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. The information provided herein should not be used for diagnosis or treatment of any medical condition. It is provided for your general information and is not a substitute for medical care or supervised medical treatment. A licensed physician should be consulted for diagnosis and treatment of any and all medical conditions.