Adverse drug reaction is an injury caused by taking medication. Adverse drug reactions 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 Adverse drug reactions is the concern of the field known as pharmacovigilance. An adverse drug event (ADE) refers to any injury occurring at the time a drug is used, whether or not it is identified as a cause of the injury. An Adverse drug reaction is a special type of ADE in which a causative relationship can be shown. Adverse drug reactions are only one type of medication-related harm, as harm can also be caused by omitting to take indicated medications.