In the case of a drug reaction, this response is harmful, and sometimes causes serious symptoms. An allergic reaction does not usually occur the first time a person is exposed to the drug. It is only after the body learns to recognize the substance that an immune system reaction is triggered. Almost any drug can cause a reaction. However, allergic reactions are quite rare considering the number of drugs commonly prescribed.
Drug reactions can vary from mild to severe and life threatening.
Most of the time, the allergic response takes the form of hives, a skin rash, or breathing difficulties. One type of drug reaction known as serum sickness develops a week or more after exposure to the medication or vaccine. There is no way a person can prevent the development of a drug allergy. Once a reaction has occurred, an individual should avoid that particular drug. Rarely, a person will need to take a drug even if he or she is allergic to it.