Drug allergies involve allergic reactions to medications. As with all allergies, a person does not show any symptoms during the initial exposure to a drug. Instead, the body goes through the process of sensitization, where the immune system perceives the drug as a harmful invader upon first encounter and begins producing specific immunoglobulin E (IgE) antibodies to attack the drug the next time it appears in the body.

During this subsequent exposure, the body experiences an allergic reaction, where mast cells and other cells coated with IgE release histamine and other chemicals that attempt to fight off the drug while producing the symptoms associated with the allergy. 

Most drug allergies manifest themselves within minutes, hours, days or weeks of taking a drug. Hives or an itchy rash are typical of many allergic reactions and can be easily treated with medications. However, in rare circumstances a drug will provoke anaphylaxis, an allergic reaction involving two or more body systems. Anaphylaxis can lead to the potentially fatal anaphylactic shock if not treated immediately.

Serum sickness is a drug allergy with symptoms that do not appear until a week or more after exposure to a medication or antiserum. Antiserum is a preparation of serum (the clear fluid portion of blood that contains antibodies) used in immunizations such as tetanus or rabies shots. 

Some people also react to the iodine found in contrast dyes that are used during x-ray procedures.

A person who reacts to a drug is not necessarily allergic to it. Most drug reactions are idiosyncratic, meaning they are simply side effects of the drug that do not involve the formation of antibodies. An uncomfortable side effect of a drug – such as nonallergic Hives – is not necessarily the result of an allergy.