Causes and Risks 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. Please read it.