Advanced Web 2.0 communication technologies have facilitated health-related information (HRI) sharing on the Internet. Especially, the COVID-19 pandemic and lockdowns around the world have forced more people to turn to the Internet for HRI. A better understanding of users' sharing content and sharing behavior can help communicators improve health literacy, raise community awareness, and facilitate social support exchanges. This paper reports the results of a systematic review of online HRI sharing literature, including key research topics, theories and methods used in past studies, and key factors of sharing behavior across online platforms. Following the PRISMA procedure for a systematic review, 58 articles were identified and analyzed using keyword matching, thematic analysis, and expert review. Guided by the platform theory, our findings differentiated five types of online platforms that differently influenced online users' sharing content and sharing purposes, including micro-blogs, social network sites, online health communities, social question and answer sites, and Wikis. The findings also clarify five main research topics and applicable theories used in each topic, including personal health sharing, health-related knowledge sharing, general health message diffusion, outcomes of HRI sharing, and exploratory research. Key factors of sharing behavior and potential sharing outcomes are also reviewed and summarized in the research framework developed from the motivation theory. Our study contributes to the understanding of online sharing behavior and provides implications for health communicators to develop effective health campaigns. Potential research directions are also identified and discussed.