What is the definition or description of: drug reaction?

Unintended effect: When an unintended effect, previously known or unknown, occurs after a drug is used.

Drug reaction: A drug reaction is an unintended health consequence of taking a medication. This includes known pharmacological side effects as well as known idiosyncratic effects. Drug allergy is a bit different and involves suspected immunologic response to the drug. An allergy can involve hives, anaphylaxis, angioedema, itchy or blistering rash, joint pain, blood disorders and even organ failure.