An adverse drug reaction (ADR) is a harmful, unintended result caused by taking medication.^{: 1.1 Adverse Drug Reaction (ADR)} ADRs may occur following a single dose or prolonged administration of a drug or result from the combination of two or more drugs. The meaning of this term differs from the term "side effect" because side effects can be beneficial as well as detrimental. The study of ADRs is the concern of the field known as pharmacovigilance. An adverse event (AE) refers to any unexpected and inappropriate occurrence at the time a drug is used, whether or not associated with the administration of the drug.^{: 1.2 Adverse Event (AE)} An ADR is a special type of AE in which a causative relationship can be shown. ADRs are only one type of medication-related harm, as harm can also be caused by omitting to take indicated medications.