Doxy-PEP & STIs:
What You Need to Know

Doxy-PEP stands for doxycycline post-exposure prophylaxis. It is the use of doxycycline — a common, inexpensive antibiotic that has been around for decades — taken after a sexual exposure to prevent certain bacterial STIs.^[14]

The concept is similar to HIV PEP: you take it after a potential exposure, not before. The window is 72 hours — the sooner the better, with the most benefit seen when taken within 24 hours.

Doxycycline is not a new drug. It has been used for decades to treat acne, malaria prevention, and bacterial infections. What is new is the intentional use of a single 200mg dose after sexual exposure as a prevention strategy.

The clinical evidence is strong — and for syphilis and chlamydia specifically, the results have been striking. Four major randomized trials have now tested doxy-PEP, and the picture that emerges is clear for some groups and still uncertain for others.

The earliest signal came from the IPERGAY substudy in France (Molina et al., 2018), which found about a 70% reduction in chlamydia and syphilis among men who have sex with men (MSM) already using HIV PrEP.^[1]

The landmark DoxyPEP trial (Luetkemeyer et al., published in the New England Journal of Medicine, 2023) studied MSM and transgender women in San Francisco and Seattle who had HIV or were on PrEP. It found:

A larger French trial, DOXYVAC (Molina et al., 2024), confirmed these results in a different population of MSM on PrEP: about a 79% reduction in chlamydia and syphilis combined, and a 33% reduction in gonorrhea.^[3]

But doxy-PEP has not shown the same benefit in every group studied. A trial in Kenya (dPEP Kenya, Stewart et al., 2023) tested doxy-PEP in 449 cisgender women on HIV PrEP and found no significant reduction in STIs compared with standard care.^[4] The likely reason wasn’t biology — it was adherence. Hair sample testing, which can objectively detect recent doxycycline use, found the drug in only about 29% of samples from women assigned to take it, even though most reported taking it. Researchers believe low real-world uptake, not lack of effectiveness, best explains the null result — but they note more research is needed to fully understand why the drug did not work as expected in this trial.^[4]

Real-world data are catching up to the trials, too. A San Francisco sexual health clinic that rolled out doxy-PEP broadly reported roughly a 60% overall drop in STIs after 96 weeks, including a 78% drop in syphilis and 67% drop in chlamydia among people using it — evidence that the trial results hold up outside a research setting.^[5]

In 2024, the CDC issued final clinical guidelines recommending that providers discuss doxy-PEP with MSM and transgender women who have had a bacterial STI in the past 12 months.^[6] The evidence base continues to grow, but so far it is strongest for this specific population.

This is one of the most important things to understand about doxy-PEP. It does not protect against everything — and it does not replace condoms, PrEP, or regular testing.

The CDC’s 2024 clinical guidelines recommend that providers discuss doxy-PEP with cisgender MSM and transgender women who have had a bacterial STI — syphilis, chlamydia, or gonorrhea — diagnosed in the past 12 months.^[6] Doxy-PEP is not recommended for everyone — appropriate use matters both for your health and for public health antibiotic stewardship.

Doxy-PEP is simple in concept but requires some care in practice. Here is how it works:

STIs and HIV are not separate concerns. They are deeply intertwined in ways that matter for everyone — whether you are HIV-positive, on PrEP, or neither.

For people living with HIV: Active STIs can temporarily increase your viral load, even if you are on ART and otherwise undetectable. Syphilis in particular has been shown to elevate HIV viral load in the genital tract, potentially affecting transmission risk. STIs also cause inflammation that can accelerate immune damage and complicate treatment. Getting tested and treated promptly is essential.

For people at risk of HIV: STIs that cause sores or inflammation — syphilis, herpes, gonorrhea — significantly increase HIV acquisition risk. An active syphilis infection can increase HIV risk by 2 to 5 times.^[9] Controlling STIs is a core component of HIV prevention.

For everyone: The STI picture is a genuine mix of good and bad news. CDC provisional data show more than 2.2 million STIs reported in the U.S. in 2024, up 13% since 2015.^[10] The good news: primary and secondary syphilis — the most infectious stages — declined about 22% in 2024, the second year in a row of decline, likely reflecting expanded testing, self-tests, and doxy-PEP use.^[10] The harder news: congenital syphilis, passed from a pregnant person to their baby, rose again — up about 2% in 2024, the 12th consecutive annual increase, with nearly 4,000 cases reported and a roughly 700% rise since 2015.^[10] This remains a public health emergency that does not get enough attention.

Doxy-PEP is not without legitimate debate. Being informed means understanding the concerns, not just the benefits.

This is the most significant public health concern. Widespread use of doxycycline could accelerate resistance — not just in STI-causing bacteria, but in the broader microbiome. Gonorrhea already has resistance to multiple antibiotic classes. Some researchers worry that broad doxy-PEP use could worsen the gonorrhea resistance crisis.^[12]

Encouragingly, the San Francisco real-world data followed resistance patterns for 96 weeks and found doxy-PEP’s benefit for chlamydia and syphilis held up over time, even as researchers continue to watch gonorrhea resistance closely.^[5] The CDC guidance attempts to balance benefit against risk by targeting high-risk populations rather than recommending universal use, and recommends ongoing surveillance of resistance patterns.^[12]

Some researchers have raised concerns that doxy-PEP could lead to reduced condom use — similar to concerns raised about PrEP when it launched. The DoxyPEP trial itself did not find evidence of significant behavioral change among participants, but this remains a concern worth monitoring in real-world settings.^[2]

Doxycycline is generally well-tolerated at single doses. Common side effects include nausea and gastrointestinal upset (reduced by taking with food). Sun sensitivity can be significant, particularly in Florida’s climate. Rare but serious reactions include severe skin reactions and esophageal damage if taken without adequate water.^[8]

Doxycycline has few significant interactions with modern ART regimens, but always disclose all medications to your prescriber. Antacids, calcium supplements, and iron can reduce doxycycline absorption and should not be taken together.

Doxy-PEP requires a prescription and a conversation with a healthcare provider. Here is how to make that conversation productive:

Not all providers are familiar with doxy-PEP, particularly outside major metro areas. If your provider is unfamiliar, you can reference the 2024 CDC Clinical Guidelines on Doxycycline Post-Exposure Prophylaxis (Doxy-PEP) as a starting point for the conversation.^[6] Sexual health clinics and HIV specialty providers are most likely to be up to date.

If you are in Florida and need help finding a provider, use our care locator to find HIV clinics, sexual health services, and Ryan White providers near you.