Personalised medicine is a discipline that integrates pharmaceutical development with existing knowledge about the genetic and phenotypic factors that influence drug response. This integration enables tailoring therapies to individual patient’s characteristics to help improve their safety and efficacy. Personalised medicine implies a shift in the way drugs are delivered as it requires new forms of testing that assess the patient´s eligibility for a drug. This poses new challenges, both at a regulatory and reimbursement level. Personalised medicine has reached higher commercial success and clinical uptake in drugs under development than old and off-patent drugs. This paper uses the case of TPMT testing to illustrate the reasons why personalised medicine for off-patent drugs is less used at a clinical level than personalised medicine for drugs under development.