Doxy-PEP: how the antibiotic taken after sex works, who US guidance covers and why resistance is a worry

An antibiotic taken after sex, in the way a morning-after pill is taken, has become part of routine sexual health care in some American cities in about four years. NPR reported on October 11 from San Francisco, where the approach began and where officials widened their guidance again in May. The method is called doxy-PEP.

Its supporters and its critics agree on the main facts. It works well against two infections and partly against a third. It also means giving antibiotics to healthy people on a large scale, and the first evidence of what that does to drug resistance is now in. Where they differ is on whether the trade is worth it, and for whom.

What is doxy-PEP?

The name is short for doxycycline postexposure prophylaxis. Doxycycline is a long-established antibiotic. The federal guideline describes it this way: Doxycycline, a broad-spectrum tetracycline antimicrobial, is well absorbed and tolerated. Broad-spectrum means it acts against many kinds of bacteria, and tetracyclines are the drug family it belongs to.

Postexposure prophylaxis is the second half. The Centers for Disease Control and Prevention (CDC) defines it in the guideline. Postexposure prophylaxis (PEP) involves taking a medication to prevent an infection after a possible exposure and is a common strategy for prevention of HIV and other infections. HIV is human immunodeficiency virus. Using an antibiotic this way has precedent, too. Doxycycline is used as PrEP or PEP to prevent infections such as malaria and Lyme disease, PrEP being pre-exposure prophylaxis, the same idea applied before exposure.

Doxy-PEP targets three infections, all caused by bacteria and all spread through sex: syphilis, chlamydia and gonorrhea. The case for a new tool is that there are few others. No vaccines and few chemoprophylaxis options exist for the prevention of bacterial sexually transmitted infections (STIs) (specifically syphilis, chlamydia, and gonorrhea). Chemoprophylaxis means using a drug to prevent a disease. The burden is uneven. These infections have increased in the United States and disproportionately affect gay, bisexual, and other men who have sex with men (MSM) and transgender women (TGW).

The regimen is simple. In the guideline’s words, the recommended dose of doxy PEP is 200 mg and should not exceed a maximum dose of 200 mg every 24 hours. It is taken once, as soon as possible after sex and no later than 72 hours.

How well doxy-PEP worked in trials

The guideline rests on three randomized trials, two in France and one in the United States. Its summary of them: in three large randomized controlled trials, 200 mg of doxycycline taken within 72 hours after sex has been shown to reduce syphilis and chlamydia infections by >70% and gonococcal infections by approximately 50%. Gonococcal means caused by the gonorrhea bacterium.

The American trial, run in San Francisco and Seattle, shows what those percentages mean in practice. It enrolled 501 men who have sex with men and transgender women who had had one of the three infections in the past year. Some were living with HIV and some were taking medicine to prevent it. Participants were randomly assigned in a 2:1 ratio to take 200 mg of doxycycline within 72 hours after condomless sex or to carry on with standard care, and everyone was tested every three months.

Among those taking HIV-prevention medicine, an STI was diagnosed in 61 of 570 quarterly visits (10.7%) in the doxycycline group and 82 of 257 quarterly visits (31.9%) in the standard-care group. Roughly one check-up in ten turned up an infection with doxy-PEP, against nearly one in three without it. The results in people living with HIV were similar. The trial’s authors concluded that the combined incidence of gonorrhea, chlamydia, and syphilis was lower by two thirds with doxycycline postexposure prophylaxis than with standard care. Incidence is the rate of new cases.

The protection was not equal across the three. Chlamydia and syphilis fell by roughly three quarters or more. Gonorrhea fell by a little over half.

The CDC put the same result in terms of pills. Persons in the study took, on average, an additional 43 doses of doxycycline to prevent an average of 1.3 bacterial STIs annually.

The known side effects of the drug are of two kinds. Adverse effects most associated with doxycycline include photosensitivity and gastrointestinal symptoms including esophageal erosion and ulceration. Photosensitivity is skin that burns easily in the sun, and the esophagus is the tube from mouth to stomach, which the tablets can irritate. In the American trial, five grade 3 adverse events and no serious adverse events were attributed to doxycycline. Grade 3 means severe but not life-threatening.

One trial had a different result, and it was the only one in women. Researchers gave doxy-PEP or standard care to Kenyan women 18 to 30 years of age who were taking HIV-prevention medicine. A total of 449 participants underwent randomization. After a year there had been 50 infections in the doxy-PEP group and 59 in the comparison group, a difference small enough to be chance.

The likeliest reason emerged from the participants’ hair, which retains traces of drugs. Among 50 randomly selected participants in the doxycycline-PEP group, doxycycline was detected in 58 of 200 hair samples (29.0%). Many of the women assigned to take the drug, it appears, were not taking it. The CDC reads the trial as suggesting that nonadherence might have been the reason for lack of efficacy, while allowing that biological differences have not been excluded. Nonadherence means not taking a medicine as intended.

Who the US guidance on doxy-PEP covers

The CDC’s recommendation, issued in June 2024, follows the trial populations closely. CDC recommends that MSM and TGW who have had a bacterial STI (specifically syphilis, chlamydia, or gonorrhea) diagnosed in the past 12 months should receive counseling that doxy PEP can be used as postexposure prophylaxis to prevent these infections. The decision is meant to be made between patient and prescriber.

The focus on people with a recent infection is deliberate. Infections are concentrated in a minority, and the guideline cites the arithmetic. In one study of 2,981 MSM, 25% of persons accounted for 76% of bacterial STIs. Aiming the antibiotic at that minority prevents the most infections for the fewest doses.

For everyone else the guideline is noncommittal. It says that clinical data to support doxy PEP in other populations (i.e., cisgender women, cisgender heterosexual men, transgender men, and other queer and nonbinary persons assigned female at birth) are limited. Cisgender describes people whose gender matches the sex recorded at birth. Prescribers are told to use their judgment.

The recommendation comes with conditions. Persons who are prescribed doxy PEP should undergo bacterial STI testing at anatomic sites of exposure at baseline and every 3-6 months thereafter, and the need for it is to be reviewed on the same schedule.

Some places have gone further. NPR reported that at one San Francisco clinic, doctors offer it to anyone they think could benefit, regardless of sex or sexual orientation, and that city officials updated local guidance to match this approach in May. Hyman Scott, a doctor at the clinic, explained the reasoning by reference to the slow early uptake of HIV prevention medicine. “We cannot make the same mistakes with rollout that we made with PrEP,” Scott said.

What happened when cities rolled out doxy-PEP

Trials show what a drug can do under supervision. The question afterward is what happens across a city.

San Francisco issued its guidance in October 2022, and researchers compared the infections reported over the next 13 months with what earlier trends had projected. The group studied was large: there were 6694 cases of chlamydia, 9603 cases of gonorrhea, and 2121 cases of early syphilis among MSM and transgender women during the analytic period. By the end, chlamydia cases were about 50% below projection and early syphilis about 51% below. Gonorrhea went the other way, rising faster than projected. The authors concluded that the doxyPEP guideline release was associated with decreases in reported cases of chlamydia and early syphilis, but not gonorrhea.

NPR’s more recent figures for the city show the same shape. Across San Francisco last year, syphilis went down 24%. Chlamydia rates dropped 18%. But gonorrhea only dipped slightly, 5%.

A health insurer’s records tell a matching story at the level of individuals. Among people taking HIV-prevention medicine at Kaiser Permanente in Northern California, 2253 (19.5%) were dispensed doxyPEP in the first 14 months it was available. In that group, the share testing positive for chlamydia in a given three-month period fell from 9.6% before they started to 2.0% afterward.

Neither study was a trial. Citywide cases can fall for other reasons, and people who choose doxy-PEP may change in other ways at the same time. The San Francisco team said as much. Further analyses are needed to assess whether declines are sustained and monitor for adverse consequences, including antimicrobial resistance.

The antibiotic resistance concern with doxy-PEP

Every use of an antibiotic gives bacteria a chance to adapt to it. Doxy-PEP means repeated doses for people who are not ill, and the worry has always been what that would select for.

Annie Luetkemeyer of the University of California, San Francisco, who led the American trial, does not dismiss it. “Need to be thoughtful about how we’re using it. Antibiotics are an incredible resource. We all worry about antimicrobial resistance,” Luetkemeyer told NPR.

The first sizable numbers have come from King County, Washington, which includes Seattle and which issued its own guidance in 2023. Among men who have sex with men diagnosed with gonorrhea there, the share of infections resistant to tetracyclines had been steady at about 27% from 2017 to early 2023. By the middle of 2024 it was 70%. Men who took more than three doses a month were more likely to have a resistant strain, though any use at all was not linked to one.

Doxycycline is not used to treat gonorrhea, so that rise does not by itself make the infection harder to cure. It does help explain why doxy-PEP protects least against gonorrhea, and it suggests that protection may shrink further.

The effect reaches beyond the target infections. The same study swabbed the nose and throat of 703 clinic patients. Doxy-PEP users were less likely to carry Staphylococcus aureus, a common bacterium that can cause skin and bloodstream infections, but more likely to carry a tetracycline-resistant form of it, 18% against 8%. The authors described the pattern as suggesting that doxy PEP impacts off-target bacteria. Of the wider picture they wrote that its impact on antimicrobial resistance and the microbiome is uncertain. The microbiome is the community of microbes that live in and on the body.

The CDC anticipated this. Its guideline judged potential harms to be low in the short-term and unknown but potentially concerning in the long-term. And it noted a gap that remains. There are no studies to date on long-term, intermittent use of doxycycline and the microbiome.

Why countries disagree about doxy-PEP

Faced with the same trials, health authorities have reached different conclusions. The CDC guideline records that certain countries have issued varying position statements on the practice of doxy PEP ranging from recommending against widespread use because of concerns about antimicrobial resistance to conditional or case-by-case use.

Australia is the clearest contrast with the United States. A national consensus conference there agreed that doxy-PEP should be considered primarily for the prevention of syphilis, with any drop in other infections treated as a side benefit. Some participants wanted it limited to syphilis alone, arguing that the risk of increasing antimicrobial resistance outweighed any potential benefit from reductions in other bacterial STIs in the target population. The statement’s overall position was that doxy-PEP may not be suitable as a population-level intervention and should instead be used more selectively.

The reasoning turns on how serious each infection is. Vincent Cornelisse, a Sydney doctor and co-author of the statement, set it out for NPR. Cornelisse’s point was that untreated syphilis can cause blindness, deafness and brain damage, while chlamydia in men is usually far milder. “I would argue that for gay men, there is no population-level benefit for getting chlamydia rates down,” Cornelisse said.

American advocates of wider use answer that the alternative on offer has not worked. Critics tell Luetkemeyer that condoms are the answer. “And if we just say, the only way you can protect yourself from HIV, from pregnancy, from STIs is using condoms, we’re going to fail,” Luetkemeyer said.

What the doxy-PEP evidence cannot yet show

Long-term effects. The trials lasted about a year. Nobody knows what years of intermittent doses do to resistance in other bacteria or to a person’s own microbes.

Whether the benefit lasts. If resistance keeps rising, protection against gonorrhea may fade, and the trials cannot say whether chlamydia or syphilis will follow.

Women and heterosexual men. One trial in women failed, probably because the pills were not taken. No trial has tested the approach in heterosexual men. Offering it to these groups goes beyond the evidence.

Resistance and cause. The King County figures show resistance rising after guidance was issued in spring 2023. They cannot prove that doxy-PEP drove all of the rise, and the most resistant strains had begun increasing in 2021.

Do not stop or change a prescribed medicine without talking to your doctor.

Whether doxy-PEP is appropriate for a particular person is assessed by their doctor.

Doxy-PEP prevents infections now at the price of a resistance risk that may be borne later, and not only by those who take it, and how to weigh one against the other is a judgment on which careful authorities have come down differently.