Allergic reactions to drugs are common. Some allergic reactions occur after a person has been receiving a drug for some time, while other reactions occur shortly after a person takes the drug and are known as “immediate drug hypersensitivity.” Immediate drug hypersensitivity reactions can vary from mild skin rashes to life-threatening reactions associated with difficulty breathing, shock, and even death. Sometimes it is difficult to confirm whether a person has had a true immediate hypersensitivity reaction to a specific drug or type of drug. This lack of certainty can be problematic when the patient needs the specific drug or a related drug again and there is no easy substitute. With some drugs, such as penicillin, doctors can test for allergic reactions by administering small amounts of the drug under the patient's skin in a procedure called skin testing. However, skin testing is not an option for other types of drugs. In some cases, the only way to know whether the patient is really allergic to the drug is to give the drug again and see what happens. This is called a drug provocation test.