Abstract Online health communities (OHCs) function as significant platforms that people use to obtain information and emotional support. Despite many studies on user behavior and relationships, little attention has been paid to user identities and how different layers of identities are interwoven. To address this potential research gap, this study examined users' relationship‐layer identities and their evolution by elaborating on the communication theory of identity (CTI) and social support theory. Additionally, based on our previous study on users' personal‐layer identities in OHCs, we investigated how users' relationship‐layer identities interacted with their personal‐layer identities. This study classified users' posts and replies into providing informational support, seeking informational support, providing emotional support, seeking emotional support, and companionship using the bidirectional encoder representation from transformers (BERT), with F1‐scores above 0.848. Through social network analysis, this study found that users of OHCs constructed their relationship‐layer identities more through informational interactions than through emotional interactions. Users with various personal‐layer identities presented different relationship‐layer identities. Users' relationships were more initiated by information exchange, and users with more interactions had more companionship activities. OHCs provided efficient communication channels for people to exchange social support.