Mandatory drug testing (MDT) is a policy that requires people in prison to provide a sample to be tested for the use of illicit drugs. Drug injectors are one group of individuals who spend time in prison. Drawing on qualitative research carried out with male and female drug injectors this article considers their views and experiences of MDT. Five broad themes arose from the analysis of these data. These themes include people's experiences of the test, their strategies to evade drug detection, punishments for testing positive, the effect of MDT on patterns of drug use, and, finally, the notions of power and risk are considered in relation to MDT. The article concludes with a discussion on the worth of this policy.Mandatory Drug Testing (MDT) is a policy that requires, by law, people resident in prison to provide a urine sample or other type of specimen (non-pubic hair, sweat or saliva but not blood or semen) for the testing of illicit drugs (Criminal Justice and Public Order Act 1994, ch. 33). It was first introduced in a small number of prisons in February 1995 and by March 1996 was extended to include all British prisons. This article considers MDT in relation to the lives of drug injectors in prison. The first part of this article looks at drug injectors in prison. The second part provides an overview of MDT policy. The third and substantive focus of the article draws on a qualitative study that examines the lives of male and female drug injectors as they move inside and outside prison. The article provides an analysis of drug injectors' views and experiences of MDT. The areas that are covered here include drug injectors' experiences of being tested, their strategies employed to evade detection, punishments for testing positive and the effect of MDT on drug use. Finally, MDT is situated within a wider context of power and risk in prison. The findings are discussed in relation to the existing literature and policy implications suggested.