These Morning-After Pills May Prevent S.T.I.’s, Researchers Say

Sexually transmitted infections have soared in recent years in the United States, prompting an urgent search for solutions. New research suggests that a widely available antibiotic, taken after sex, may help stem the tide.

A single dose of doxycycline taken within 72 hours of unprotected sex dramatically cuts the risk of a bacterial S.T.I., studies have found. The approach seems most effective for preventing chlamydia and syphilis, and slightly less so for preventing gonorrhea.

The strategy has been shown to work among trans women and men who have sex with men who are at high risk for acquiring an S.T.I. But the pills have not shown a benefit in cisgender women (whose gender identity matches the sex assigned at birth).

With the resurgence of S.T.I.s, researchers and health officials are eager to find new prevention strategies. Researchers have also found that a meningitis vaccine halved the incidence of gonorrhea. The two diseases are caused by closely related bacteria.

The most recent data were presented last month in Seattle at the Conference on Retroviruses and Opportunistic Infections.

“While condoms work really well when people use them, they’re not always accessible to people in their relationships,” said Dr. Jenell Stewart, an infectious diseases physician at Hennepin Healthcare and the University of Minnesota. “We need to have solutions beyond that.”

The Centers for Disease Control and Prevention does not yet recommend post-exposure doxycycline to prevent S.T.I.s. But based on the strength of the new evidence, some cities like San Francisco are already offering the antibiotic to those at high risk of infection with chlamydia, syphilis and gonorrhea. Health officials are calling it “doxy-PEP.”

“For those who have a lot of S.T.I.s, I think this is certainly something I hope that the C.D.C. can provide some guidance,” said Dr. Annie Luetkemeyer, an infectious disease physician at Zuckerberg San Francisco General Hospital and Trauma Center.

Syphilis was nearly eliminated in the United States in 2000; rates of gonorrhea, too, were declining around that time. But the infections have resurged, in large part because of the shuttering of sexual health clinics across the country.

Between 2017 and 2021, syphilis cases shot up by 68 percent and gonorrhea cases by 25 percent. About half of those new infections were in teens and young adults.

The incidence of S.T.I.s also rose in women, and Black women in particular. Rates of congenital syphilis, acquired during gestation from an infected mother, increased by nearly 200 percent during the period.

On any given day in 2018, about one in five Americans had an S.T.I., the C.D.C. has estimated. Although many infections are easily treated with antibiotics, many people remain undiagnosed because they may not have symptoms or have poor access to health care.

Left untreated, syphilis can damage the reproductive system, brain, heart and other organs. Congenital syphilis can even lead to death; nearly 7 percent of infants with syphilis in 2020 did not survive.

The new studies looked primarily at the use of doxycycline in men who have sex with men, who account for more than 40 percent of S.T.I.s in the United States. Generally, the participants were given a supply of the antibiotic and told to take two pills within three days of a potential exposure to an S.T.I.

A study in 2017 found that post-exposure doxycycline sharply reduced the rate of chlamydia and syphilis, but not gonorrhea, in this group.

“We were very cautious at the time because we didn’t want everyone to use the strategy before it has been confirmed,” said Dr. Jean-Michel Molina, a professor of infectious diseases at Paris Cité University, who led the study.

The results were bolstered by those from another study, presented last year at a conference on H.I.V. That trial found that doxycycline after sex decreased the incidence of syphilis and chlamydia among participants by more than 80 percent, and that of gonorrhea by about 55 percent.

The approach seemed so effective that a data safety monitoring board advised the researchers to halt the trial and offer doxycycline to all participants.

The latest studies largely confirmed these promising results. In one of them, Dr. Molina and his colleagues tested doxycycline in 232 men who have sex with men, who were already enrolled in a larger study of H.I.V. prevention.

The men who took doxycycline after sex were 84 percent less likely to contract chlamydia or syphilis and about half as likely to contract gonorrhea, compared with those who did not get the antibiotic.

A separate arm of that trial found that men who got 4CMenB, a vaccine against bacterial meningitis made by GlaxoSmithKline, were half as likely to get gonorrhea. It’s unclear how long the protection might last, and the vaccine is not approved for this purpose in the United States, Dr. Luetkemeyer said.

But the vaccine is considered to be safe and effective, and it is often offered to travelers, young adults and military recruits. “There’s a lot of enthusiasm in high-risk populations to get more data and to start using the meningococcal B vaccine,” Dr. Luetkemeyer said.

Although these studies included only men who have sex with men, she and other researchers said they expected the results to be applicable to men of any sexual orientation.

But the antibiotic did not prevent S.T.I.s in cisgender women, at least in Kenya.

Dr. Stewart and her colleagues enrolled 449 young women in Kisumu who were taking daily pills to prevent H.I.V. and tested them for S.T.I.s every three months. The number of infections among those given a supply of doxycycline was roughly the same as among those who did not get the drug.

The results were “a huge disappointment,” said Dr. Stewart. The researchers are now trying to understand why the women did not benefit, she said. It may be because of anatomical differences in how the drug is metabolized or because of the high prevalence of antibiotic-resistant bacteria in Kenya.

The idea that doxycycline might be widely used to prevent S.T.I.s has provoked some worry about antibiotic resistance. The drug has been used for decades to treat chlamydia and syphilis without the emergence of resistant strains, but antibiotic-resistant gonorrhea is already a serious concern in many parts of the world.

In France, where some of the studies were conducted, about 65 percent of infections are resistant to tetracyclines, a class of antibiotics that includes doxycycline. In Kenya, every bacterial sample from the small number of infected women in the trial turned out to be resistant.

In the United States, about 26 percent of gonorrhea cases in 2018 were resistant to tetracyclines. So far, however, doxycycline’s use to prevent S.T.I.s does not seem to contribute to drug resistance, Dr. Luetkemeyer said.

Doxycycline is already widely used to prevent malaria and to treat acne and rosacea. It would be recommended for preventing S.T.I.s only to those who might anyway frequently take antibiotics to treat infections, she noted.

“This was a very high-risk group — 30 percent per quarter had one or more S.T.I.,” she said of the participants in her trial. “That is not the general population.”

The C.D.C. is reviewing the latest data and expects to make new recommendations on the use of post-exposure doxycycline. In the meantime, the agency has advised health care providers to offer only doxycycline, not other antibiotics, for prevention, and only to gay and bisexual men and transgender women, for whom there is evidence of effectiveness.

Patients should be counseled about potential side effects, like gastrointestinal trouble and sensitivity to light, the agency said.