Trends of bacterial sexually transmitted infections with doxycycline post-exposure prophylaxis among people using pre-exposure prophylaxis for HIV

Background People using pre-exposure prophylaxis (PrEP) for HIV are at high risk of acquiring sexually transmitted infections (STIs). We aimed to assess the impact of doxycycline post-exposure prophylaxis (doxy-PEP) on the incidences of STIs among PrEP users in the real-world setting. Methods From February 2022 to November 2025, participants aged ≥18 years receiving HIV PrEP were prospectively enrolled. STI testing at baseline and follow-up included syphilis serology and multiplex real-time PCR assays for Chlamydia trachomatis , Neisseria gonorrhoeae and Mycoplasma genitalium . Beginning in November 2023, doxy-PEP was provided during follow-up visits. STI incidence rates before and after the first documented doxy-PEP prescription were compared within the same participants. Results A Total of 397 participants were included, with a median age of 32 Years. All participants were cisgender men; 90.9% Were men who have sex with men, 7.3% Were bisexual cisgender men And 1.8% Were heterosexual cisgender men. At enrolMent, 3.3% (13/397) Had reactive syphilis serology, 15.5% (60/386) Tested positive for C. trachomatis , 13.7% (53/386) For N. gonorrhoeae And 5.2% (20/386) For M. genitalium . Compared with the pre-doxy-pep period, the post-doxy-pep period had significantly lower incidence rates of syphilis (2.0 Vs 6.4 Per 100 person-years; incidence rate ratio (Irr) 0.3, 95% CI 0.1 To 0.8) And Chlamydia (8.8 Vs 31.9 Per 100 person-years; irr 0.3, 95% CI 0.2 To 0.4). No significant difference was observed for N. Gonorrhoeae Infection (21.0 Vs 26.3 Per 100 person-years; irr 0.8, 95% CI 0.6 To 1.1) Or M. genitalium Infection (12.7 Vs 9.5 Per 100 person-years; irr 1.3, 95% CI 0.8 To 2.3). Conclusion In this real-world HIV PrEP cohort, doxy-PEP was associated with significant reductions of syphilis and chlamydia.