药物遗传学是基于人群的遗传变异研究该人群对药物的遗传反应的分别。人们早已知道人群里的不同人对同一种药物的反应不同，这是受药物影响的分子受体的不同或清除药物的代谢酶的差异造成的。药物遗传学是在分子水平上研究这些差异的科学。通过对人群中存在的不同的分子受体进行识别和分类，然后系统研究药物对其影响，人们有希望预测或抑制药物对不同亚人群的作用。药物遗传学的应用包括减少副作用，定制药物，改善临床实验以及挽救一些由于对少数人群会产生严重副作用而被禁用的药物。

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.