Adverse effects, poisoning and monitoring

A new medicine reaches the market after pre-clinical testing in animals and cells and three phases of clinical trials in humans. Those trials are designed to answer a specific question: is this drug effective and safe enough in a defined group of patients? That design has limits. First, numbers: a trial may include a few hundred to a few thousand carefully selected patients, so an adverse effect that occurs in one person in ten thousand will usually be missed. Second, time: trials run for months, while a drug prescribed for a chronic disease may be taken for years, so late effects cannot appear during the trial. Third, selection: trials commonly exclude pregnant women, children, the very old, and patients with kidney, liver or heart disease, and they often exclude patients taking many other medicines — precisely the people who will receive the drug most often in ordinary practice. Fourth, conditions: trials are conducted with close monitoring and clear instructions, whereas ordinary use involves missed doses, self-medication, alcohol and interactions with other products. Phase IV, the post-marketing surveillance phase, exists to close this gap. Once the drug is licensed and used widely, reports of efficacy and toxicity arrive from practitioners and are reviewed, and the licence can be renewed, restricted or cancelled. Rare reactions, interactions and effects in previously excluded groups surface at this stage. This is why no drug is ever fully tested at the moment of approval, and why reporting suspected reactions is part of using medicines responsibly rather than an optional administrative task.

The words used for drug harm are not interchangeable, and each carries a different question. A side effect is a predictable, often unavoidable effect at normal doses; it is usually known before the drug is marketed and may be managed rather than avoided, as when a sedating medicine is taken at night. An adverse drug reaction is a harmful and unintended response at normal doses; the emphasis is on harm and on the dose being normal, which separates it from poisoning. An allergic reaction is an immune response; it may occur at any dose, may appear only on a second exposure, and can range from a rash to life-threatening swelling or difficulty in breathing. Poisoning or overdose is harm from too much drug, whether accidental or deliberate. An interaction is not a property of one drug alone but of the combination: another medicine, a food or a disease state changes what the body does to the drug or what the drug does to the body. A further practical distinction is between a reaction that is a known effect of the class and one that is unexpected. Known effects are managed by warning, monitoring and dose adjustment; unexpected effects are the ones that must be reported, because they may reveal something that was not known when the drug was licensed. Using these words precisely is not pedantry: the name you give an event decides whether you observe, adjust, stop, treat as an emergency or report.