Doxy-PEP (doxycycline post-exposure prophylaxis) is a single dose of the antibiotic doxycycline taken after condomless sex to lower the risk of certain bacterial STIs. The standard dose is 200 mg taken as soon as possible within 72 hours CDC. It cuts the risk of syphilis and chlamydia, but it doesn't protect against HIV or other viruses.
doxycycline, within 72 hours after sex
syphilis, chlamydia, some gonorrhea
MSM & trans women with a recent bacterial STI
evidence still limited for others
| Item | Value |
|---|---|
| Dose | 200 mg — doxycycline, within 72 hours after sex |
| Reduces | bacterial STIs — syphilis, chlamydia, some gonorrhea |
| For | specific groups — MSM & trans women with a recent bacterial STI |
| Not for | everyone — evidence still limited for others |
What is doxy-PEP and how does it work?
Doxy-PEP uses doxycycline, a tetracycline antibiotic that's been around for decades to treat infections like chlamydia and early syphilis. With prophylaxis it comes down to timing. You take a dose after a sexual exposure so the drug is circulating during the window when bacteria you may have just been exposed to would be establishing an infection, and hitting those organisms early can stop an infection before it takes hold. It's the same logic as HIV post-exposure prophylaxis (PEP) applied to bacteria rather than a virus.
Bacterial is the key word. Doxy-PEP works against the bacteria that cause syphilis, chlamydia, and — less reliably — gonorrhea. It does nothing against HIV, herpes, HPV, hepatitis, or any other virus. Antibiotics don't act on viruses, so doxy-PEP covers one specific slice of the STI picture. For a plain-language primer on the concept, see our overview of what is doxypep? antibiotic to prevent stis.
How well does doxy-PEP work?
In studies, doxy-PEP has reduced new cases of syphilis and chlamydia among people taking it after sex CDC MMWR, 2024. The effect on gonorrhea is more mixed. Some studies showed a benefit and others didn't, largely because gonorrhea has grown more resistant to tetracyclines in many places. The protection is strong and consistent for syphilis and chlamydia, less predictable for gonorrhea.
It also isn't perfect for any of these. It lowers risk without zeroing it out. Think of it as a backstop layered on top of other prevention rather than a force field that lets you drop everything else. Chlamydia in particular tends to come back when a partner isn't treated, so doxy-PEP doesn't replace the need for partner treatment and follow-up — more on chlamydia reinfection and retesting if that's a concern for you.
How to use doxy-PEP and who it's for
The protocol is simple and worth getting exactly right. Take 200 mg of doxycycline as a single dose as soon as possible after sex, ideally within 72 hours of the exposure. Sooner is better, and beyond about three days the benefit drops off. Don't take more than that amount in any 24-hour period, even if you have sex more than once. It's a per-day cap, not a per-encounter dose.
In practice, this looks like keeping a small supply on hand and taking one dose after condomless sex. People often take it with food and a full glass of water to reduce stomach upset, and stay upright for a bit afterward, since doxycycline can irritate the esophagus. It can also make you more sensitive to sunburn, so sunscreen is worth remembering.
Doxy-PEP is not recommended for everyone. Current CDC guidance is for providers to offer and discuss doxy-PEP with gay and bisexual men and transgender women who have had at least one bacterial STI — gonorrhea, chlamydia, or syphilis — in the past 12 months. That group has the clearest evidence of benefit. For other groups, including cisgender women, there isn't yet enough data to weigh the benefits against the harms, so it isn't broadly recommended. That doesn't mean it can never be considered; the conversation should be individualized with a clinician.
Cost and how to get doxy-PEP
Doxy-PEP requires a prescription. You'd typically get it through a primary care provider, a sexual-health or LGBTQ-focused clinic, or sometimes a telehealth service that handles sexual health. Doxycycline itself is an old generic drug, so the medication is generally inexpensive, and many clinics that serve higher-risk patients are set up to prescribe it as part of a prevention package alongside HIV PrEP and routine STI screening.
A common mistake is buying leftover doxycycline online or using a friend's pills without a clinician knowing your STI history, allergies, and other medications. Seeing a provider makes sure doxy-PEP fits your situation and that you're also getting tested on a sensible schedule. You can start the conversation and arrange screening when you get tested.
What doxy-PEP does NOT protect against
Because doxy-PEP only works on bacteria, it leaves several common infections completely uncovered:
- HIV — a virus; doxy-PEP does nothing here, so HIV PrEP or PEP remains separate and essential.
- Herpes (HSV) — a virus that causes recurrent genital or oral sores; unaffected by doxycycline.
- HPV — a virus that can cause genital warts and several cancers; prevented by vaccination, not antibiotics.
- Hepatitis B and C — viral liver infections; not touched by doxy-PEP.
- Gonorrhea — partial and inconsistent protection at best, because of tetracycline resistance.
There's also a longer-term unknown: antibiotic resistance. Using an antibiotic broadly and repeatedly can push bacteria to adapt, and the full impact of widespread doxy-PEP on resistance isn't yet settled. CDC continues to monitor resistance as doxy-PEP is used, and frames it as a targeted tool for specific higher-risk groups rather than a pill for everyone.
How doxy-PEP fits with condoms, vaccines, and testing
Doxy-PEP works best as one layer in a stack, with each layer covering a gap the others leave open:
| Prevention tool | What it covers | What it misses |
|---|---|---|
| Doxy-PEP | Syphilis, chlamydia, partial gonorrhea | All viruses (HIV, HSV, HPV, hepatitis) |
| Condoms | Broad coverage of bacterial and many viral STIs | Skin-to-skin infections (HSV, HPV) on uncovered areas |
| Vaccines (HPV, hep B) | HPV-related cancers/warts, hepatitis B | Everything else |
| HIV PrEP | HIV | All other STIs |
| Regular testing | Catches infections early so they're treated and not spread | Doesn't prevent exposure itself |
Routine STI screening is the piece people skip most often, and it catches what slips through. If you're using doxy-PEP, regular testing tells you whether it's working and flags the infections it can't prevent. Timing matters too. Testing too soon after an exposure can miss an early infection, so check when to test after exposure before you book.
When to talk to a clinician
Bring up doxy-PEP if you've had a bacterial STI in the past year and you have condomless sex, or if you simply want to know whether it's a reasonable fit for your situation. Also see a clinician if you develop symptoms — sores, discharge, burning with urination, pelvic or testicular pain, a new rash — because doxy-PEP prevents infection but doesn't treat one, and an active infection needs proper diagnosis and a full treatment course. A provider can confirm eligibility, review drug interactions and allergies, and set you up with the right testing rhythm.