Background Online communities are known to break down barriers between supposed experts and non‐experts and to promote collaborative learning and ‘radical trust’ among members. Young people who self‐harm report difficulties in communicating with health professionals, and vice versa. Aim We sought to bring these two groups together online to see how well they could communicate with each other about self‐harm and its management, and whether they could agree on what constituted safe and relevant advice. Methods We allocated 77 young people aged 16–25 with experience of self‐harm and 18 recently/nearly qualified professionals in relevant health‐care disciplines to three separate Internet discussion forums. The forums contained different proportions of professionals to young people (none; 25%; 50% respectively) to allow us to observe the effect of the professionals on online interaction. Results The young people were keen to share their lived experience of self‐harm and its management with health professionals. They engaged in lively discussion and supported one another during emotional crises. Despite registering to take part, health professionals did not actively participate in the forums. Reported barriers included lack of confidence and concerns relating to workload, private–professional boundaries, role clarity, duty of care and accountability. In their absence, the young people built a vibrant lay community, supported by site moderators. Conclusions Health professionals may not yet be ready to engage with young people who self‐harm and to exchange knowledge and experience in an anonymous online setting. Further work is needed to understand and overcome their insecurities.