ABSTRACT A key selling point of pharmacogenetics is the genetic stratification of either patients or diseases in order to target the prescribing of medicine. The hope is that genetically ‘tailored’ medicines will replace the current ‘one‐size‐fits‐all’ paradigm of drug development and usage. This paper is concerned with the relationship between difference and justice in the use of pharmacogenetics. This new technology, which facilitates the identification and use of difference, has, we shall argue, the potential to lead to injustice either by (...) the inappropriate use of difference or through the inappropriate failure to use difference. We build on empirical data from a detailed study of the range of options for the development of pharmacogenetics to present a consideration of the ethical issues that surround patient and disease stratification. In it we explore the ways in which the use of pharmacogenetics may lead to the creation of new, genetically stratified, forms of difference and new forms of injustice based on these divisions. We also examine the ways in which existing forms of difference and social stratification may interact with the use of pharmacogenetics. In conclusion, we suggest how an understanding of these ethical issues could usefully inform future policy discussions. (shrink)