Recent research on open-label placebos, or placebos administered without deception or concealment, suggests that they can be effective in a variety of conditions. The current research sought to unpack the mechanisms underlying the treatment efficacy of open-label placebos.A health care provider induced an allergic reaction in 148 participants via a histamine skin prick test. Participants were then exposed to 1 of 4 conditions additively leveraging various mechanisms of open-label placebo treatments: a supportive patient-provider relationship, a medical ritual, positive expectations, and a rationale about the power of placebos.There were no main effects of condition on allergic responses. However, participant beliefs about placebos moderated the effect of open-label placebo treatment condition on physiological allergic reactions: the condition including all 4 components of open-label placebos (a supportive patient-provider relationship, a medical ritual, positive expectations, and a rationale about the power of placebos) significantly reduced physiological allergic reaction among participants with a strong belief in placebos compared with participants in the control group.Participants' beliefs about placebos interact with information from the provider to reduce physiological allergic reactions in response to an open-label placebo treatment. This study underscores the importance of measuring and understanding how participants' beliefs influence outcomes of treatment, and furthers our understanding of when and how open-label placebo treatments work. (PsycINFO Database Record (c) 2019 APA, all rights reserved).