This dissertation is an ethnographic study of contemporary gay Taiwanese men's sexual health with a focus on the circulation of HIV prevention medicine and blood management. In the 2010s, the governance of HIV/AIDS has undergone a significant shift, moving from biomedical treatment to prevention: pre-exposure prophylaxis (PrEP) is prescribed for HIV-negative individuals to prevent sexually contracting HIV. PrEP engenders a new serological condition, "HIV negative, on PrEP." By signaling the absence of virus and personal use of HIV biomedicine, "HIV negative, on PrEP" implies that this medicine works at the molecular level of human blood to suppress viral incubation and replication, and entails a medical and social urgency of constantly bringing the drug into an individual's body. This dissertation asks what it means to be “HIV negative, on PrEP” in the neoliberal, transnational context of drug consumption and regulation. In this project, I argue that serostatus associated with HIV biomedicine should not be seen as a fixed scientific category about one’s wellbeing, but instead a dynamic process of becoming “HIV negative, on PrEP.” I tell the story of how gay men, governments, AIDS advocates, pharmaceutical companies, and other social actors utilize "HIV negative, on PrEP" as a means to redefine sexual health during a time when drugs are newly introduced and not yet widely available or financially accessible. In doing so, I unearth the socio-economic tensions, health inequalities, and hegemonic oppressions against gay men amid the HIV biomedical prevention regime. A multi-sited ethnography conducted in Taiwan and Thailand from 2016 to 2019, this dissertation traces PrEP’s social trajectory and gay men’s socio-sexual practices to document the transformation of sexual health in four main chapters: government-led medical support programs, the AIDS advocacy organizations initiated drug-delivery model, gay men’s medical tourism to Thailand, and gay men’s sexual communication through smartphone social apps. Drawing on the theories and methods from the science and technology studies (STS), new feminist materialism, medical anthropology, and media studies, I offer an expansive and performative interpretation of health, safety, risk, and other taken-for-granted notions in public health, illustrating how gay Taiwanese men have undergone a biomedical and social transformation of blood management and body modification. In moving toward self-health enhancement, their bodies and sexualities have become intertwined with the economies of pharmaceutical innovation, governmental regulation, and personal mobility and pleasure. Ultimately, this dissertation contributes to the emerging scholarship of “Queer STS” by addressing the broader issues of the politics of self-medication, the marketization of HIV medicine, and the making of queer sexuality in the digital environment.