Diagnoses are powerful tools that fulfill various practical and symbolic functions. In this paper, I examine how a contested diagnosis called orthorexia nervosa has been taken up by users on Instagram, where tens of thousands of posts engage with the topic, many of them from individuals who identify with the condition. I put scholarship on medicalization and diagnosis in conversation with literature on subcultures to foreground the subversive work that is enabled through this diagnosis. Drawing on more than 350 hours of online ethnographic fieldwork and 34 in-depth interviews, I examine how participants construct a shared identity, draw on common language and norms, and undertake collective practices, as they negotiate dominant understandings of health. I show how they draw on the legitimacy endowed by the diagnostic label to validate and make sense of experiences of suffering but also to counter dominant health-seeking discourses, practices, and aesthetics in an online space where these are highly visible and valued. I also discuss some ways Instagram as a digital platform shapes its uptake by this community in meaningful ways. On the one hand, participants draw heavily on the language and framing of medicine to make sense of their fraught experiences with food and their bodies, effectively advocating for the medicalization of their own suffering while also creating a sense of community and shared identity. However, on the other hand, they actively use the diagnosis and the recovery process enabled through it to effectively resignify dominant beliefs, values, and practices that are experienced as injurious, including some that are particularly prevalent on Instagram.