This study experiments with TikTok videos to promote clinical trial participation. More specifically, it examines how short-form video sources (doctors vs. prior volunteers for clinical trials) influence perceived source credibility, self-efficacy, and behavioral intention to participate in clinical trials. Findings from this online experiment (N = 396) showed that doctor sources led to greater behavioral intention through enhancing source credibility compared to prior volunteer sources. Alternatively, prior volunteer sources increased behavioral intention via enhanced self-efficacy for participants with low trust in doctors. These findings contribute to the understanding of how short-form video sources act as heuristic cues, leading to persuasion outcomes. Overall, we recommend featuring doctors when using video-based messages to promote clinical trial participation. Also, this study emphasizes the need for health communication practitioners to consider prior volunteers as spokespersons when targeting populations with low baseline trust in doctors.