While risk of harm is an important focus for whether clinical research on humans can and should proceed, there is uncertainty about what constitutes harm to a trial participant. In Phase I trials on healthy volunteers, the purpose of the research is to document and measure safety concerns associated with investigational drugs, and participants are financially compensated for their enrollment in these studies. In this article, we investigate how characterizations of harm are narrated by healthy volunteers in the context of the adverse events (AEs) they experience during clinical trials. Drawing upon qualitative research, we find that participants largely minimize, deny, or re-attribute the cause of these AEs. We illustrate how participants’ interpretations of AEs may be shaped both by the clinical trial environment and their economic motivation to participate. While these narratives are emblematic of the larger ambiguity surrounding harm in the context of clinical trial participation, we argue that these interpretations also problematically maintain the narrative of the safety of clinical trials, the ethics of testing investigational drugs on healthy people, and the rigor of data collected in the specter of such ambiguity.