Could a Morning-After Pill Prevent Sexually Transmitted Infections?

San Francisco has formally recommended doxyPEP, which works like Plan B but targets STIs

close-up round, pink, hairy bacteria
Illustration of Neisseria gonorrhoeae bacteria Alissa Eckert / SCIENCE PHOTO LIBRARY via Getty Images

A new morning-after pill doesn’t aim to prevent pregnancy. Instead, the drug may reduce the risk of contracting sexually transmitted infections (STIs).

The antibiotic—called doxyPEP for doxycycline post-exposure prophylaxis—works similarly to pregnancy-preventing morning-after pills like Plan B, except that it targets syphilis, chlamydia and gonorrhea.

In May, a clinical trial of the drug closed early, because its results were so promising that researchers thought it would be unethical to continue giving unmedicated placebos to control group participants. The trial studied doxyPEP’s use in men who have sex with men and transgender women, given that these populations have the highest risk of infection. It found that taking a single dose within 72 hours of oral, anal or vaginal sex significantly reduced the risk of contracting syphilis, chlamydia and gonorrhea, according to a press release.

Researchers shared the preliminary results in July at the 24th International AIDS Conference. Among HIV-negative participants, the doxyPEP treatment was 88 percent effective at preventing chlamydia, 87 percent effective against syphilis and 55 percent effective against gonorrhea. For HIV-positive individuals, the numbers differed slightly—74 percent for chlamydia, 77 percent for syphilis and 57 percent for gonorrhea.

Scientists have studied doxycycline’s effect on STIs for years, but significant progress has been made only recently, report Jamie Smyth and Caitlin Gilbert for the Financial Times. Last week, the San Francisco Department of Health released the first formal guidance in the United States recommending the use of doxyPEP to prevent bacterial STIs. San Francisco has some of the country’s highest rates of syphilis, the STI with the highest severe disease potential.

“We're really hopeful that doxyPEP can help us turn the tide on the STI epidemic that we've been struggling with in San Francisco and across the country for so long,” San Francisco City Clinic medical director Stephanie Cohen tells the Bay Area Reporter’s Liz Highleyman. “We believe this is a really important, innovative public health strategy for STI prevention.”

The antibiotic doxycycline has been around for more than four decades and is commonly used to treat and prevent acne and Lyme disease, as well as to prevent malaria, reports Jon Cohen for Science. The drug has minimal side effects beyond digestive issues and an increased sensitivity to sunlight.

The morning-after strategy to prevent STIs isn’t new, either—it dates back to the mid-20th century, reports Vox’s Keren Landman. During the 1940s, the U.S. Navy researched whether penicillin prevented gonorrhea among men, if taken after sex. Today, doxyPEP contains antivirals that are similar to HIV PEP—a treatment that has effectively prevented infections after HIV exposure from sex or needles—and aids in stopping an infection before it really takes effect, Vox writes.

Though studies on the drug’s efficacy have mainly focused on men who have sex with men and transgender women, an ongoing trial in Kenya aims to assess if doxyPEP works in cisgender women as well, per Vox.

However, doxyPEP treatment has raised concerns among some experts who say prescribing the drug for STI prevention may increase antibiotic resistance to other infections that doxycycline treats or prevents, including malaria, per the Financial Times.

“Having more options to prevent bacterial STIs among high-risk groups is certainly needed and the study data for preventing syphilis and chlamydia is quite convincing,” Cindy Liu, chief medical officer at the Antibiotic Resistance Action Center at George Washington University, tells the publication. “But we need to address the concern that this could trigger a rise in doxycycline-resistant MRSA and undermine the antibiotic’s usefulness. I think we need more data on this.”

Despite these concerns, some experts say doxyPEP could help reduce STI transmission more broadly, reports Vox. Jeffrey Klausner, an infectious disease doctor at the University of Southern California, tells the publication that even if the treatment was exclusively used among men who have sex with men, doxyPEP would ultimately benefit everyone at risk of contracting STIs. “It would reduce transmission among the people at highest risk for repeated infections, thus making onward transmission through other sexual networks less likely,” Vox writes.

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