Idiosyncrasy, in medicine and nutrition, is an individual’s peculiar, unpredictable reaction to a food, drug, or substance — a response that most people do not share and that dose alone does not explain. One diner’s violent migraine from red wine or cheese, another’s racing heart from a single coffee, the rare haemolysis a drug triggers in G6PD-deficient patients — all are idiosyncratic. The term acknowledges the limits of population science: beneath every average response curve lie individuals whose genetics, gut flora, or history make them outliers. Modern pharmacogenomics and personalised nutrition are, in a sense, the systematic study of idiosyncrasy — mapping why the same food or drug is one person’s remedy and another’s poison.