An antibiotic taken after sex, rather than after a diagnosis, is now part of US prevention guidance for some people. CDC’s 2024 doxy-PEP guideline rests on strong trial results in gay and bisexual men and transgender women, a trial in cisgender women that found no benefit, and open questions about resistance.
- What it is: a single dose of doxycycline after sex to lower the chance of a bacterial STI.
- Targets: syphilis, chlamydia and gonorrhea; it is an antibiotic, so it has no effect on viruses such as HIV.
- Recommended for: gay, bisexual and other men who have sex with men and transgender women with a bacterial STI in the past 12 months.
- Not recommended (no recommendation possible): cisgender women, cisgender heterosexual men, transgender men and nonbinary people, for lack of evidence.
- Follow-up: STI testing at baseline and every 3-6 months.
On June 6, 2024, CDC published its first clinical guidelines on doxycycline postexposure prophylaxis, shortened to doxy-PEP, in its MMWR Recommendations and Reports series. The idea still generates confusion: some people assume it works like HIV PEP, some assume anyone can use it, and some worry it will breed resistant bacteria. Each of those points has a specific answer in the guideline and the trials behind it.
How the evidence arrived: a timeline
| When | What happened | What it showed |
|---|---|---|
| 2018 | IPERGAY substudy published (Lancet Infectious Diseases) | In this open-label substudy, doxy-PEP reduced chlamydia risk by 70% and syphilis risk by 73%, with no significant difference for gonorrhea, as summarized by CDC |
| February 2023 | ANRS DOXYVAC results presented as a conference abstract | Trial stopped early for efficacy; CDC reports adjusted hazard ratios of 0.49 for gonorrhea, 0.11 for chlamydia and 0.21 for syphilis |
| April 6, 2023 | DoxyPEP trial published in the New England Journal of Medicine | Among 501 men who have sex with men and transgender women, combined STI incidence fell by about two thirds |
| October 2, 2023 | CDC posts draft guidelines for 45 days of public comment | Clinicians and the public could comment before the guideline was finalized |
| December 21, 2023 | dPEP Kenya trial published in NEJM | In 449 cisgender women, no significant reduction in STIs |
| June 6, 2024 | Final CDC guideline published (MMWR Recomm Rep, vol. 73, no. RR-2) | Strong recommendation for MSM and transgender women with a recent bacterial STI; no recommendation for other groups |
What the trials found
The US DoxyPEP study (Luetkemeyer et al., NEJM 2023) enrolled men who have sex with men and transgender women who either took HIV pre-exposure prophylaxis (PrEP) or were living with HIV, and who had had gonorrhea, chlamydia or syphilis in the previous year. Participants were randomized 2:1 to doxycycline after condomless sex or to standard care, with STI testing every quarter.
In the PrEP group, an STI was found at 10.7% of quarterly visits with doxycycline versus 31.9% with standard care, a relative risk of 0.34. Among people living with HIV, the figures were 11.8% versus 30.5%, a relative risk of 0.38. The reductions held for each infection: in the PrEP group the relative risks were 0.45 for gonorrhea, 0.12 for chlamydia and 0.13 for syphilis. Five grade 3 adverse events and no serious adverse events were attributed to doxycycline.
The Kenyan trial (Stewart et al., NEJM 2023) asked the same question in women aged 18 to 30 who were taking HIV PrEP. It found 25.1 STIs per 100 person-years with doxycycline and 29.0 with standard care, a relative risk of 0.88 that was not statistically significant. Hair testing in a random sample detected doxycycline in only 29% of samples from the doxycycline group, which suggests many participants were not taking it. Every gonorrhea isolate in that trial was resistant to doxycycline.
Both the investigators and CDC point to low use as a likely factor, but the trial cannot prove that better adherence would have worked. That uncertainty is why women are not covered by the recommendation.
What the guideline specifies
The core recommendation in the CDC guideline says providers should counsel gay, bisexual and other men who have sex with men and transgender women who have had at least one bacterial STI (syphilis, chlamydia or gonorrhea) in the past 12 months about the benefits and harms of doxycycline 200 mg once within 72 hours of oral, vaginal or anal sex, not exceeding 200 mg per 24 hours, and should offer it through shared decision-making. Elsewhere it says the dose should be taken ideally within 24 hours and no later than 72 hours.
That is a guideline statement about what clinicians offer, not a self-treatment plan. Doxy-PEP is a prescription; your clinician decides whether it fits and writes the instructions for your situation. STI prevention is not among the uses listed on the doxycycline label we checked, and the guideline itself does not address FDA labeling for this use.
CDC rates the evidence for its main recommendation as high quality and the recommendation as strong. A footnote adds that, although not directly studied, doxy-PEP “could be discussed” with men who have sex with men and transgender women who have not had a bacterial STI in the past year but expect sexual activities known to raise exposure risk.
Known: In men who have sex with men and transgender women with a recent STI, doxy-PEP cut chlamydia and syphilis sharply and gonorrhea by roughly half in the DoxyPEP trial. Serious side effects were not attributed to it in that trial. It does not target HIV or other viral infections, which is why CDC pairs it with HIV testing and PrEP.
Not yet known: Whether it works in cisgender women if taken consistently; whether it helps cisgender heterosexual men, transgender men or nonbinary people, who were not studied; how much it will drive resistance in gonorrhea and in bacteria that live on the skin and in the gut over years; and what long-term repeated use does to the microbiome. CDC says the guideline will be updated as data arrive.
Who it is and isn’t recommended for
Recommended: gay, bisexual and other men who have sex with men and transgender women with syphilis, chlamydia or gonorrhea diagnosed in the past 12 months. CDC’s doxy-PEP page for clinicians repeats that group and says evidence does not support use in other populations, including cisgender women.
No recommendation: the guideline states that “no recommendation can be given at this time” for cisgender women, cisgender heterosexual men, transgender men and other queer and nonbinary people. It notes that data for these groups are limited and asks clinicians to use clinical judgment and shared decision-making.
A separate conversation: people who are pregnant or could become pregnant, and anyone whose child might be exposed. The doxycycline label warns that tetracycline-class drugs during the last half of pregnancy and in childhood up to age 8 may permanently discolor teeth. The doxy-PEP guideline itself does not address pregnancy.
What doxy-PEP does not prevent
Doxycycline is an antibiotic, and the trials measured only syphilis, chlamydia and gonorrhea. It does not prevent HIV, which is why the guideline calls for HIV screening and linkage to PrEP for HIV-negative people. It is not a substitute for vaccines: CDC’s list of services to offer alongside doxy-PEP includes mpox, hepatitis A and HPV vaccination. Our page on bacterial vs. viral infections and antibiotic resistance explains why an antibiotic has no effect on viruses.
It also does not replace treatment. Someone with symptoms, or a partner’s positive test, still needs testing and a full treatment course for the specific infection; our guide to chlamydia and gonorrhea testing and treatment covers that.
The resistance question
Using an antibiotic on a schedule tied to sex, rather than to a diagnosis, raises a fair worry. CDC’s guideline names it directly: doxy-PEP may facilitate antimicrobial resistance in STI bacteria and in other bacteria. In the DoxyPEP trial, tetracycline-resistant gonorrhea occurred in 5 of 13 cultured cases in the doxycycline groups and 2 of 16 in standard care. Among participants carrying Staphylococcus aureus, CDC reports that tetracycline-resistant strains rose from 5% to 13% at 12 months, even as overall carriage fell.
CDC’s conclusion is that current data suggest overall benefit, but that resistance and microbiome effects “will need to be monitored.” Its clinician page says CDC will use multiple data systems to watch for increases in resistance and that “questions remain about potential long-term development of antimicrobial resistance.” This is one reason the guideline asks clinicians to reassess the need for doxy-PEP every 3-6 months instead of renewing it indefinitely.
Testing and follow-up
The guideline asks for bacterial STI testing at the body sites exposed at the start and every 3-6 months, HIV screening for HIV-negative people according to current recommendations, and a check on whether doxy-PEP is still needed at the same interval. It frames doxy-PEP as one part of a broader sexual health approach that includes risk-reduction counseling, STI screening and treatment, vaccination and linkage to HIV PrEP.
Side effects named in the guideline include sensitivity to sunlight and gastrointestinal symptoms such as nausea, vomiting, diarrhea and irritation or ulceration of the esophagus. Our article on doxycycline side effects describes which ones are common and which need care.
Questions to ask a clinician
- Based on my history over the past year, do I fall into the group CDC recommends doxy-PEP for?
- If I am outside that group, what does the evidence say for someone like me, and does our state or city guidance differ?
- How will testing work, and which sites should be swabbed every 3-6 months?
- Should I be on HIV PrEP, and am I up to date on mpox, hepatitis A and HPV vaccines?
- Do any of my medicines, supplements or a possible pregnancy change whether doxycycline is right for me?
- What symptoms mean I should get tested and treated instead of relying on doxy-PEP?
- When will we review whether I still need it?
Sources
- Bachmann et al., CDC Clinical Guidelines on the Use of Doxycycline Postexposure Prophylaxis for Bacterial STI Prevention, United States, 2024. MMWR Recomm Rep 2024;73(No. RR-2)
- CDC: Doxy PEP for bacterial STI prevention (clinician page)
- Luetkemeyer et al., Postexposure Doxycycline to Prevent Bacterial STIs, NEJM 2023 (PubMed)
- Stewart et al., Doxycycline Prophylaxis to Prevent STIs in Women, NEJM 2023 (PubMed)
- DailyMed: Doxycycline hyclate label
Evidence checked November 2, 2026. General information only; doxy-PEP is a prescription decision made with a clinician.