Sexually Transmitted Infections Among Men Who Have Sex with Men Using Daily and Event-Driven HIV Pre-Exposure Prophylaxis: A Retrospective Cohort Study

Abstract Background HIV pre-exposure prophylaxis (PrEP) has reduced HIV incidence among men who have sex with men (MSM), but bacterial sexually transmitted infections (STIs) remain common, with limited data comparing regimens. Methods A retrospective cohort study was conducted at Melbourne Sexual Health Centre, Australia (2024–2025). MSM aged ≥18 reporting PrEP use (daily, event-driven, or switching) tested for chlamydia, gonorrhoea, or syphilis were included. Positivity was assessed at first consultation (baseline); incidence among those with ≥2 tests ≥30 days apart. Logistic and Poisson regression estimated associations and incidence rate ratios. Results Among 3,229 PrEP users (median age 33, IQR 29–39), 51.0% were daily, 43.2% event-driven, and 5.8% switchers. At baseline, 33.7% (765/2,268) had any STI, with lower positivity among event-driven users (29.8%) than daily users (37.0%) or switchers (37.6%; p=0.001), primarily driven by differences in gonorrhoea positivity. After adjustment, event-driven PrEP use was independently associated with lower odds of baseline STI positivity (aOR 0.75, 95% CI 0.62–0.90), while Medicare ineligibility and recent partners had higher odds. Over 700.6 person-years, any-STI incidence (39.4/100 person-years) did not differ across regimens. Conclusion Although event-driven users had lower baseline STI positivity, incidence was similar across regimens, suggesting baseline differences reflect underlying risk profiles rather than ongoing risk.