All drugs elicit a response by affecting function.
Derived from this paradigm is the concept that heterogeneity in function will, in turn, produce heterogeneity in drug response. Functional variation may result from variation in intrinsic or heritable factors or in extrinsic or environmental factors. Pharmacogenetics is the branch of pharmacology that deals with the influence of heritable factors upon drug response. The area developed as it became increasingly clear that genetic factors play a role in idiosyncratic drug response. In the past 30 years, more than 100 examples of idiosyncratic drug response have been documented.