药物不良反应（英语：adverse drug reaction，简称ADR）是患者在使用某种药物的治疗疾病的时候产生的与治疗无关的作用，而这种作用一般都对患者的治疗不利。不良反应是药物所具有的两重性之一，完全没有不良反应的药物是不存在的。同时，不良反应的发生也是有一定比例的，不是所有使用该药物的患者都会出现不良反应；每个出现不良反应的患者之间，其出现的不良反应表现和程度也不一定相同，存在着很大的个体差异性。不良反应包括6个方面，即副作用、毒性反应、变态反应、继发性反应、后遗效应和致畸作用。

An adverse drug reaction (ADR) is an injury caused by taking a medication. 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 expression differs from the meaning of "side effect", as this last expression might also imply that the effects can be beneficial. The study of ADRs 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 ADR is a special type of ADE in which a causative relationship can be shown.