Drug allergies occur when the immune system recognises a drug as a foreign substance and produces antibodies to attack it. Certain drugs are more likely to cause allergic reactions, including antibiotics, non-steroidal anti-inflammatory drugs (NSAIDs) and chemotherapy drugs. Some people may be more susceptible to drug allergies due to genetic factors, while most people usually develop an allergy after repeated exposure to a drug.The symptoms of a drug allergy can vary widely, but usually include a skin rash, hives, itching and swelling of the face, tongue or throat. More severe symptoms may include difficulty breathing, wheezing, chest tightness and anaphylaxis, a life-threatening reaction that can cause a sudden drop in blood pressure, tachycardia and loss of consciousness.