Online health communities (OHCs) provide knowledge for users, enabling conversations across a broad range of health topics. The development of OHCs depends on users’ motivations to share health knowledge. Yet little literature has explored how perceived benefits and costs affect users’ motivations for sharing both general and specific knowledge. Based on social exchange theory, we propose a research model that comprises intrinsic benefits (sense of self-worth, satisfaction), extrinsic benefits (social support, reputation, and online attention), cognitive cost, and executional cost to investigate the effects of these factors on users’ motivations for general and specific knowledge sharing. We compare the different effects of these factors on users’ motivations for knowledge sharing. Results demonstrate positive effects of intrinsic and extrinsic benefits on users’ motivations for general and specific knowledge sharing. Differences exist in the negative effects of cognitive and executional costs on users’ motivations for general and specific knowledge sharing. This study contributes to promoting the enrichment of online health knowledge and provides implications for the development of OHCs.