Drug allergy overview
Editor-In-Chief: C. Michael Gibson, M.S., M.D.  Associate Editor(s)-in-Chief: Charmaine Patel, M.D. 
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A drug allergy, or immunologic drug reaction is a reaction to a drug that specifically occurs through immunologic activation in response to a drug. Drug allergy is a sub-classification of adverse drug reactions. Adverse drug reactions are classified into two categories; Type A (predictable), and Type B (unpredictable). Drug allergy falls under Type B adverse drug reactions, along with pseodoallergic drug reactions.
Drug allergies are classified using two main sets of criteria. One is set forth by the World Allergy Association, and classifies the reaction as to whether it occurs within one hour of treatment, or after one hour. The other set of criteria are based on the type of immunologic reaction that has occurred and the mediators involved.
An allergic reaction will not occur on the first exposure to a substance. The first exposure allows the body to create antibodies and memory lymphocyte cells for the antigen. However, drugs often contain many different substances, including dyes, which could cause allergic reactions. This can cause an allergic reaction on the first administration of a drug. For example, a person who developed an allergy to a red dye will be allergic to any new drug which contains that red dye. Medications can cause allergic reactions through various mechanisms. The drug can either act as a direct antigenic particle, or it can cause activation of immune cells by the direct interaction with immune cell receptors.
The types of drugs that can cause drug allergies vary. Drugs containing sulfa are common in causing drug allergy reactions. Other common drugs implicated in leading to an allergic reaction are antibiotics, insulin, and iodinated drugs.
Differentiating Drug Allergy from other Diseases
A drug allergy is different from an intolerance. A drug intolerance, which is often a milder, non-immune-mediated reaction, does not depend on prior exposure. Most people who believe they are allergic to aspirin are actually suffering from a drug intolerance.
Epidemiology and Demographics
Adverse drug reactions may occur in up to 10% of the worldwide population, and also affect up to 20% of patients who are hospitalized. Of all the cases of fatalities caused by anaphylaxis, drugs may be responsible for 20% of the deaths.
There are known risk factors for the development of drug allergies. Some are based on the patient and include; female gender, being young or middle aged, genetics, presence of certain viral infections (HIV and EBV), and history of atopy or prior drug allergies. Other factors are based on the quality of the drug, and include; high molecular weight compounds, intravenous and intramuscular routes of administration, and frequent, prolonged, high doses of medication.
In general, screening for drug allergies is discouraged. To screen for drug allergy, one would have to be exposed to the drug, which may mean putting the individual at an uneccesary risk for a severe drug reaction through a screening test. Screening for drug allergy may be useful in patients who are in need of a specific medication, but are susceptible or carry risk factors for having a severe allergic reaction upon exposure to the drug. Testing is usually reserved as a confirmatory diagnostic test in individuals who have already been shown to have an allergic reaction to a drug.
Natural History, Complications and Prognosis
In general, any allergic reaction will worsen with continued exposure to the offending agent. Anaphylaxis will most certainly progress to death if untreated, and the same is true for the severe allergic reactions such as Stevens-Johnson syndrome and toxic epidermal necrolysis.
History and Symptoms
Symptoms of a drug allergy can range from mild itching and rash, to serious exfoliative dermatological conditions that can be life threatening. Symptoms can also include the respiratory system, to cause wheezing and trouble breathing, as well as cause anaphylaxis and anaphylactic shock.
Findings on physical examination will vary widely in drug allergy, depending on the specific manifestation of the type of drug allergy (Stevens-Johnson Syndrome, toxic epidermal necrolysis), and the severity of the allergic reaction. A full, comprehensive physical examination should be performed to distinguish drug allergy from any other causes and conditions, and particular attention should be given to the respiratory and cardiovascular systems to quickly recognize impending life-threatening reactions.
Drug allergy and it's associated conditions is primarily a clinical diagnosis based on the patient history, and through physical exam. Certain laboratory findings may be seen during the acute phase of the reaction, but are not always specific. Skin testing and biopsies can be performed when there is not a clear diagnosis.
The management strategies for drug allergy include both acute and long-term treatment, as well as drug desensitization and graded challenges for patients in whom the drug they are allergic to is absolutely neccesary for treatment.