Background/Objectives: SDU is frequent among MSM. It relates to both riskier sexual behaviours and HIV prevention uptake. Evidence on SDU prevalence and associated factors among MSM in Portugal is limited. We estimated SDU prevalence and examined its correlates and outcomes.

Methods: Cross-sectional analysis of entry visits from the Lisbon Cohort of MSM (2011-2023), including cisgender MSM aged 16 or older who responded to SDU questions. SDU is the use of ketamine, GHB/GBL, cannabis, MDMA, amphetamines, methamphetamine, LSD, cocaine, synthetic cathinones, heroin, or methadone just before/during sex in the past year. Prevalence and 95% confidence intervals (CI) were calculated. Adjusted odds ratio (aOR) to assess associations of sociodemographic, sexual/drug use behaviours, PEP/PrEP awareness, and violence related to sexual orientation/gender identity (SOGI) with SDU were estimated using logistic regression models. Further models examined associations of SDU with PEP/PrEP use and recent HIV testing. All adjusted for sociodemographic and visit year factors.

Results: Among 11,147 participants, SDU prevalence was 25.4% (95%CI 24.6-26.2%). Higher odds of SDU were observed in younger than 45 years (15-24, aOR 1.8; 25-34, aOR 1.9; 35-44, aOR 1.4), born abroad (Brazil, aOR 1.5; Other countries, aOR 1.5), unemployed (aOR 1.3), SOGI-related victimized (aOR 1.8), using alcohol just before/during sex (aOR 7.0), using poppers/Viagra/Cialis just before/during sex (aOR 7.6), anal sex debut before age of consent (aOR 1.8), higher number of non-steady partners (≥ 10, aOR 3.0), meeting non-steady partners in virtual venues only, sex-on-premises venues only or in both (aOR 1.4, 2.3 and 2.7 each), condomless anal intercourse (CAI) due to alcohol/drugs influence (aOR 4.7), and awareness of PEP/PrEP (aOR 1.4). As a possible exposure, SDU was associated with CAI (aOR 1.6), PEP/PrEP use (aOR 1.4), HIV testing (aOR 1.7), and current HIV reactive rapid test result (aOR 1.4). All aORs had p ≤ 0.001.

Conclusions/Recommendations: One in four reported SDU. SDU clustered among younger, migrant, unemployed and SOGI-related victimised MSM. SDU was contemporaneously associated with both higher engagement in PrEP/PEP uptake and HIV testing and higher prevalence of CAI and reactive HIV tests. This suggests that SDU identifies MSM in sexual networks characterised by simultaneous high prevention uptake and persistent HIV risk. These findings support targeted integrated harm-reduction and safer-sex strategies to MSM engaging in SDU in Lisbon.

Funding: Lab: DOI 10.54499/UID/04750/2025 & 10.54499/LA/P/0064/2020. PhD: 2024.02483.BD.