A drug allergy is the abnormal reaction of your immune system to a medication. Any medication — over-the-counter, prescription or herbal — is capable of inducing a drug allergy. However, a drug allergy is more likely with certain medications.
A drug allergy is not the same as a drug side effect, a known possible reaction listed on a drug label. A drug allergy is also different from drug toxicity caused by an overdose of medication. There are many ways in which people can react to drugs and medicines but not all of these are allergy related, which can cause confusion. Some people are genuinely allergic to certain drugs, but this is quite rare. Most drug ‘reactions’ are due to the various issues described below.