APA Dictionary of Psychology

any unintended, harmful, and potentially fatal response to a drug. Reactions may be genetically determined (as in the case of hypersensitivity), in which case they are highly individual and can be difficult to predict, or they may arise through interactions with other prescribed or nonprescribed drugs or with dietary items (as in the case of monoamine oxidase inhibitors). This term sometimes is used synonymously with side effect, but an adverse drug reaction more properly denotes an unexpected negative occurrence, whereas side effects may be positive or negative and are usually anticipated. Also called adverse drug event (ADE); adverse event; adverse reaction.