You've probably seen the headlines about syphilis and chlamydia creeping back up. For many people, sexual health has quietly become a bigger worry than it was a decade ago. Something new may be starting to change that picture.
Taking a single 200 mg dose of the antibiotic doxycycline within 72 hours after sex is linked to syphilis diagnoses coming in 55% below projected levels across Australia.
That is the finding of a new real-world study published in Nature Health on October 21, 2026, the biggest look yet at the approach known as doxy-PEP.
Chlamydia diagnoses also tracked 36% below projections over the same period. Gonorrhea rates did not show a significant drop.
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What the real-world data actually showed
Researchers at the Burnet Institute in Melbourne analyzed anonymous data from more than 78,000 gay and bisexual men attending 29 clinics across Australia. The window covered the 27 months after national doxy-PEP guidance took effect in September 2023.
The guidance, from Australia's main sexual health specialist body, recommends considering doxy-PEP for at-risk gay and bisexual men, especially those with a recent STI.
It arrived as Australian syphilis diagnoses had already climbed more than 50% over the previous decade.
Lead author Dr. Michael Traeger said earlier trials had shown doxy-PEP could cut an individual's risk. What the field had not known, he said, was whether the benefit held up across a whole population, and the Australian data suggests it is beginning to.
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The study is observational, not a randomized trial, so it shows an association rather than proof of cause. Even so, the size and the three-year window make it the strongest real-world signal to date.
Why gonorrhea remains the open question
Three large randomized trials had already shown that doxy-PEP cuts syphilis and chlamydia by more than 70% and gonorrhea by around 50%. The flat gonorrhea result in Australia likely reflects something researchers in the US have been watching closely.
A growing share of gonorrhea strains now carry a gene that makes them resistant to tetracycline antibiotics, the family doxycycline belongs to.
In parts of the US, that resistance has climbed sharply since doxy-PEP use expanded. Recent work in The Lancet Infectious Diseases also raises the concern that doxycycline may select for resistance to ceftriaxone, a key gonorrhea drug.
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Where that leaves people in the United States
In the US, the CDC first recommended doxy-PEP in June 2024, for gay and bisexual men and transgender women who have had a bacterial STI in the past 12 months.
The agency advises STI testing every 3 to 6 months for anyone taking it, and a check-in on whether to continue doxy-PEP at each visit. It is prescription only, so the conversation starts at a doctor's office or sexual health clinic.
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This article is made and published by Aksel Kryhlmand, who may have used AI in the preparation.