Adverse drug reactions represent a major health problem. an adverse effect is an abnormal, harmful, undesired and/or unintended side effect. A focus on drug interactions. adverse drug reaction An adverse drug reaction (abbreviated ADR ) is a term to describe the unwanted, negative consequences sometimes associated with 6. adverse effect (medicine) In medicine , an adverse effect is an abnormal, harmful, undesired and/or unintended side-effect, although not necessarily