Adverse Drug Reaction (ADR)
An adverse drug reaction is a noxious, unintended response to a medicine given at normal doses, and unlike an adverse event the term carries a causal judgement inside it.
An adverse drug reaction is a response to a medicine that is noxious and unintended and occurs at doses normally used in humans. The definition contains a causal claim, which is what separates it from an adverse event: anything unpleasant that happened during treatment, causality unresolved. The Rawlins-Thompson scheme splits reactions into Type A, augmented pharmacology that is dose-dependent and predictable from the mechanism, and Type B, idiosyncratic reactions that are dose-independent and unpredictable from what the drug is known to do.
Injectable peptides supply clean examples of both. The nausea, vomiting and delayed gastric emptying that dominate GLP-1 receptor agonist labels are Type A: they follow from the receptor being engaged, become more frequent as dose rises, and ease as tolerance develops or the dose falls. Anaphylaxis to a peptide, or antibody-mediated loss of response, is Type B: it depends on the individual immune system, not on how much drug was given.
The distinction decides what happens next. A Type A reaction is a tolerability problem, and slower escalation or a lower maintenance dose usually keeps the drug usable. A Type B reaction generally ends the drug and often the class, since rechallenge risks a worse event. It also predicts how population rates behave: Type A reactions accumulate as more people reach the top dose, while Type B rates stay small and flat.
The error you meet most often runs the other way, reading a trial adverse event table as a list of reactions the drug caused. Placebo arms in obesity trials report headache, fatigue and gastrointestinal symptoms at double-digit rates, so an unadjusted count overstates harm. Its mirror image is forum talk calling every reported problem a side effect of a bad batch, an attribution needing the same causality work as any other.