Myth of shellfish, iodine, and CT contrast allergy link debunked

By Steven Rothrock, MD, FACEP, FAAP on May 29, 2013

CT scans are some of the most commonly ordered tests in medicine. Contrast media or “dyes” are often administered intravenously (IV) before a CT scan is performed to make blood vessels and tissues with high blood flow stand out or appear brighter than surrounding tissue. There is a widely held misconception that persons who are allergic to shellfish are also allergic to IV contrast administered during CT scans. Iodine has been proposed as the common agent to both of these substances responsible for this allergy. Several large reviews have exposed the myth behind this misconception.1-4

Are those allergic to seafood, allergic to the iodine within seafood?

No one is allergic to iodine. This element is found throughout everyone’s body within their thyroid gland, breast tissue, and many other areas.1-4  Without iodine in our bodies, we cannot live. Both fish and shellfish also contain iodine as do many foods including yogurt, milk, bread, macaroni, eggs, corn and many beans.  Allergic reactions to shellfish and fish have been shown to be due to proteins within their muscles (tropomyosins, parvalbumin) and not their iodine content.1-5

On a related note, the iodine within topical cleansing solutions, Betadine and Povidine, does not cause allergic like symptoms. Like IV contrast, reactions or allergies that occur are not due to the iodine but are due to other substances within these solutions.4,5 When side effects occur, they also have been shown to be due to skin irritation and not allergy in most cases. A prior reaction to these solutions does not mean that IV contrast needs to be avoided in the future.5

What about iodine in IV contrast used for CT scans?

Many of the contrast materials used in CT scans are made from molecules that are related to iodine (triiodinated benzoic acid). Adverse reactions to these contrast materials have been shown to occur via a non-allergic mechanism.1,2 Reactions are not due to the iodine but are due to other substances within the contrast.1,2

A true allergic reaction or anaphylaxis occurs from release of the immunoglobulin IgE after exposure to an allergy inducing substance (allergen). IgE is produced following a prior exposure to an allergen. With IgE induced reactions, you must have been exposed to the substance in the past.4

Adverse reactions to contrast dyes are not mediated or caused by IgE. They occur because the chemicals within the contrast (not iodine) directly cause a release of histamine and other substances into the bloodstream and thus, are not truly allergic reactions. The terms anaplylactoid and non-allergic anaphylaxis are used to describe these reactions. With non-IgE adverse reactions, no prior exposure is needed and reactions are not truly allergic although symptoms are similar to true allergies.

What does a prior shellfish allergy mean?

Individuals who are allergic to any single substance (e.g. fish, foods, medicines) have an increased chance they might be allergic to another substance compared to those who have never had a prior allergy. Adverse reactions (non-allergic) to contrast media are doubled or tripled in these individuals. However, severe reactions to IV contrast still occur in less than 1% with allergies to other substances like shellfish. People with asthma, heart disease, kidney disease, or taking a type of blood pressure medicine (β blocker) are also at increased risk for adverse reactions to contrast media.1-6

How common are contrast reactions?

Many individuals receiving IV contrast have mild warmth, flushing of the skin, and nausea during or after administration. Pain on injection also can occur especially if higher doses are used or IV injection occurs too rapidly.6 Each of these is a normal reaction to receiving contrast material and not a true adverse reaction.

Typically used contrast media (low osmolar contrast media) cause mild to moderate side effects (e.g. itching, minor swelling) in 0.2 to 0.7% of individuals. Serious reactions with breathing difficulty or low blood pressure occur in 0.01 to 0.02% of individuals. Importantly, prior mild to moderate reactions do not predict severe reactions in the future and experts recommend administering IV contrast after premedication with steroids and antihistamines in these individuals.5,6 Serious reactions can be treated with fluids, asthma medicines, steroids, and antihistamines.6 Individuals with prior reactions to IV contrast need to have their physicians consider not using contrast or pre-medicating with steroids and antihistamines to decrease side effects depending upon the importance of the test, the severity of the prior reaction and other individual patient factors.

Expert recommendations

The American College of Radiology states that the predictive value of specific allergies to shellfish or other products is “unreliable.” “While a significant number of health care providers (mistakenly) continue to inquire specifically into a patient’s history of allergy to seafood, especially of shellfish, there is no evidence to support this practice.”6

The American Academy of Asthma, Allergy, and Immunology states that “it is a myth that shellfish allergy means it is unsafe to receive iodine dyes. If you have a shellfish allergy, you can most likely get radiocontrast medical procedures, unless you have a separate allergy to them.”7

Despite the clear evidence that no one is allergic to iodine and there is no direct link between shellfish allergy and use of contrast during CT scans, this myth has been hard to put to rest. In a survey of patients presenting to a pediatric clinic with suspected seafood allergy, 92% of parents mistakenly believed that iodine was the cause of that allergy.1 In a study from 2005, over one third of radiologist and one half of cardiologists would not use contrast media or would alter their treatment for patients undergoing procedures requiring the use of IV contrast (e.g. CT scans and heart catherizations) in those with shellfish allergy. The authors of this study called this practice antiquated and recommended that this myth “take its rightful place in oblivion.”1

The takeaway message

It’s a good thing no one is allergic to iodine. It is an essential part of our body and without it we would die. There is no link between shellfish, iodine, and contrast allergy.  Anyone, regardless of their prior allergies, can have a reaction to IV contrast. Fortunately, these reactions are usually mild and when severe can be treated with simple medications.


  1. Beaty AD, Lieberman PL, Slavin RG. Seafood allergy and radiocontrast media: are physicians propagating a myth? Am J Med 2008; 121: 158: e1-4.
  2. Coakley FV, Panicek DM. Iodine allergy: an oyster without a pearl. AJR 1997; 169: 951-952.
  3. Huang SW. Seafood and iodine: an analysis of a medical myth. Allergy Asthma Proc 2005; 26: 468-469.
  4. Schabelman E, Witting M. The relationship of radiocontrast, iodine, and seafood allergies: a medical myth exposed. J Emerg Med 2010; 39: 701-707.
  5. Katelaris CH. 'Iodine allergy' label is misleading. Australian Prescriber 2009; 32: 125-128.
  6. American College of Radiology Committee on Drugs and Contrast Media. Manual on Contrast Media. ACR 2012; Version 8.
  7. Pondgee T. Shellfish allergy is not a shell game. American Academy of Allergy, Asthma, & Immunology.