Background <p>The rapid expansion of energy drink markets over the past two decades has sparked growing policy debate surrounding their regulation, due to their potential adverse health effects, particularly among young people. In this context, mandatory warning labels have emerged as a promising public health intervention. The present work aims to explore the effect of warning labels on emerging adults’ purchase intention and perceived healthfulness of energy drinks in Uruguay.</p> Methods <p>An online randomized controlled trial was implemented using a convenience sample of 340 emerging adults. Commercial energy drink cans, varying in brand (market-leading vs. lesser-known brand) and sugar content (regular vs. zero-added sugar variant), were used as stimuli. Participants were randomly assigned to either the control group (no warning label) or one of the four intervention groups, each of which was exposed to a warning label condition (Insomnia-graphic, Insomnia-textual, Tachycardia-graphic, Tachycardia-textual). The questionnaire comprised six main sections: (i) purchase intention; (ii) perceived effects of energy drink consumption and perceived healthfulness; (iii) perception of warning labels; (iv) comparison of warning labels; (v) socio-demographic characteristics; and (vi) energy drink consumption habits and perceived risk.</p> Results <p>The graphic warning referring to tachycardia led to a significant reduction in purchase intention and perceived healthfulness compared with the control condition; no significant effect was observed for the remaining warning label conditions. In addition, warning labels were consistently associated with an increased attribution of the negative effects they highlighted, whereas no statistically significant effects on positive outcomes were observed. Results also provide emerging insights into the role of branding, consumption patterns and risk perception in shaping perceived healthfulness and purchase intention for energy drinks. Regarding participants’ perception of warning labels, the graphic tachycardia warning emerged as the most effective condition.</p> Conclusions <p>Results support the use of warning labels as a public health strategy to increase awareness of the potential harms associated with energy drink consumption and suggest that their impact may extend even to products with strong market presence and high consumer recognition.</p>