr 2008;31:8-10). In ophthalmology, it has been customary to use acetazolamide tablets for raised intraocular pressures not responding to local therapy. I note that your article does not mention acetazolamide.

I would be grateful for your advice about possible allergic reactions to acetazolamide. My concern relates to one patient who had a severe anaphylactic reaction, presumed to be due to acetazolamide.

Roger McGuinness
The Eye Institute
Bondi Junction, NSW

Dr William Smith, one of the authors of the article, comments:

The essential question is whether a patient who has a history of an allergic reaction to a sulfonamide antibiotic (sometimes inappropriately referred to as 'sulfur allergy') is at increased risk of an allergic reaction to acetazolamide compared to a patient with a history of allergy to an unrelated drug, or with no drug allergy history.

It is known that being allergic to one drug increases the risk of allergy to other drugs, regardless of the structural difference or similarity of the second drug. In fact the more drugs one is allergic to, the greater the risk that one will have a reaction to any other drug. This is a separate issue to cross-reactive allergy, which depends on the structural relatedness of the drug, such that the immune system, primed to respond to one drug, will react with a second structurally similar drug.

Firstly, acetazolamide, although a sulfonamide, is not a sulfonylarylamine sulfonamide and is therefore thought to be not sufficiently structurally similar to sulfonamide antibiotics to be cross-reactive to the immune system. Secondly, a survey of patients with a history of sulfonamide antibiotic allergy did not show an increased incidence of allergic reactions to non-antibiotic sulfonamides (including acetazolamide) above that conferred by a history of allergy to unrelated drugs.¹

The patient who had anaphylaxis to acetazolamide constitutes anecdotal evidence. It is most likely that this allergic reaction was coincidental and not specifically related to a previous history of allergy to sulfonamide antibiotics.

Current expert opinion, based on the evidence, would be that a history of sulfonamide antibiotic allergy should not be considered an absolute contraindication to the use of acetazolamide. (I acknowledge that this is contrary to the current product information; it would be wise for medicolegal reasons to employ caution in such patients.) Doctors should always be prepared to deal with allergic reactions to the medications they prescribe, although these reactions are rare. Intravenous drugs carry a risk of causing more severe anaphylaxis although not at any greater incidence compared with oral administration. The risk of such reactions will be increased above background levels in patients with a history of allergy to other drugs, particularly multiple other drugs, whether sulfonamide or not.

Editor, – I agree that the term 'sulfur allergy' (Aust Prescr 2008;31:8-10)is misleading in relation to allergic reactions to sulfonamide drugs and the confusion is contributed by the American custom of substituting 'f' for 'ph'.

John Walker
Ear, Nose and Throat Specialist
Edgecliff, NSW

Editor, – I was interested in the article on sulfur allergy (Aust Prescr 2008;31:8-10),not only for its content but by the metamorphosis of 'sulphur' to 'sulfur'. I acknowledge that language is in a constant state of flux but is this spelling an editorial decision or are we now all to use the American pharmacopeia for drug nomenclature?

Ross MacPherson
Clinical Associate Professor
Department of Anaesthesia and Pain Management
Royal North Shore Hospital
Sydney

The Therapeutic Goods Administration publishes the Australian Approved Terminology for Medicines (at www.tga.gov.au/docs/html/aan.htm). For more than a decade 'sulfur' has been the Australian approved name.