Medications can have various side effects, including allergic reactions. These occur if the immune system considers the medication to be a harmful substance, and fights it. A drug is not the same as a drug intolerance, which is where your body's metabolism can't process the medication properly.
People who have a drug are often no longer sure which medications they can take, and which medications they can’t take. And it isn’t always easy to find alternatives to the medications you’re allergic to. So drug allergies not only cause acute symptoms, but may also delay treatments. Some people have to use less effective medications instead.