After the turn of the millennium, HIV clinical researchers pivoted from developing and testing new antiretrovirals (ARVs) for treatment, to reconfiguring the same molecules for pre-exposure prophylaxis (PrEP). In 2012, Truvada became the first HIV therapy to also be approved by the FDA for PrEP, regarded as a magic bullet that promised to end the epidemic. However, six years after its approval, it continues to be inaccessible to those who are most vulnerable. In this article, I critically analyze HIV PrEP clinical trials, dissecting the novel techniques researchers use to demonstrate efficacy. I argue that in making sense of the interplay between adherence to a prophylactic regimen and risk for HIV, biomedical HIV prevention research has revealed a new subject of biopolitics, Homo adhaerens. In the early 2000s, clinical researchers operating in the Global South identified Homo adhaerens as the ideal subject, one who embodies both high-risk behavior and diligent adherence to a daily oral regimen. I trace the construction of Homo adhaerens to the United States, where I listen closely to activists engaged with the ongoing DISCOVER trial of PrEP. Activists either aspire for Homo adhaerens as a standard, making the liberal argument that expanding access could make PrEP successful, or they rebuke the framework of clinical research that produces narrow understandings of adherence, efficacy, and universality. Ultimately, I argue that by failing to grapple with the social realities that underlie poor adherence, PrEP clinical trials produce knowledge that is not useful for those who are most vulnerable.