After years of rising sexually transmitted infection (STI) rates in Australia, new Burnet Institute-led research has found promising evidence that doxycycline post-exposure prophylaxis (doxyPEP), an antibiotic taken after sex, may be helping to reduce syphilis and chlamydia diagnoses among gay and bisexual men.
The study, published in Nature Health, found that syphilis diagnoses were 55 per cent lower and chlamydia diagnoses 36 per cent lower than expected, based on projections of earlier trends, in the 27 months following the introduction of national doxyPEP guidance in September 2023.
The findings come as Australia continues to grapple with high STI rates. National syphilis and gonorrhoea diagnosis rates have risen by more than 50 per cent over the past decade, with almost 6,000 syphilis diagnoses recorded in 2025 alone, according to the latest Kirby Institute surveillance data.
DoxyPEP involves taking the antibiotic doxycycline within 72 hours after sex to reduce the risk of bacterial STIs.
Researchers analysed data from more than 78,000 gay and bisexual men attending 29 sexual health and general practice services across Australia between January 2022 and December 2025. The findings provide the first national-level evidence that suggest changes in doxyPEP prescribing for gay and bisexual men were associated with sustained reductions in syphilis and chlamydia diagnoses.
Lead author Dr Michael Traeger, Deputy Head of the HIV and STI Prevention Group at Burnet Institute, said the findings suggest that doxyPEP could play an important role in reducing STI diagnoses among gay and bisexual men.
“These findings suggest we're beginning to see those benefits reflected in sustained declines across Australian sexual health services."
The research used data from the Australian Collaboration for Coordinated Enhanced Sentinel Surveillance of Sexually Transmissible Infections and Blood Borne Viruses (ACCESS), a national surveillance collaboration between Burnet Institute, the Kirby Institute and the National Reference Laboratory (NRL).
In September 2023, the Australasian Society for HIV, Viral Hepatitis and Sexual Health Medicine (ASHM) released national guidance recommending that doxyPEP be considered for gay and bisexual men at increased risk of STIs, particularly those with a recent STI diagnosis.
Developed in consultation with clinicians, researchers and community representatives, the guidance has been supported by community organisations working to raise awareness of doxyPEP, improve access and support its uptake.
While the findings are promising, researchers found no significant reduction in gonorrhoea diagnoses, highlighting the need for additional prevention strategies and ongoing monitoring of antibiotic resistance.
The findings follow the World Health Organization's first recommendation on doxyPEP in 2026, which emphasises targeted use alongside comprehensive sexual health care, regular STI testing and antibiotic-resistance monitoring.
Publication: Traeger et al. Population-level impact of doxycycline post-exposure prophylaxis on bacterial sexually transmitted infections. Nature Health (2026).
