An allergic reaction to a medication is an overreaction by the body's immune system to a substance (allergen) in a medication a person has taken. An allergic reaction to a medication may cause symptoms that range from a minor rash to severe anaphylactic shock, depending on the person and the type and dose of the medication.

A medication allergy is different from an adverse medication reaction, such as a medication side effect or a reaction when taking more than one medication. Because symptoms and treatments vary, a health professional should determine whether a person has a medication allergy or an adverse reaction. A severe medication allergy can be life-threatening; an adverse reaction usually is not.

A person who has been diagnosed with a medication allergy should wear a medical alert bracelet or other identification and should not take that medication again.