Health-related message boards are common digital health platforms with great practical and sociological relevance. In this work, we present an investigation of the topological structure of interactions in such forums. We visualise the interactions among users and between users and forum administrators using a simple yet effective network visualisation tool called the Bitmap of Sorted Adjacency Matrix (BOSAM) algorithm. We apply the BOSAM algorithm to six health-related message boards of various scopes and sizes. The outcome reveals major differences between the user interaction networks of these forums. The results of our analysis correlate closely with the characteristics of the respective message boards, including their topic coverage, presence of user communities, commercial or community nature and management style.