Most people in the developed world will sooner or later be given prescription drugs to treat common diseases or to reduce the risk of getting them. Almost everyone who takes medicines will, at some stage, encounter those that do not work as well as they do in other people or even that cause an adverse reaction. Pharmacogenetics seeks to reduce the variation in how people respond to medicines by tailoring therapy to individual genetic make-up. It seems increasingly likely that investment in this field might be the most effective strategy for rapidly delivering the public health benefits that are promised by the Human Genome Project and related endeavours.