New research has found an antibiotic that’s been around for decades could help prevent sexually transmitted bacterial diseases, according to a recent University of Washington clinical trial that proved so effective in certain populations it ended early.

In the study, UW researchers teamed up with infectious disease experts from the University of California, San Francisco, to find new strategies to reduce transmission of gonorrhea, chlamydia and syphilis — bacterial sexually transmitted infections that have been on the rise in the United States and around the world for more than five years, said Dr. Connie Celum, a UW professor of global health, medicine and epidemiology who co-led the research.

Because the trial ended early and analysis continues, the results have not yet been published and researchers declined to share more specific data.

The findings were strong, Celum said. In about a year and a half of observation, she and other researchers found overwhelming evidence that a dose of doxycycline helps prevent bacterial STIs in men who have sex with men and transgender women — populations with disproportionately high rates of gonorrhea, chlamydia or syphilis, she said.

“We need new tools to try and interrupt STI transmission,” Celum said. “While not life-threatening, they do cause significant morbidity in women and men. … This is one of the first new interventions that could have applicability.”

In King County, rates of bacterial STIs have steadily risen for years, said Dr. Julie Dombrowski, a co-investigator of the study who also helps run Public Health — Seattle & King County’s HIV/STD program. In 2019, rates of syphilis, gonorrhea and chlamydia among men who have sex with men reached their highest levels since data became available in 1992, according to the county’s annual report.


Bacterial STI rates have shown “massive increases” among the entire population, Dombrowski said.

The clinical trial initially recruited about 550 participants, who had already had an STI in the past year, from public health clinics in San Francisco and Seattle. Those who received doxycycline — a widely available, though not commonly used, antibiotic used to treat some bacterial infections — took 200 milligrams of the medication within 72 hours of having unprotected sex, Celum said. Those in the “control arm” of the study didn’t receive the antibiotic.

Before beginning the trial, researchers were required to specify how strong data had to be in order to stop the study early, she said.

Last month, an independent data safety monitoring board, which reviews the findings twice a year, confirmed researchers had a “clear finding.” Participants who took doxycycline had substantially fewer infections than those who didn’t, including those with HIV or taking HIV PrEP (pre-exposure prophylaxis), Celum said.

The research showed that “if you have the antibiotic in your body at the time of, or shortly after … being exposed to gonorrhea, chlamydia, or syphilis, the infection won’t happen,” she said.

The antibiotic, which would still require a prescription, was generally safe and well tolerated, she said.


Despite encouraging early results, more work is needed to understand the medication’s effectiveness on different populations and whether doxycycline could lead to antimicrobial resistance, which occurs when bacteria changes over time and no longer responds to medicine, Celum said.

“One of the next steps will be for groups, like the CDC and policy groups, to really review the data,” she said. In addition, several other related studies are underway, including ones in Kenya — which is also led by UW investigators — Canada, Australia and France.

While adding doxycycline as an infection prevention tool is a promising first step in reducing STIs, Celum said more diagnostic testing and screenings are needed.

“There’s a push now, hopefully a growing push, to try and move away from syndromic management, which is when you have to have symptoms before getting treated for STIs,” she said. “And 90% of people, both men and women, who have chlamydia and gonorrhea do not have symptoms.”

Celum and her team will present their findings at the International AIDS Conference in Montreal in late July, after continuing to observe participants this summer.

“Because there’s been insufficient evidence to this in many parts of the world, it’s really important we have interventions that work,” Celum said. “… It takes a long time to develop new vaccines, so in the meantime, this could be a way to reduce the burden of STIs.”