Drug testing plays a key role in youth substance use treatment in Sweden. Young people treated for substance use problems are routinely required to leave urine samples, and there is often controversy between patients and staff around its relevance. Still, there is a lack of research on how young people make sense of this practice. This article contributes to this knowledge through an ANT-inspired (Actor Network Theory) analysis of how youth enact urine testing in their treatment experiences. We attempt to tease out what kind of sociomaterial object urine testing is according to youth, and how it affects their lives. The study is based on interviews with 25 previous patients (mean age 17). The analysis shows that the participants enacted urine testing as both a stable object that creates binaries in knowledge networks (use or nonuse), and as a flickering object that appears in and affects diverse drug-body-treatment assemblages (even outside the clinic). The participants had internalized the importance of leaving negative samples to get discharged and avoid adult surveillance. They described a practice that made substance use a demarcated, individual and treatable problem, and also, often contrary to their own self-understandings, devalued their ability to be honest about and regulate their conduct. Through establishing substance use as a simplified either/or phenomenon and through attributing patients with the agency to become nonusers only, urine testing appears counter-productive if treatment is to strengthen informed decision making and responsibility among soon to be adults.