Causes, Incidence and Risk Factors Drug allergies occur when there is an allergic reaction to a medication. This is caused by hypersensitivity of the immune system, leading to a misdirected response against a substance that does not cause a response in most people. The second or subsequent exposure causes an immune response . Reactions to drugs are uncommon, but almost any drug can cause an adverse reaction.
Most drug allergies cause minor skin rashes and hives.
Serum sickness is a delayed type of drug allergy that occurs a week or more after exposure to a medication or vaccine. Penicillin and related antibiotics are the most common cause of drug allergies. An examination of the skin may show hives , skin rash , or angioedema . Difficulty breathing , wheezing , and other symptoms may indicate anaphylactic reaction . Skin testing may confirm allergy to penicillin-type medications.