Prescribing doxyPEP after STI diagnosis is more efficient than targeting specific groups of people

Dr Michael Traeger at CROI 2023.
Dr Michael Traeger at CROI 2023. Photo by Roger Pebody.

Prescribing doxycycline post-exposure prophylaxis (doxyPEP) to gay and bisexual men and transgender women after a sexually transmitted infection (STI) diagnosis instead of focusing on specific groups “at higher risk” would reduce STI spread while minimising the number of people needing to take the antibiotic, according to a study presented at the 30th Conference on Retroviruses and Opportunistic Infections (CROI 2023) in Seattle yesterday.

“Specifically in our cohort, we estimate that prescribing doxyPEP for 12 months following an STI diagnosis could have averted 42% of all STIs,” Dr Michael Traeger of Harvard Medical School said while presenting the study’s findings.

Three clinical trials have shown that prescribing the antibiotic doxycycline to men and transgender women within 72 hours of having condomless sex reduces STIs. However, not all STIs are created equal. The trials indicate relatively high effectiveness at preventing chlamydia (as high as 88%) and syphilis (as high as 87%) in men and transgender women, whereas doxyPEP is much less effective at preventing gonorrhoea in these groups. One study found the drug prevented around 50% of gonorrhoea infections, while another presented found zero benefit.



Antibiotics, also known as antibacterials, are medications that destroy or slow down the growth of bacteria. They are used to treat diseases caused by bacteria.


An umbrella term for people whose gender identity and/or gender expression differs from the sex they were assigned at birth.


A sexually transmitted infection caused by the bacterium Treponema pallidum. Transmission can occur by direct contact with a syphilis sore during vaginal, anal, or oral sex. Sores may be found around the penis, vagina, or anus, or in the rectum, on the lips, or in the mouth, but syphilis is often asymptomatic. It can spread from an infected mother to her unborn baby.


Having sex without condoms, which used to be called ‘unprotected’ or ‘unsafe’ sex. However, it is now recognised that PrEP and U=U are effective HIV prevention tools, without condoms being required. Nonethless, PrEP and U=U do not protect against other STIs. 


Single-celled micro-organisms.

The differences in doxyPEP effectiveness observed in the trials likely depend on a variety of factors such as how soon after exposure participants took the medication and whether they were living with HIV. DoxyPEP’s ability (or inability) to reduce gonorrhoea infections is highly dependent on whether a particular strain of bacteria has developed antibiotic resistance. Furthermore, widespread use of antibiotics has the potential to exacerbate drug resistance in gonorrhoea and other STIs, leading to questions about how best to use doxyPEP to prevent the spread of STIs in communities of gay and bisexual men and transgender women.

While the three trials show reduced STIs at the individual level, Traeger’s analysis aimed to investigate how community spread might shrink in response to different doxyPEP prescribing strategies. To accomplish this, Traeger and his colleagues evaluated various hypothetical prescribing strategies divided into two categories. The first category focused on strategies that target particular groups such as gay and bisexual men or transgender women living with HIV or those taking PrEP.

Alternatively, prescribing strategies could focus on STI diagnoses, for example by providing access to the preventive drug for 12 months after people have been treated for an STI, or targeting those who have had multiple infections. A key component of the analysis was also to identify more efficient strategies, meaning those requiring the least the number of individuals taking the drug to prevent an infection.

To accomplish this, the research team evaluated STI test data for more than 10,500 people who accessed care between 2015 and 2020 at a large health centre in Boston that specialises in care for LGBTQ+ people. Participants were mostly gay (87%) and White (71%) but included bisexual men (7%), transgender women (4%), and nonbinary people (2%). In addition, the cohort included people taking PrEP (54%) and people living with HIV (14%).

The researchers estimated how applying the hypothetical prescribing strategies would have reduced STI incidence in the Boston cohort, assuming the effectiveness of the antibiotic was the same as in the DoxyPEP study.

As expected, the more people that were prescribed doxyPEP, the more STIs were averted. However, Traeger argued that giving doxyPEP to everyone may not be the most efficient approach. Making doxyPEP available to everyone in the study group after condomless sex would have prevented 70% of STIs, but would also have required all 10,500 people to take the drug – potentially increasing concerns about developing resistance.

Strategies based on specific groups of people were less efficient, needing a higher proportion of the group to avert infections. For example, prescribing doxyPEP to all people living with HIV and all people taking HIV PrEP had the potential to prevent 60% of STIs in a year, but 68% of the clinic cohort would be taking the antibiotic.

In contrast, strategies focusing on STI diagnoses would see smaller proportions of people needing doxyPEP. If doxyPEP was prescribed after any STI diagnosis, 42% of new STIs could have been avoided. Prescribing doxyPEP to those who’d been diagnosed with two STIs in the past 12 months would have averted 23% of the groups’ infections with just 14% of people taking it.

“Of course the overall impacts will decrease when prescribing to less people – there's no silver bullet – but you do get this trade off between doxyPEP prescribed and the impact,” Traeger said during a press briefing.

One positive impact of this approach could be more efficiently reducing the spread of specific STIs. By giving doxyPEP after syphilis, 25% of subsequent syphilis infections could be avoided by prescribing doxyPEP to just 9% of the cohort.

Traeger pointed out that the effectiveness of these strategies may differ in the real world and in larger populations, but argued that their results point to ways of weighing benefits against potential harms when setting clinical prescription guidelines for doxyPEP. “Specifically, our analysis suggests that the most efficient doxyPEP prescribing strategies are based on STI history rather than PrEP use or HIV status.”


Traeger MW et al. Potential impact and efficiency of doxy-PEP among people with or at risk of HIV. Conference on Retroviruses and Opportunistic Infections, Seattle, abstract 122, 2023.

View the abstract on the conference website.