A drug allergy happens when the body's immune system overreacts to a substance (allergen) in a medicine that the person has taken. This causes an allergic reaction. Symptoms can include hives or welts, shortness of breath, rash, swelling, redness, and blisters.

A drug allergy can cause anaphylaxis in certain people. Anaphylaxis is a severe whole-body (systemic) reaction that can be deadly.

Symptoms of an allergic reaction can occur within an hour or so of taking the medicine. Or they can take days or weeks to appear.

A side effect is a symptom that is caused by the way a drug works. Or it's caused by some ingredient in the drug. It can be hard to tell the difference between an allergy and a side effect.

Because the treatment varies, a doctor should figure out if someone has a drug allergy or a side effect from a drug. Treatment of drug allergies includes not taking the medicine that causes the reaction and taking medicine to relieve symptoms. In severe cases (anaphylaxis), emergency care is needed. People who have a drug allergy should wear a medical ID tag (such as a medical alert bracelet).