An adverse drug reaction (ADR) is associate degree injury caused by taking a medicine. ADRs might occur following one dose or prolonged administration of a drug or result from the mixture of 2 or additional medicine. The that means of this expression differs from the that means of "side effect", as this last expression may also imply that the results are often useful. The study of ADRs is that the concern of the sphere referred to as pharmacovigilance. associate degree adverse drug event (ADE) refers to any injury occurring at the time a drug is employed, whether or not or not it's known as a reason behind the injury.