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There is a need to identify prescribing criteria that maximise the number of STIs averted while minimising excessive use.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Design<\/jats:title>\n                    <jats:p>In this prospective longitudinal cohort study with repeated measures and biobehavioural data collection, participants completed a questionnaire and tested for STIs at each visit.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Setting<\/jats:title>\n                    <jats:p>Community-based, population-level study conducted in three large Canadian cities between February 2017 and July 2023.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Participants<\/jats:title>\n                    <jats:p>2449 GBM were recruited through respondent-driven sampling (RDS); 1998 had \u22651 follow-up visit, contributing 7551 person-years of observation. Eligible participants were aged \u226516 years, cis- or transgender men, reported sex with another man in the past 6\u2009months and resided in Montreal, Toronto or Vancouver.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Primary and secondary outcome measures<\/jats:title>\n                    <jats:p>Adjusted rate ratios (aRR) of STIs, accounting for RDS recruitment, loss to follow-up and confounding were estimated using generalised estimating equations (GEE) Poisson regression. For identified STI risk factors, the proportions of STIs averted through doxy-PEP prescription (based on the efficacy of doxy-PEP for each bacterial STI) and the number needed to treat (NNT) for 1 year to avert one STI, assuming 100% adherence, were calculated.<\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      Among 1998 participants, the combined incidence rate of any\n                      <jats:italic>C. trachomatis<\/jats:italic>\n                      ,\n                      <jats:italic>N. gonorrhoeae<\/jats:italic>\n                      and syphilis infection was 29.5 (95%CI 27.3 to 31.9) per 100 person-years. STI risk factors that had the most impact as doxy-PEP criteria were history of any of the three STIs in the past 12\u2009months (P12M) (aRR=2.0, 95% CI 1.8 to 2.2, 36% STI averted, NNT=2.1); \u226510\u2009male sexual partners in the past 6\u2009months (P6M) (aRR=3.8, 95% CI 3.0 to 4.9, 41% STI averted, NNT=2.4); HIV-pre-exposure prophylaxis (PrEP) use P6M (aRR=1.7, 95% CI 1.5 to 2.0, 29% STI averted, NNT=2.5); use of any chemsex-related substance P6M (aRR=1.2, 95% CI 1.1 to 1.4, 28% STI averted, NNT=2.6); and group sex event attendance P6M (aRR=1.2, 95% CI 1.1 to 1.3, 27% STI averted, NNT=2.3). Reporting\u226510\u2009male sex partners P6M represented the most useful criterion for syphilis prevention (52% syphilis infections averted, NNT=20). Prescribing doxy-PEP to GBM having any of the following STI risk factors, namely, \u22651 bacterial STI P12M, \u226510\u2009male sex partners P6M, or HIV-PrEP use P6M, would substantially increase the proportion of all STI diagnoses potentially averted (60%) with minimal increase of the NNT (2.7).\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Conclusion<\/jats:title>\n                    <jats:p>This work informs on the impact of various doxy-PEP clinical prescribing criteria and demonstrates the benefit of focusing on any of the following three criteria: \u22651 bacterial STI P12M, \u226510\u2009male sex partners P6M or HIV-PrEP use P6M.<\/jats:p>\n                  <\/jats:sec>","DOI":"10.1136\/bmjopen-2025-111357","type":"journal-article","created":{"date-parts":[[2026,5,26]],"date-time":"2026-05-26T09:05:32Z","timestamp":1779786332000},"page":"e111357","update-policy":"https:\/\/doi.org\/10.1136\/crossmarkpolicy","source":"Crossref","is-referenced-by-count":1,"title":["Incidence and risk factors of\n                    <i>C. trachomatis<\/i>\n                    ,\n                    <i>N. gonorrhoeae<\/i>\n                    and syphilis among a cohort of urban Canadian gay, bisexual and other men who have sex with men, 2017\u20132023: informing the potential impact of doxycycline prophylaxis"],"prefix":"10.1136","volume":"16","author":[{"given":"Gilles","family":"Lambert","sequence":"first","affiliation":[{"name":"Direction R\u00e9gionale de Sant\u00e9 Publique de Montr\u00e9al, Montreal, Qu\u00e9bec, Canada"},{"name":"Institut National de Sant\u00e9 Publique du Qu\u00e9bec, Montr\u00e9al, Qu\u00e9bec, 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