Study highlights benefits and risks of doxy-PEP for STI prevention

Taking a dose of the oral antibiotic doxycycline after a high-risk sexual encounter has dramatically reduced the incidence of sexually transmitted infections (STIs) in places where the strategy is being tried.

Despite its effectiveness, the new strategy, known as doxy-PEP, may come with risks, especially with chronic use. Experts worry about the impact on the community of gut bacteria, also known as the microbiome, and the potential that the antibiotic will give rise to resistant strains of bacteria. 

Now, using metagenomic sequencing to see the impact of doxycycline on the gut microbiome of those who took it frequently for six months, UC San Francisco researchers have found both reassurance and possible cause for concern. 

Doxy-PEP did not have much impact on the overall composition of bacterial communities in gastrointestinal tracts. But scientists noted signs of resistance building against tetracycline, the class of antibiotic that doxycycline belongs to, which could make it less effective. 

San Francisco's high-profile STI prevention role 

Doxy-PEP is short for doxycycline post-exposure prophylaxis, and it involves taking two 100-milligram pills within 72 hours of condomless sex.

Prompted by early promising clinical trial results, San Francisco became the first city in the country in October of 2022 to recommend doxy-PEP to gay and bisexual men and transgender women with a history of having unprotected encounters with multiple partners.

But the widespread use of antibiotics raises concerns about resistance and the potential harmful impact on gut health, specifically the balance of bacteria and other microbes. Disruption can lead to diarrhea, nausea, fever and abdominal pain; and until now, there had been very limited research into these side effects. 

More research is needed to determine which gut bacteria are turning on these tetracycline-resistant genes to know whether this will lead to more doxycycline-resistant infections among both individuals taking doxy-PEP and in the greater community.

"Right now, it looks like the pros outweigh the cons," Langelier said. "Especially given the dramatic rise in STIs, in particular syphilis, over the past decade."

Source:

University of California - San Francisco

Journal reference:

Chu, V. T., et al. (2024). Impact of doxycycline post-exposure prophylaxis for sexually transmitted infections on the gut microbiome and antimicrobial resistome. Nature Medicine. doi.org/10.1038/s41591-024-03274-2.