For several medical treatments there is considerable scientific and medical uncertainty about the relative benefits and risks they imply. The literature indicates social support as an important factor in health uncertainty management and the concept has gained considerable interest in the areas of behavioral medicine and health psychology research. Despite the growing importance of online health communities (OHCs), there is still a dearth of research evaluating the interactive and reciprocal nature of social support exchange that may occur in the online environment and, to the best of the authors’ knowledge, no single study has specifically focused on online support interactions in contexts characterized by treatment uncertainty. This study aims to generate knowledge on how OHCs can serve as platforms of social support for groups of individuals that are subject to treatment uncertainty. To attain this objective, we build on social capital theory and adopt a netnographic and a dual case-study approach to analyze peer-to-peer social support interactions in OHCs where users face treatment uncertainty. Findings suggest that OHCs can be a place for extending the social support networks of people facing treatment uncertainty as multiple typologies of peer-to-peer social support are remotely exchanged in the virtual communities. However, we also found that the information available in these forums can be subject to imprudent processes of selection by forum participants who try to conform online discussion to suit the forums’ shared and accepted narratives highlighting the benefits of these uncertain treatments, while minimizing the respective risks. From a theoretical perspective, this study has implications for health uncertainty management literature, by highlighting collective processes of uncertainty management not captured in well-established theories of uncertainty management, which are mostly focused on individual and interpersonal processes of uncertainty management in dyad relationships, and not group dynamics. From a managerial practice perspective, the study provides insights for public health, private and not-for-profit health promotion institutions and managers of the OHCs, that can be used to program community-based social marketing interventions intended to enhance the quality and increase the volume of social support available in OHCs where users may face treatment uncertainty.