Most drugs exert their effects by interacting with their receptors in a reversible manner. The interaction of an agonist with its receptor leads to a conformational change in the receptor that results in a signal or stimulus. The stimulus then initiates other actions that ultimately result in a biological response. The events between generation of the stimulus and the final response constitute what is called the response chain. This can be a simple direct process such as the opening or closing of an ion channel, or it may involve a long transduction process that includes a cascade of several events and the action of second messengers.

The overall intensity of drug response is a function of two types of properties: drug-specific and tissue- or system-specific properties. The drug-specific properties are the ability of the drug to interact with the receptors (affinity) and the ability of the drug to produce a stimulus per unit receptor (intrinsic efficacy). The tissue-specific properties are the number or density of the receptors and the process that converts the initial signal into a response. Ideally, a pharmacodynamic model should separate these two types of properties.