What Is A Drug Allergy, And What Are The Most Common Symptoms?

Adverse effects from drugs are extremely common and can be divided into two broad categories. Drug intolerance represents a pharmacologic effect of a medication that creates undesirable symptoms. A good example is nausea and vomiting after taking codeine cough syrup or insomnia after using pseudoephedrine. These intolerances do not represent allergies and will be relatively consistent each time the drug is taken. The other major category of reactions is an  immunologic sensitivity induced by the drug, broadly referred to as a drug allergy. Drug allergies are much less common, accounting for about 5–10% of adverse reactions to medications.

In order for patients to develop an allergy to a drug, they need to have taken the medication before. Once they have taken the drug on one or more occasions, their immune system is then able to form a specific type of response to the drug that causes them  to have allergic symptoms when they take it again. The most common mechanism by which the immune system reacts to a drug is the development of IgE antibodies. These antibodies lead to allergic reactions which come on very quickly, usually within minutes to hours after taking a drug. The severity of a reaction is related to the amount of drug taken, the route of delivery (injectable medications cause worse reactions than oral), and the level of allergic sensitivity. The most common symptoms caused by an IgE reaction to a drug are itching of the skin along with hives and/or swelling. Occasionally, patients will develop severe generalized reactions that  extend to other organs beside the skin, consisting of  wheezing, throat swelling, abdominal cramping, diarrhea, dizziness, and possibly a large drop in blood pressure (shock).

Other immunologic reactions to drugs are not related to IgE antibodies but are instead caused by immune cells. These reactions, referred to as cell-mediated immune reactions, typically consist of a diffuse pinpoint rash that involves much of the body and begins several days after starting the medication. Another less common mechanism is caused by IgE antibodies, called an “immune complex reaction.” The immune complexes, which consist of a clump of IgE antibodies bound to the drug, circulate in the bloodstream and then become lodged in body tissues, particularly the skin and kidney. The small hemorrhages caused by these immune complexes are manifested as small bloody dots or bruises in the skin or bleeding into the kidney.