Distinguishing Hazards and Harms, Adverse Drug Effects and Adverse Drug Reactions

The terms 'adverse drug effects' and 'adverse drug reactions' are commonly used interchangeably, but they have different implications. Adverse drug reactions arise when a compound (e.g. a drug or metabolite, a contaminant or adulterant) is distributed in the same place as a body tissue (e.g. a receptor, enzyme, or ion channel), and the encounter results in an adverse effect (a physiological or pathological change), which results in a clinically appreciable adverse reaction. Both the adverse effect and the adverse reaction have manifestations by which they can be recognized: adverse effects are usually detected by laboratory tests (e.g. biochemical, haematological, immunological, radiological, pathological) or by clinical investigations (e.g. endoscopy, cardiac catheterization), and adverse reactions by their clinical manifestations (symptoms and/or signs). This distinction suggests five scenarios: (i) adverse reactions can result directly from adverse effects; (ii) adverse effects may not lead to appreciable adverse reactions; (iii) adverse reactions can occur without preceding adverse effects; (iv) adverse effects and reactions may be dissociated; and (v) adverse effects and reactions can together constitute syndromes. Defining an adverse drug reaction as "an appreciably harmful or unpleasant reaction, resulting from an intervention related to the use of a medicinal product" suggests a definition of an adverse drug effect: "a potentially harmful effect resulting from an intervention related to the use of a medicinal product, which constitutes a hazard and may or may not be associated with a clinically appreciable adverse reaction and/or an abnormal laboratory test or clinical investigation, as a marker of an adverse reaction.