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Antibiotic after sex slashes STD rates in study

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Just one dose of the antibiotic doxycycline taken after sex halved the number of chlamydia and syphilis cases in San Francisco, promising new research shows.

In the study, gay and bisexual men and transgender women who had a history of sexually transmitted infections or multiple sex partners were given a supply of the antibiotic and asked to take two 100-milligram pills within 72 hours of unprotected sex. The strategy is known as doxy-PEP.

What happened?

In results presented Monday at the Conference on Retroviruses and Opportunistic Infections in Denver, new cases of and early syphilis—but not gonorrhea—dropped drastically over the course of a year.

"It's not subtle, it is very fast and we're seeing the beginning of it, not the end," researcher Dr. Hyman Scott, a medical director at the San Francisco Department of Public Health, told the New York Times. "This is what we want for STI prevention."

Syphilis cases have reached their since 1950, the CDC reported in January. Left untreated, syphilis damages the heart and brain and can cause blindness, deafness and paralysis. Meanwhile, chlamydia infection rates plateaued nationwide in 2022, but there were still nearly 1.7 million cases.

"These data are simply looking at numbers of STI cases at different time points, but make me feel hopeful that doxy-PEP may be able to reduce STIs at a population level," Dr. Ina Park, an STI expert at the University of California, San Francisco, told the Times. She was not involved in the study.

So far, the evidence supports the case for doxycycline use only in gay and bisexual men and transgender women.

But, "the majority of STIs in the United States occur in cisgender women," Dr. Jonathan Mermin, director of the CDC's National Center for HIV, Viral Hepatitis, STD and TB Prevention, told the Times.

"Studies of whether doxy-PEP works in cisgender women should be implemented as quickly as possible," he stressed.

After releasing draft guidelines for doxy-PEP in October, the CDC plans to issue its final recommendations in the next few months, Mermin noted.

The San Francisco Department of Public Health had already begun recommending doxy-PEP for gay and and a year before the agency's draft guidelines were published.

In the latest research, tracked monthly rates of chlamydia, gonorrhea and early syphilis before and after the introduction of doxy-PEP in November 2022. They also compared the numbers with the rate of STI infections in cisgender women.

Over a 13-month period, new cases of chlamydia and early syphilis across the city dropped by 50 percent, compared with the expected numbers.

Unfortunately, cases of chlamydia in cisgender women steadily increased.

"The fact that we did not see declines in STIs in other populations not recommended for doxy, specifically cis women, strengthens the conclusion that the decline in chlamydia and early syphilis cases is related to [the] doxy-PEP rollout," said researcher Madeline Sankaran, an epidemiologist at the San Francisco Department of Public Health who presented the findings, the Times reported.

While the results are promising, Mermin cautioned that they may have been skewed by the 2022 outbreak of mpox, during which high-risk groups curtailed their sexual activity.

More information: Abstract: Sustained Reduction of Bacterial STIs During the DoxyPEP Study Open-Label Extension

Visit the U.S. Centers for Disease Control and Prevention for more on STDs.

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