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) to be tested for the presence of illicit drugs. Drawing on qualitative research carried out with male and female drug injectors, the article considers their views and experiences of MDT. Five broad themes arose from the data analysis: 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 the notions of power and risk in relation to MDT. The article questions the effectiveness of MDT in dealing with drug use in prison: It encourages people to evade detection, has the potential to shift drug using patterns from “soft” to “harder” forms of drug taking, and could discourage drug users from approaching prison staff with drug problems for fear of being tested, found positive and punished. In short, methods used to test people for drugs, the types of results MDT obtains and its effectiveness in tackling drug use in prison brings the policy into question. Table, references