doxyPEP After Sex Drove a Sharp STI Decline in Australia

An antibiotic taken after sex may be behind one of the sharpest STI declines Australia has recorded in years. Syphilis diagnoses came in 55% lower and chlamydia 36% lower than earlier trends predicted, across 27 months after national prevention guidelines endorsed doxyPEP for gay and bisexual men. The finding comes from a Burnet Institute-led study published in Nature Health, and it is the first national-level evidence that the strategy is moving the needle beyond individual clinical trials, according to the researchers. Randomized trials had already shown that doxyPEP cuts a person's odds of syphilis and chlamydia; the open question was whether real-world prescribing would shift national numbers.

doxyPEP is a dose of doxycycline taken within 72 hours after condomless sex. In September 2023, the Australasian Society for HIV, Viral Hepatitis and Sexual Health Medicine released national guidance recommending that clinicians consider it for gay and bisexual men at higher risk of STIs, particularly those with a recent infection. The recommendation targeted the people facing the greatest risk rather than the general public, and the World Health Organization issued its own first recommendation on the strategy this year. Prescriptions climbed steadily after the guidance landed. A 2024 survey of more than two thousand men who have sex with men found that about one in seven had used doxyPEP in the previous year.

How the researchers measured the drop

The team drew on ACCESS, a national surveillance collaboration between the Burnet Institute, the Kirby Institute at UNSW Sydney and the National Reference Laboratory. It analyzed records from more than 78,000 gay and bisexual men who attended 29 sexual health services and general practices between January 2022 and December 2025. Projecting the pre-guidance trend forward, the authors write, syphilis and chlamydia both fell well below expected levels, while gonorrhea showed no significant change. The drop was visible quickly: syphilis began falling steeply within months of the guidance, while the chlamydia decline followed later and more gradually.

Separate surveillance backs up the picture. Kirby Institute data show infectious syphilis diagnoses fell by roughly a third among both HIV-negative and HIV-positive gay and bisexual men over the past three years, with chlamydia declining as well and gonorrhea staying relatively flat. "The welcomed decline in syphilis and chlamydia among these groups likely reflects increasing use of doxycycline post-exposure prophylaxis," said Professor Andrew Grulich, who heads the HIV Epidemiology and Prevention Program at the Kirby Institute. Both datasets cover the same period, which makes the association harder to dismiss.

The antibiotic-resistance question

Researchers are quick to flag the tradeoff. Wider use of doxycycline raises the possibility of resistance emerging, in the STI organisms themselves and in unrelated bacteria. Dr Vincent Cornelisse, an adjunct associate professor at the Kirby Institute, put the balance in plain terms: "These studies importantly highlight the potential population-level benefit of large-scale implementation of doxy-PEP among people who are at risk of bacterial STIs." He added that resistance data from trials so far look fairly reassuring, but the question deserves continued attention.

The backdrop is a decade of rising rates. National syphilis and gonorrhea diagnoses rose by more than half over the past ten years, and nearly six thousand syphilis cases were recorded last year. Men account for 78% of syphilis and 73% of gonorrhea diagnoses in Australia, which is why prevention efforts keep centering gay and bisexual men. These trends line up with rising doxyPEP uptake in the same years the guidance took effect.

The Australian guidance deliberately stayed narrower than the approach taken in San Francisco, recommending time-limited use for those at highest risk of syphilis, with ongoing review. Dr Michael Traeger, the study's lead author, noted that the syphilis drop arrived earlier than the chlamydia decline, which could mean the drug also works as treatment-as-prevention by cutting onward transmission from recent infections. For now, the researchers say the next job is watching resistance and keeping testing rates high enough to see what happens next. Monitoring continues through ACCESS, the same clinic network that tracked the decline. Read the full report here: Medical Xpress on the doxyPEP study, and the Burnet Institute announcement: Burnet Institute on the findings.