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Allergic and Pseudoallergic Drug Reactions




Upon completion of the chapter, the reader will be able to:

  1. Describe the potential incidence of allergic and pseudoallergic reactions and why it is difficult to obtain accurate numbers.

  2. Describe the Gell and Coombs categories of reactions.

  3. Identify the classes of drugs most commonly associated with allergic and pseudoallergic reactions.

  4. Recommend specific treatment for a patient experiencing anaphylaxis.

  5. Recommend an approach to drug selection in patients with multiple drug allergies.

  6. Describe drug desensitization procedures for selected drugs.


Allergic and pseudoallergic drug reactions are reported together. They are rarely confirmed by testing, making statistical analysis imprecise, with both over-reporting and under-reporting. KEY CONCEPT Approximately 5% to 10% of adverse drug reactions are allergic or immunologic; however, allergic and pseudoallergic reactions represent 24% of reported adverse drug reactions in hospitalized patients.1,2 These reactions are costly and cause considerable morbidity and mortality. Between 10% and 20% of hospitalized patients incur drug reactions (7% in the general population), with about one-third possibly due to hypersensitivity; however, most of these reactions are not reported, especially in pediatrics.1,3,4 Patients experiencing an allergic drug reaction in the hospital have increased costs of $275 to $600 million annually.5 This financial burden can occur due to several reasons, including increased indirect cost of (a) time and lost labor; (b) use of more costly alternative medications; and (c) treatment failures. Outpatient rates are not well studied and are much harder to collect. Relying on a patient’s history without an attempt to verify the relationships between drugs taken and symptoms experienced results in confusion. Health care professionals and patients use the term “drug allergy” in such a general way that it is not medically useful and, further, perpetuates a level of fear and concern in the public and in medical practice that is inappropriate and costly. This same confusion and anxiety sometimes lead medical personnel to ignore or forget “drug allergy” with potentially catastrophic results. Clearly, an understanding of how allergic and pseudoallergic reactions occur and how they might be managed or prevented is important to health care professionals and their patients.


Drug allergies are immune responses resulting from different mechanisms of immunologic recognition and activation, and reactions are produced by multiple physiologic pathways. This produces a confusing spectrum of clinical pictures and complex pathophysiologic mechanisms. The Gell and Coombs classification has been used for decades and still provides a framework for thinking about mechanisms of immunologic drug reactions, as shown in Table 54–1.6,7

Table 54–1Reaction Classification, Clinical Symptoms, and Potential Causative Drugs6,7

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