A drug allergy is an immunologic reaction to a medication and should be distinguished from a non-immunologic adverse drug reaction. All medications potentially have side effects, which may or may not be related to the drug's mechanism of action. Side effects may include nausea, vomiting, diarrhea, muscle aches, headache, fatigue, drowsiness, or insomnia.

A drug allergy is not a side effect and may result in itching, rash, hives, facial swelling, difficulty breathing, and a drop in blood pressure. Typically, an allergic reaction to a drug will not occur the first time the medication is taken. The immune system needs to be exposed (or sensitized) to the drug before a reaction can develop.

Sensitization to one drug may lead to an allergic reaction to another drug in the same class, such as for penicillin-derived antibiotics or non-steroidal anti-inflammatory drugs (NSAIDs). If you are concerned that you have a drug allergy, please consult with your primary care physician, who may decide that you may need evaluation by an allergist.