Most, if not all, medications have some sort of side effect or reaction that varies from person to person. In some cases these reactions happen because of a drug allergy, and are not the typical side effects. For this reason, it is important to know what side effects to expect from medications, so you can identify possible allergic reactions.
Simply put, an allergy is the immune system overreacting to an otherwise harmless substance, like medication. Drug allergies not only develop when you take a medication by mouth but can also develop when you rub medication directly on the skin or have it administered by injection.