Abstract
The growing reliance on online communities has reshaped how individuals articulate, validate, and navigate psychological distress. However, the value of online peer support spaces remains insufficiently studied, particularly in the context of underrecognized conditions such as Premenstrual Dysphoric Disorder (PMDD). Here we investigated r/PMDD, a Reddit-based peer support community comprising 17,332 participants over a 12-year period (2012-2024), contextualized by their activity across 112 other mental health subreddits. We found a substantial decrease in the association of PMDD with depression and anxiety, evident both in general cross-community activity trends and at the individual level shortly after users become active in r/PMDD. Despite PMDD's clinical classification as an affective disorder, users frequently discussed both psychological and physical symptoms. These discussions largely aligned with DSM-5-TR diagnostic criteria, though substantial heterogeneity was evident across individuals. Users clustered around distinct treatment types, with SSRI antidepressants, contraceptives, and complementary medicine as the most prominent. These three medication types were negatively associated between each other, indicating a compartmentalized approach to treatment. Moreover, users posting about SSRI antidepressants in r/PMDD exhibited higher cross-community activity across most disorders compared to those posting about contraceptives. The findings underscore the value of online peer support communities as a complement to clinical understanding of diagnostically complex conditions like PMDD, particularly in relation to comorbidity patterns, symptomatology, and treatment.