Background: Social media platforms such as Reddit have become important spaces where individuals articulate their distress, seek support, and explore alternative ways of understanding mental health outside traditional institutional frameworks. These environments provide an opportunity to examine mental health discourse at scale, offering perspectives that extend beyond traditional clinical and research settings. Objective: This study aims to examine the structure of mental health communities on Reddit by identifying patterns of association between mental disorders reflected in user activity and assessing how these relationships align with established diagnostic categories in the ICD (International Classification of Diseases). Methods: We manually curated 114 Reddit communities focused on specific mental health conditions from the 20,000 most active subreddits in 2022. Each community was labeled into 49 disorders and categorized under 9 ICD diagnostic categories within the group of mental and behavioral disorders, collectively known as the F codes. We constructed a disorder association network by identifying statistically significant user overlaps based on coposting across subreddit pairs using a bipartite configuration model, with Bonferroni-corrected significance (P<.001). We analyzed the connectivity of the network within and across diagnostic categories, examining inter- and intracategory links. Finally, we compared the structure of disorder associations inferred from Reddit with the ICD classification derived from diagnostic criteria using hierarchical clustering. Results: The inferred Reddit network of psychopathology revealed an interconnected structure (density=0.135), with all but 6 disorders forming a single giant component that spans across all 9 diagnostic categories. The most prominent disorders by number of users included hyperkinetic disorders (85,000), depressive episodes and recurrent depressive disorders (73,000), habit and impulse disorders (69,000), pervasive developmental disorders (52,000), and generalized anxiety disorder (44,000). In terms of connectivity, posttraumatic stress disorder (17/48 of all possible connections), obsessive-compulsive disorder (16/48), and depersonalization-derealization disorder (15/48) emerged as the most central in the network of positive disorder associations, while schizotypal disorder, avoidant personality disorder, and agoraphobia were the most central when accounting for the association strength. At the level of disorder categories, several disorders, such as bipolar disorder and premenstrual dysphoric disorder, displayed high intercategory associations but weak intracategory ties, indicating blurred diagnostic boundaries. The network of negative coposting associations revealed a divergence from the expectations of past research; for instance, addiction-related communities (eg, alcohol and opioids) were negatively associated with much of the broader mental health discourse. Finally, hierarchical comparisons showed moderate overlap between the Reddit network of disorder associations and the ICD network of diagnostic criteria, both in pairwise edge similarity (13% of edges present in both networks) and overall clustering (Adjusted Rand Index=0.295). Conclusions: Reddit-based mental health communities reveal a complementary structure of disorder associations shaped by lived experience, often diverging from formal diagnostic criteria and exhibiting patterns of association that do not align with established diagnostic boundaries.
