藥物遺傳學是基于人群的遺傳變異研究該人群對藥物的遺傳反應的分別。人們早已知道人群里的不同人對同一種藥物的反應不同，這是受藥物影響的分子受體的不同或清除藥物的代謝酶的差異造成的。藥物遺傳學是在分子水平上研究這些差異的科學。通過對人群中存在的不同的分子受體進行識別和分類，然后系統研究藥物對其影響，人們有希望預測或抑制藥物對不同亞人群的作用。藥物遺傳學的應用包括減少副作用，定制藥物，改善臨床實驗以及挽救一些由于對少數人群會產生嚴重副作用而被禁用的藥物。

Pharmacogenomics is the study of the stratification of the pharmacological response to a drug by a population based on the genetic variation of that population. It has long been known that different individuals in a population respond to the same drug differently, and that these variations are be due to variations in the molecular receptors being affected by the drug, or to differences in metabolic enzymes that clear the drug. Pharmacogenomics is the science of studying these variations at the molecular level. By identifying and classifying all the tolerable variations of a molecular receptor known to exist in a population, and then performing systematic studies of the effect of the drug on each of the variants, one can hope to predict or constrain the use of the drug to different subgroups. Applications of pharmacogenomics include reducing side effects; customized drugs; improved clinical trials; and the rescue of some drugs that have been banned due to severe side effects in a small percentage of the eligible population.