New data shows that cases of bacterial STIs have dropped dramatically in San Francisco. The decline is greatest among men who have sex with men (MSM) and transgender women. Public health officials in the city believe doxy PEP has helped drive these declines.
Doxy PEP is an STI prevention strategy. Users take the antibiotic doxycycline after condomless oral or anal sex to prevent chlamydia, gonorrhea, or syphilis. The strategy is intended for men who have sex with men (MSM) and transgender women who have sex with men. Research has shown that in this group it can reduce the risk of chlamydia by 88%, syphilis by 87%, and gonorrhea by 55%. Research on cisgender women who have penis-in-vagina sex has not found doxy PEP to be effective. Doxy PEP does not prevent HIV or other viral STIs like HPV or herpes.
San Francisco was an early adopter of this strategy, and it seems to be paying off. Between 2024 and 2025:
- cases of chlamydia dropped by 18%
- cases of syphilis dropped by 24%
- cases of gonorrhea dropped by 5%.
Dr. Stephanie Cohen, director of STI and HIV prevention and control for the San Francisco Health Department, said in a statement, “The remarkable drop in STI cases in San Francisco is due in large part to the trailblazing work that made the rollout of doxy PEP possible.”
San Francisco Health Officer Dr. Susan Philp added, “We are excited to see the progress being made in lowering STI diagnoses in San Francisco. We encourage the public to keep up the momentum by taking actions — such as getting tested — to protect their sexual health.”
The Centers for Disease Control and Prevention (CDC) recommends that providers discuss doxy PEP with MSM and transgender women who have sex with men. The CDC guidelines note that people in these communities who have had an STI in the last year are good candidates for doxy PEP.
Does doxy PEP increase antibiotic resistance?
There are concerns that widespread use of doxy PEP could increase antibiotic resistance in the bacteria that cause these STIs. The bacteria that cause gonorrhea, for example, has already shown resistance to full classes of antibiotics. In fact, existing resistance to doxycycline may be why doxy PEP is not as effective against gonorrhea.
The fear is that that increasing the use of doxycycline will give bacteria more opportunity to evolve antibiotic resistance. An early study did find that certain types of antibiotic-resistant gonorrhea were becoming more common. More research needs to be done to determine whether an increase in doxy PEP use is behind this rise.
In the meantime, experts continue to recommend doxy PEP. And data like what we see in San Francisco suggest that it’s having a positive impact on the STI epidemic.