A drug allergy is an allergy to a drug, most commonly a
medication. Medical attention should be sought immediately if an
allergic reaction is suspected. 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. 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.