Iodine Dye Allergy

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At least once a week, we hear a patient tell us that they are scared to eat fish because they had an allergic reaction to iodine. The next part of the story involves the Radiology procedure they underwent when they had the reaction. Let’s untangle two myths and shed some light on this:

1) Radiocontrast Media (RCM) that contains organic iodine may cause adverse reactions such as urticaria, angioedema, bronchospasm, laryngospasm and shock. These reactions are not truly allergic in origin. Instead, a non-allergic pathway triggers what looks exactly like an allergic reaction. While we don’t know the exact mechanism, it is apparently related to the high osmolarity (concentration of particles) of these agents. If you have had this type of reaction and need another procedure done, your physician will probably recommend a lower osmolar preparation to decrease the risk of another reaction.

The risk can be further decreased by pre-medicating with antihistamines (e.g., Allegra, Claritin/Clarinex or Zyrtec) and steroids (e.g., prednisone). I don’t recommend Benadryl anymore for outpatient pre-medication protocols. The risk of tolerating the procedure just fine, but then falling asleep and crashing your car into a tree on the way home must also be considered. Why would you fall asleep? Don't forget that most over the counter sleeping pills are made of diphenhydramine (the active ingredient in Benadryl).

2) Seafood may contain iodine. However, seafood allergy is not caused by iodine; rather, to specific proteins in fish and shellfish (e.g., parvalbumin, tropomyosin) that also do not contain iodine. Thus, fish or shellfish allergy does not indicate a sensitivity or allergy to iodine.

So how did the two myths get tangled? Allergic reactions tend to occur in people who already have other allergic reactions. So the person who has allergies (e.g., shrimp) is at higher risk to have another unrelated allergic reaction than the person with no history of allergies.

Many thanks to the American Academy of Allergy, Asthma and Immunology for providing this update. Their website is a terrific source for physicians and health consumers.

David B. Engler, MD

Note: Information contained in this article should not be considered a substitute for consultation with a board-certified allergist to address individual medical needs.