If someone develops pruritus, rash, hives or difficulty breathing after taking certain medication, that person may have a drug allergy. In case of drug allergy, body’s immune system become sensitized to a substance (usually a protein) in the medication.
Immune system perceives it as a foreign invader and produce a specific reaction. Allergic reactions can vary from hives to life threatening allergic reactions (anaphylaxis). True allergy to drugs are not common. A number of times common side effects of medications are perceived as drug allergy.