Doxycycline Post-Exposure Prophylaxis Cuts Chlamydia and Syphilis by Half, but Risks Antibiotic Resistance

Doxycycline post-exposure prophylaxis, or Doxy-PEP, is used to reduce the risk of sexually transmitted infections (STIs) like chlamydia and syphilis. Taken soon after unprotected sex up to 72 hours after the risky encounter, it can lower the chances of infection. However, researchers at the University of California, San Francisco (UCSF) are examining its long-term effects on gut health and antibiotic resistance.

Doxycycline

Doxycycline Credit: SFAF

What is Doxy-PEP?

Doxy-PEP involves taking two 100-milligram tablets of doxycycline within 72 hours after high-risk sexual encounters. This method targets gay and bisexual men and transgender women with multiple partners. Data from March 2024 indicates that Doxy-PEP cut the rates of chlamydia and syphilis by about 50% in San Francisco within one year.

Research Methodology

UCSF researchers used metagenomic sequencing to study the impact of doxycycline on gut bacteria. They analyzed individuals who regularly used the medication over six months, looking for changes in gut bacteria and signs of antibiotic resistance.

Findings on Gut Microbiome and Resistance

The study produced mixed results. While the overall gut bacteria composition remained stable, an increase in tetracycline resistance genes was noted among users. Dr. Chaz Langelier, the lead author, stated, “Doxy-PEP does not seem to have a major impact on the gut microbiome, but it affects antimicrobial resistance in gut bacteria.”

Among the 100 Doxy-PEP users and 50 control patients, a rise in antibiotic-resistance genes was observed. Those who continued using Doxy-PEP showed an increase in gene expression. Dr. Langelier added, “We do not see significant changes in the gut bacterial community, but doxy-PEP users accumulate more tetracycline resistance genes over time.”

Potential Risks and Side Effects

Long-term use of doxy-PEP is associated with risks, including diarrhea, nausea, fever, and abdominal pain. The emergence of antibiotic resistance may reduce doxycycline’s effectiveness against infections.

Antibiotics can also disturb the balance of gut bacteria, leading to severe conditions like Clostridium difficile infections. A healthy gut is important for overall well-being, and any disruption can have serious effects.

Future Research Directions

More research is needed to identify which gut bacteria activate tetracycline-resistant genes in Doxy-PEP users. Understanding this could help manage the risks of antibiotic resistance and guide the safe use of Doxy-PEP.

Conclusion

Doxy-PEP offers benefits in preventing STIs, especially for high-risk groups. However, the risks of antibiotic resistance and gut health disruption must be considered. As Doxy-PEP usage increases, healthcare providers and patients should remain aware of these issues. While reducing STI rates may currently outweigh the risks, continued research is essential.

References

Chu, V.T., Glascock, A., Donnell, D. et al. Impact of doxycycline post-exposure prophylaxis for sexually transmitted infections on the gut microbiome and antimicrobial resistome. Nat Med (2024). https://doi.org/10.1038/s41591-024-03274-2

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