Two studies published in this month’s JAMA Internal Medicine offer encouraging findings about the use of doxy PEP. One study in San Francisco found lower rates of syphilis and chlamydia after doxy PEP was introduced than would have been expected. Another study in Northern California found fewer case of chlamydia, syphilis, and gonorrhea among PrEP users who also used doxy PEP. While there’s not enough data to prove a direct causal connection yet, the authors of both studies believe that the early doxy PEP roll out in parts of California is behind decreasing STI rates in the state.

Doxy PEP is a new STI prevention strategy. Users take the antibiotic doxycycline after condomless oral or anal sex to prevent chlamydia, gonorrhea, or syphilis. Early studies showed that it can reduce the risk of syphilis and chlamydia by 80% or more. It can also reduce the risk of gonorrhea by 50% when used by men who have sex with men (MSM) and transgender women who have sex with men. Research on cisgender women who have penis-in-vagina sex has not found doxy PEP to be effective.

San Francisco was an early adopter of this strategy. One of the new studies looked at bacterial STI rates in that city after the introduction of doxy PEP. It compared those numbers to projected rates based on past data. Researchers looked at real and projected numbers over a 13-month period that ended in November 2023. They found that chlamydia was down 50% compared to projections and early syphilis was down 51%. Gonorrhea cases, however, were higher than the projected rates for that period.

The other study was conducted among Kaiser Permanente patients in Northern California. It included 11,551 people who were taking PrEP to prevent HIV. The researchers found that those who also took doxy PEP had fewer cases of all bacterial STIs. Specifically, quarterly positive tests were down by 79% for chlamydia, 80% for syphilis, and 12% for gonorrhea. In contrast, rates of these STIs remained stable among those participants who took PrEP but did not use doxy PEP.

Researchers noted that the declines in gonorrhea cases only included infections of the urethra or anus. There was no decline in cases of gonorrhea of the throat. More research is needed to understand why doxy PEP is ineffective against throat infection.

There are concerns about antibiotic resistance and whether widespread use of doxy PEP could add to the problem. 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. The authors of both studies agree that this issue needs to be further investigated.

The Centers for Disease Control and Prevention (CDC) offer guidelines on doxy PEP use. CDC recommends that providers discuss doxy PEP with men who have sex with men and transgender women who have sex with men—especially those who’ve had an STI in the last year.

These new studies provide a glimpse of how use of doxy PEP among MSM and transgender women could positively impact STI rates on a population level.