Allergic drug reaction is an unintended side effect of a drug and it ranges from minor rash to life threatening reaction.
However, an allergy reaction to a drug is often misdiagnosed, over-diagnosed, or not reliably documented e.g. penicillin allergy. This becomes a problem when penicillin or related drugs e.g. Amoxycillin is essential and required by doctors to treat certain diseases and there is no better alternative. In such scenarios, a controlled allergy testing is required to see if the documented allergy is really true or not. If the testing shows that there is no allergy, then the drug can be safely used again.