Background: In the context of social marketing, the effectiveness of prevention messages is a major issue. The main objective of the present study was to assess the effect of prevention messages framing on self-efficacy reinforcement in order to improve intentions to reach or maintain sufficient weight in a non-clinical sample. It thus focuses on testing the mediating role of self-efficacy. Methods: Two hundred and thirty-three university student women were randomly assigned to one of the two conditions (gain-framed versus loss-framed message). They were exposed to a short persuasive message and surveyed on self-efficacy and intention to maintain sufficient weight. Results: Loss-framed messages elicited higher levels of self-efficacy than gain-framed messages, which led to higher intentions to reach or maintain sufficient weight. This study sheds light on the mediating role of self-efficacy. Conclusions: The results suggest ways to improve the persuasiveness of prevention campaigns, thereby opening up further research avenues.