The pharmacological effects of a drug depend on its concentration at the site of action, and therefore on the concentration in blood and on the dose.

The relationship between the concentration or dose and the corresponding effect can usually be represented mathematically as a rectangular hyperbola; when effect is plotted against log concentration or log dose, the curve is sigmoidal.

In this review we consider factors that influence a subject's susceptibility to adverse drug reactions.

Determinants of susceptibility include Immunological, Genetic, demographic (Age and Sex), Physiological and Exogenous factors (drug–drug interactions, for example), and Diseases and disorders such as renal failure, giving the mnemonic I GASPED.

Some susceptibility factors are discrete (for example, ‘all‐or‐none') and some are continuous; susceptibility can therefore be discrete or continuous; and the factors can interact to determine a person's overall susceptibility to harm.