<p>This article argues for a renewed focus on athletes’ attitudes towards legal and banned performance enhancing substances, considering the dual roles of law and science in shaping these attitudes. The article begins by setting out how the World Anti-Doping Agency (WADA) determines which substances are banned, before considering the so-called grey areas of the law which open the door for athlete moral and sporting opinions to determine how they act in relation to these substances. The research employs a qualitative methodology using structured content analysis, focusing its scope specifically on the sport of athletics. It is underpinned by a range of legal theories on international law and regulatory science. The article ultimately argues that, despite WADA’s efforts to gain and maintain epistemic control over the Prohibited Substances list and the line between clean and dirty in anti-doping, there is an inherent subjectivity to the laws which creates the opportunity for athlete agency to fit individual perceptions of morality into a malleable legal system. It concludes that anti-doping organisations may be able to regain control over this subjective system in individual sports by taking a more proactive approach to promoting legal performance enhancing substances/supplements, reducing the opportunity for athletes to use their agency but risk their careers by taking uninformed decisions in pursuit of improved performance.</p>