What Is DoxyPEP? The Morning-After Pill for STIs, Explained

DoxyPEP stands for doxycycline post-exposure prophylaxis: a single 200 mg dose of the common antibiotic doxycycline, taken within 72 hours after sex, to prevent bacterial STIs before they take hold. Think of it as a morning-after pill — but for syphilis, chlamydia, and gonorrhea rather than pregnancy.

How DoxyPEP works

Doxycycline is a broad-spectrum antibiotic that's been used safely for decades. Taken shortly after a possible exposure, it reaches the tissues where STI bacteria first establish themselves and stops the infection before it starts. The window matters: the trials dosed within 72 hours of condomless sex, and effectiveness is assumed to be higher the sooner you take it.

What the evidence shows

The pivotal DoxyPEP trial (published in the New England Journal of Medicine, 2023) enrolled men who have sex with men and transgender women, either living with HIV or on PrEP. Compared with standard care, DoxyPEP reduced quarterly incidence of chlamydia and syphilis by roughly two-thirds or more, with a smaller but meaningful reduction in gonorrhea (gonorrhea's partial resistance to tetracyclines explains the difference). Results were strong enough that San Francisco began recommending it citywide before national guidance caught up.

One honest limitation: a trial in cisgender women in Kenya did not show a protective effect, likely due to adherence and tissue-level factors — which is why current CDC guidance focuses on gay/bi men and transgender women, and use in cisgender women remains an individualized decision with your clinician.

Who should consider it

Per CDC guidance, DoxyPEP should be discussed with gay or bisexual men and transgender women who have had a bacterial STI (syphilis, chlamydia, or gonorrhea) in the past 12 months. Many clinicians also prescribe it case-by-case for people with multiple partners who haven't had a recent STI — the guidance sets a floor for the conversation, not a ceiling.

Dosing, safety, and the resistance question

The regimen is simple: 200 mg of doxycycline, once, within 72 hours after sex — and no more than 200 mg in any 24-hour period regardless of how many encounters. Full details, including what to do about multiple encounters in a weekend, are in the dosage guide. Common side effects (stomach upset, sun sensitivity) and how to avoid them are covered in the side-effects guide.

The most common concern is antibiotic resistance. It's a legitimate scientific question being actively monitored; so far, surveillance hasn't shown resistance changes that outweigh the dramatic drop in STI rates in populations using DoxyPEP, and the consensus of public-health bodies is that the benefit currently outweighs the theoretical risk for the recommended groups. Your prescriber should revisit this with you as data evolves.

DoxyPEP + PrEP: the combined strategy

DoxyPEP prevents bacterial STIs; PrEP prevents HIV. They cover completely different infections and are commonly prescribed together as a comprehensive sexual-health toolkit. If you're already on PrEP, adding DoxyPEP is usually a five-minute conversation at a routine visit.