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, ideally within 24 hours, as recommended in clinical guidance.

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] Doxycycline was detected in only about 29% of tested hair samples, suggesting low use may help explain the result. That does not establish effectiveness in cisgender women: biological differences and other explanations remain under study.^[4]

Real-world data are catching up to the trials, too. An observational study at a San Francisco sexual health clinic followed doxy-PEP implementation over 96 weeks and found sustained reductions in bacterial STIs, especially chlamydia and syphilis, with smaller effects for gonorrhea. This supports effectiveness in routine care, while continued monitoring is needed.^[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: STIs need testing and treatment for their own health effects. They do not negate U=U when HIV viral load stays below 200 copies/mL on effective ART. U=U prevents sexual HIV transmission, but it does not prevent transmission of other STIs.^[15]

For people at risk of HIV: STIs that cause sores or inflammation — syphilis, herpes, gonorrhea — significantly increase HIV acquisition risk. The size of that increase varies with the infection and circumstances; STI prevention, HIV PrEP when appropriate, and testing work together.^[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, The surveillance figures alone cannot establish how much each prevention strategy contributed.^[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]

The San Francisco clinic study found sustained benefit over 96 weeks, but it called for continued surveillance of gonorrhea and antibiotic resistance; it did not establish that resistance concerns have been resolved.^[5] CDC recommends discussing benefits and possible harms and reassessing the need for doxy-PEP every three to six months.^[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. Separate doxycycline by at least two hours from dairy products, antacids, and supplements containing calcium, iron, magnesium, or sodium bicarbonate.^[6]

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.