Prevalence, Patterns and Implications of Chemsex Substance Use Among Gay and Bisexual Men Who Have Sex With Men: A Cohort-Collaboration Between the Swiss HIV Cohort Study and the SwissPrEPared Study.

INTRODUCTION: Global awareness to chemsex, a specific form of sexualized psychoactive substance ("chems") use among gay, bisexual and other men who have sex with men (gbMSM), is increasing. We aimed to assess prevalence, patterns and implications of "chems" use on chemsex-related outcomes. METHODS: We conducted a Swiss-wide longitudinal study using individual-level data from April 2019 to December 2024 of adult gbMSM with HIV, enrolled in the Swiss HIV Cohort Study (SHCS), or taking HIV pre-exposure prophylaxis (PrEP), enrolled in SwissPrEPared. Determinants of "chems" (methamphetamine, new psychoactive stimulants, GHB/GBL) use and clinical implications were analysed using logistic regression. Longitudinal "chems" use patterns were distinguished by rule-based classification. Point-prevalences of "chems" use, including COVID-19 and mpox periods, were analysed using interrupted time series analysis. RESULTS: We analysed 107,855 follow-up visits from 13,304 gbMSM. "Chems" were used by 17.2% of gbMSM (21.4% in SwissPrEPared vs. 11.2% in the SHCS). Age below 26 (compared to 26-37, adjusted odds ratio [aOR]: 0.82, 95%-confidence interval: 0.68-0.96), over 37 (aOR for 38-48: 0.84, 0.75-0.94; aOR for over 48: 0.54, 0.47-0.62) and bisexual orientation showed lower odds (aOR: 0.78, 0.66-0.90), while participation in French-speaking SwissPrEPared centres (aOR compared to SHCS Eastern Switzerland: 1.41, 1.22-1.64) and sex with non-steady partners showed higher odds for "chems" use (aOR: 12.44, 8.85-18.17). "Chems" use was associated with depression, inconsistent condom use, high non-steady partner count and sexually transmitted infections (STIs). Comparing longitudinal use patterns, temporally restricted use, that is, recovering from stable use, reported "chems" use only once, or for a single episode shorter than 1 year, showed the lowest risk for STIs and high partner count, whereas intermittent use showed lower risk only for syphilis. "Chems" use decreased marginally during COVID-19 and mpox, with a quick return and no difference to levels before COVID-19. CONCLUSIONS: A large proportion of gbMSM use "chems," with congruent determinants across both cohorts but heterogeneous impact on sexual and mental health that varies by specific substance and longitudinal use patterns. Further understanding of underlying mechanisms driving this heterogeneity is needed before delivering mechanism-informed interventions tailored to "chems" use patterns, like STI and depression screening, harm-reduction counselling and linkage to addiction care.