Background Pre-exposure prophylaxis (PrEP) is a highly efficacious HIV prevention tool. Whether changes in sexual risk behaviours and frequency of sexually transmitted diseases (STDs) occur among individuals using PrEP is unclear. We evaluated sexual behaviours and STDs among men who have sex with men (MSM) in the open-label US PrEP Demonstration (Demo) Project.

Methods The Demo Project enrolled 557 MSM at STD clinics in San Francisco and Miami, and a community health centre in Washington, DC. Participants were tested for STDs and reported their sexual risk behaviours in the prior 3-months at baseline and weeks 12, 24, 36 and 48. Prevalence of STDs and STD incidence were assessed, and changes in reported risk behaviours and STD incidence were assessed using chi-square tests.

Results The median number of anal sex partners in the prior 3-months decreased from 5 at baseline to 4 at week 48 (p = 0.0003). While the median number of condomless receptive anal sex episodes was unchanged, the median number of receptive anal sex episodes with condoms declined from 6.5 to 2.0 (p < 0.0001). One quarter (25.7%) had an STD at baseline and 42.2% were diagnosed with ≥1 STD during the study. Extra-genital STDs were prevalent: 9.8–15.3% positivity for rectal gonorrhoea (GC) or chlamydia (CT) and 5.2–12.9% positivity for pharyngeal GC or CT at follow-up visits. Overall STD incidence was high, but did not increase over time (p = 0.96); incidence/100 person-years was 47.8 (95% CI: 41.6–54.7), 42.9 (95% CI 37.0–49.4) and 12.6 (95% CI 9.5–16.3) for CT, GC and syphilis, respectively. There were two HIV seroconversions (incidence 0.43; 95% CI 0.05–1.54), both had undetectable drug levels at the time of seroconversion.

Conclusion HIV incidence was extremely low, despite a high incidence of STDs in a PrEP demonstration project. Quarterly STD screening, including testing at extra-genital sites, is recommended for MSM taking PrEP.

Conflict of interest This work was supported by National Institute for Allergies and Infectious Diseases (NIAID) [UM1AI069496]; National Institute for Mental Health (NIMH)  [R01MH095628]; and the Miami Centre for AIDS Research (CFAR) [P30AI073961] from the National Institutes of Health (NIH). NIAID participated as a partner in protocol development, interpretation of data and gave final approval to submit the manuscript for publication. Study drug for The PrEP Demo Project was provided by Gilead, but Gilead played no role in design or conduct of the study; collection, management, analysis, or interpretation of the data; preparation or approval of the manuscript; and decision to submit the manuscript for publication.

Susan Philip has received speaker’s honorarium from Gilead. Richard Elion has received research support from Gilead and is on the speakers’ bureau for Gilead. TRIAL REGISTRATION: clinicaltrials.gov Identifier: NCT# 01632995.