Antibiotic after sex: Australia records far fewer syphilis cases

With so-called doxy-PEP, people take the antibiotic doxycycline after condomless sex to prevent bacterial sexually transmitted infections. A team including researchers from the Burnet Institute and La Trobe University in Melbourne has examined how well this works in everyday practice. The study was published on 21 October 2026 in the journal Nature Health. It analysed routine data on 78,560 gay and bisexual men aged 16 and over who were tested between January 2022 and December 2025 at 29 sexual health services and general practices in Australia's ACCESS network. According to aidsmap, Australia was among the first countries to recommend doxy-PEP; national guidance was issued in autumn 2023.

The researchers compared actual diagnoses after the guidance with the trend that would have been expected without it. Instead of the projected 5,403 syphilis cases, 2,430 were recorded – a 55 percent reduction. For chlamydia there were 13,676 diagnoses instead of 21,280, about 35 percent fewer. Gonorrhoea showed no significant change. In 16 of the clinics, the share of doxycycline prescriptions with doxy-PEP instructions rose from 0.04 to 27.7 percent. “This is the first time, other than COVID, that we've really seen these declines in more than 10 or 15 years,” lead author Michael Traeger of the Burnet Institute said, according to aidsmap, when presenting early results at the International AIDS Conference in August.

The authors point out limitations: whether individual men actually used doxy-PEP could not be identified, so the data show population-level associations rather than proof of cause. The participating clinics also mainly reflect people who already test regularly, and other changes in behaviour may play a role. Information on funding was not available to the newsroom. According to aidsmap, concerns about antibiotic resistance centre on gonorrhoea; doxy-PEP is not a very effective tool against it, and the resistance question needs further research.