Drug Allergy Overview

Drug allergies occur when your immune system mistakes a drug for a harmful substance. Prescription or over-the-counter drugs may cause allergic reactions.

To understand why people develop drug allergies, it is essential to understand how the immune system works, and how it can go wrong. Our immune system normally responds to dangerous external elements by producing a group of cells known as Immunoglobulin E (IgE) antibodies. These antibodies defend us against harmful irritants and toxins that can cause illness. 

For those with an allergy, however, the immune system gets overly sensitized to certain substances, thinking they are harmful even though they really are not. This sensitization triggers an inflammatory response—an allergic reaction. The most common symptoms of an allergic reaction to a drug are hives, skin rash, or fever.

A true allergic reaction to a drug cannot occur without an initial exposure to the drug, sensitization (when IgE is previously induced by that exposure), and then another exposure. That means you may have an allergic reaction to a drug that has caused no effects in the past.

Another thing to bear in mind is that many drugs produce side effects that are not an allergic reaction. Only about 10 percent of adverse drug events are actually allergic reactions. Nevertheless, side effects of drugs can be dangerous, and it is essential that a doctor be seen to check any significant adverse reactions. Both allergic and nonallergic drug reactions can be life-threatening.