Despite the prevalence of health communication on social media, the mechanisms by which social media affect health information exchange have received less than adequate investigation. By integrating the lenses of affordance and social support, this study examines the effect of social media affordances on health information exchange, with a particular focus on the mediation effect of social support and the boundary condition of health consciousness. Results of an online survey indicated that the affordances of information accessibility and metavoicing are positively related to both general and experiential informational support. Moreover, the affordance of association is positively related to experiential informational support and emotional support. Additionally, three types of social support have positive effects on health information exchange, and health consciousness positively moderates the effect of experiential informational support on health information exchange. This study enriches existing literature by revealing the underlying mechanisms and boundary conditions of the relationship between social media affordances and health information exchange. Furthermore, it provides new insights for practitioners to promote health information exchange on social media.