This research examines physician-driven online health community (OHC), a social media application in healthcare that engages both patients and physicians. Specifically, we theoretically hypothesize and empirically test the mutual impact between patients’ and physicians’ participation in physician-driven OHC and the impact of patients’ and physicians’ participation on patient wellbeing and the patient–physician relationship. We investigate these in the context of managing diabetes and depression, two common chronic diseases. We collected data from a leading Chinese online consultation platform. Physician-driven OHC, an important feature of the consultation platform, allows patients to further interact with physicians beyond the online consultations. By exploiting two events that separately posed “exogenous shocks” to patients’ and physicians’ participation in physician-driven OHC, we were able to estimate the causal effects of patients’ and physicians’ participation. Our results show that physicians’ participation in their OHC significantly increases patients’ participation for both diabetes and depression, but patients’ participation only increases physicians’ participation for depression. Though both patients’ and physicians’ participation significantly improve patient wellbeing and the patient–physician relationship, there are interesting nuances in the persistence of these effects over time. These findings have important implications for the self-management of chronic diseases and healthcare policy making.