Drug Allergy

Introduction

"Do you have any drug allergies?"

Often, when dispensing medications to patients (especially antibiotics or NSAIDs) or clerking CP1 form, we ask patients this question.

NOTE: 8 food-types cause about 90% of food allergy reactions.

  • Milk (mostly in children)
  • Eggs
  • Peanuts
  • Tree nuts (e.g. walnuts, almonds, pine nuts, brazil nuts and pecans)
  • Soy
  • Wheat (and other grains with gluten, including barley, rye and oats)
  • Fish (mostly in adults)
  • Shellfish (mostly in adults)

Major Food Allergens



Case Scenario

The onset of hives within a few hours of taking a new medication is easily recognized as possible drug hypersensitivity. However, many clinical presentations of drug hypersensitivity are more complex or take place in the setting of illness and/or polypharmacotherapy. For example,
  • The patient in intensive care who develops a rash while receiving multiple medications
  • The ambulatory with complex chronic diseases who develops a new and unexplained symptom while taking many medications.
The main questions that arise in such situations include:
  • Is the adverse event related to a drug?
  • If so, which drug is responsible?
  • Is the reaction due to an immune response to the drug (i.e. true drug allergy) or due to another mechanism, such as pseudoallergy?
  • If the reaction is a drug allergy, what is the likely mechanism?
  • How can necessary therapy for the underlying medical problem be continued?



Drug Allergy

Everyone reacts to drugs differently. A person who takes a certain drug may experience rashes, while another person who takes the same drug may not have any adverse reactions. Collectively, drug allergy, intolerance and nonimmune drug hypersensitivity reactions have been estimated to comprise 25% of all adverse drug events. Drug allergy is considered to be relatively rare, representing 6-10% of all adverse reactions.

Drug allergy refers to adverse drug reactions due to hypersensitivity that mediates a specific immune response directed at the drug.

  • The parent drug or its reactive metabolite serves as an antigen, which is subsequently recognized and processed by the immune system, culminating in the production of drug-specific antibodies or sensitized T lymphocytes.
  • The antigen-antibody reaction can target a variety of cells and body tissues, leading to organ-specific or more generalized systemic adverse events.
Drug Allergy vs Drug Hypersensitivity Reactions

NOTE: It is often difficult to distinguish between allergy and non-immune drug hypersensitivity reaction. Most importantly, the signs, symptoms and severity of the reaction, rather than its mechanism, should drive clinicians' decision-making.



Clinical Presentation

Immunologic drug reactions are divided into 4 categories according to the Gell and Coombs system.

Gell and Coombs system of Allergic Drug Reaction

The World Allergy Organization (WAO) has recommended dividing immunologic drugs reactions into 2 types:

  • Immediate reactions, occurring within 1 hour of the first administered dose.
    • These reactions typically present as angioedema, urticaria, bronchospasm or anaphylaxis.
  • Delayed reactions, occurring after 1 hour but usually more than 6 hours and occasionally weeks to months after the start of administration.
    • Constitute a broader category of events including maculopapular exanthems, delayed urticaria, immune-mediated blood disorders and serum skin reactions.

NOTE: Vancomycin-induced red man syndrome is thought to be nonimmune drug hypersensitivity reaction, whereas vancomycin-associated blood dyscrasias and anaphylaxis are attributed to more rare allergic reactions.



Medications Commonly Associated with Allergies

  • Antibiotics, such as penicillin, cephalosporins, sulfonamides
  • NSAIDs



Management

Recommended treatment regimens for drug allergy vary based on the signs and symptoms of the reaction and the type of allergic syndrome. Reactions mediated by nonimmune mechanisms are treated in a manner similar to those of true allergic reactions, with the choice of therapy based on the patient's signs and symptoms.

Management of Drug Allergy



Information for Patients

Patients in whom an allergic reaction to a drug develops should be educated as to the name of the specific drug, the terminology used to describe the reaction, and the likelihood of having a similar or more severe reaction upon re-exposure to the drug.

  • Allergic card may be issued or patients should be advised to wear a medical alert tag or bracelet.

Patients with a history of allergy to a drug or multiple drugs should be encouraged to ask questions about newly prescribed medications.

  • They should be educated regarding the avoidance of the causative drug and other cross-reactive drugs in the future.

In the case of a severe immediate reaction to a drug or chemical, the patient may be prescribed injectable epinephrine in an easily injectable form for use in an emergency situation.



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