DoxyPEP — Doxycycline Post-Exposure Prophylaxis for STIs

DoxyPEP is one of the biggest shifts in STI prevention in decades. It's a single 200 mg doxycycline pill taken within 72 hours after condomless sex. In MSM (men who have sex with men) and transgender women on PrEP or living with HIV, it cuts new chlamydia and syphilis cases by roughly two-thirds and gonorrhea by about half.

The CDC formally recommended it in 2024 for specific populations. Here's the practical guide.

The short answer

  • What it is: 200 mg of doxycycline taken once, within 72 hours of condomless oral, anal, or vaginal sex
  • What it prevents: Chlamydia (88% effective), syphilis (87%), gonorrhea (~55%)
  • What it doesn't prevent: HIV, HPV, herpes, hepatitis, trichomoniasis, BV, yeast — anything non-bacterial or not susceptible to doxycycline
  • Who's it for: MSM and transgender women with one or more bacterial STIs in the past year (per CDC). Other groups are being studied.
  • How often: No more than once every 24 hours, even if multiple sexual encounters
  • Side effects: Usually mild — nausea, sun sensitivity, GI upset

How doxyPEP works

Doxycycline is a tetracycline antibiotic that's been around since the 1960s. It works against the bacteria that cause chlamydia, syphilis, and (to a lesser extent) gonorrhea by inhibiting their protein synthesis.

When taken within 72 hours of exposure:

  • The drug reaches sufficient blood and tissue levels to kill bacteria before they establish infection
  • The 72-hour window is based on bacterial doubling time and pharmacokinetics
  • Single 200 mg dose is enough — multi-day courses aren't more effective

The data — what the trials showed

Major trials

  • DoxyPEP (San Francisco/Seattle, 2022) — MSM and transgender women on HIV PrEP or with HIV. 65% reduction in chlamydia, syphilis, and gonorrhea combined.
  • ANRS IPERGAY substudy (France, 2017) — First proof-of-concept; showed similar effect.
  • Kenyan trial (cisgender women, 2023) — Did NOT show benefit. Reasons under investigation; possibly adherence, biological factors at the genital tract, or different gonorrhea resistance patterns.

Why effects vary by infection

  • Chlamydia: ~88% reduction — most susceptible to doxycycline
  • Syphilis: ~87% reduction — also highly susceptible
  • Gonorrhea: ~55% reduction — modest effect because of widespread tetracycline resistance in gonorrhea

Who CDC recommends doxyPEP for (2024 guidelines)

Recommended: MSM and transgender women who have had at least one bacterial STI (chlamydia, gonorrhea, or syphilis) in the past 12 months.

Counsel about and consider: MSM and transgender women without a recent STI diagnosis but who report condomless sex with multiple partners.

Not currently recommended for:

  • Cisgender women — trial data hasn't shown clear benefit
  • Heterosexual men — limited data
  • Pediatric / pregnancy contexts

This may change. Studies in additional populations are ongoing.

How to take it

  • Dose: 200 mg doxycycline hyclate or monohydrate, taken orally
  • Timing: As soon as possible after condomless sex, ideally within 24 hours; no later than 72 hours
  • Max frequency: No more than once in 24 hours, regardless of number of sexual encounters
  • With or without food: Either is fine, but food can reduce nausea
  • Avoid: Antacids, calcium, iron, magnesium, dairy products within 2 hours (they bind doxycycline and reduce absorption)
  • Stay upright: For at least 30 minutes after taking (reduces risk of esophageal irritation)
  • Avoid intense sun: Doxycycline causes photosensitivity

What doxyPEP doesn't prevent

DoxyPEP only covers some bacterial STIs. It does not prevent:

  • HIV — use PrEP / condoms / U=U
  • HPV — get vaccinated; doxyPEP has no effect
  • Herpes (HSV) — no antibiotic prevents viral infection
  • Hepatitis A, B, C — vaccines for A and B; condoms reduce risk
  • Trichomoniasis — caused by parasite; doxycycline doesn't work
  • Mycoplasma genitalium — variable susceptibility; some strains resistant
  • Yeast and BV — not STIs but doxy doesn't help

Translation: doxyPEP is one tool among several. Layered prevention still matters.

Side effects

Most users have no problems. Possible side effects:

Common

  • Mild nausea (especially without food)
  • Sun sensitivity / easy sunburn
  • GI upset (heartburn, indigestion)
  • Mild diarrhea

Uncommon

  • Esophageal irritation if taken without enough water
  • Photoallergic reactions (rash on sun-exposed skin)
  • C. difficile infection (rare with single doses)
  • Tooth discoloration (only in children under 8 — not adults)

Drug interactions

  • Antacids, calcium, iron, magnesium, dairy — reduces absorption
  • Warfarin — doxycycline can increase warfarin effects
  • Hormonal contraception — minimal interaction, but still discussed
  • Isotretinoin (Accutane) — avoid combination (intracranial pressure risk)

Concerns about antibiotic resistance

This is the biggest debate around doxyPEP. Concerns:

What could happen

  • More resistance in gonorrhea (already a concern)
  • Resistance in commensal bacteria (e.g., staph)
  • Resistance in syphilis (rare but possible)
  • Disruption of gut and skin microbiome

What the data shows so far

  • Modest increases in tetracycline-resistant staph among doxyPEP users
  • No major changes in gonorrhea resistance patterns yet, but monitoring is ongoing
  • Chlamydia and syphilis resistance to doxycycline remains very low globally

The trade-off

DoxyPEP probably reduces total antibiotic use because it prevents infections that would otherwise require multi-day antibiotic courses. The math may favor doxyPEP overall, but vigilance is required.

How to get doxyPEP

Through a prescribing clinician

  • Your PCP, sexual health clinic, or PrEP provider
  • One-time consultation; prescription written for multiple doses (often 10-30 pills/month)
  • Refilled as needed

Through telehealth

  • Several telehealth services prescribe doxyPEP (often bundled with HIV PrEP refills)
  • Costs vary; many insurance plans cover

Cost

  • Generic doxycycline is cheap (often $10-30/month even cash)
  • Coverage by insurance is improving as CDC formally recommends it
  • 340B clinics, sexual health clinics, AHF often dispense at low cost

What to ask your doctor

  • "Am I a candidate for doxyPEP based on my situation?"
  • "How many pills should I have on hand?"
  • "Should I keep taking PrEP / using condoms? (yes — doxyPEP is additive)"
  • "How does this affect routine STI screening intervals? (still every 3-6 months for MSM on PrEP)"
  • "What about doxycycline interactions with my other medications?"

What about doxyPrEP (daily doxycycline)?

DoxyPrEP — taking doxycycline daily rather than after exposure — has been studied but is not currently recommended. Reasons:

  • More side effects from daily dosing
  • More antibiotic exposure → more resistance concern
  • Similar effect can be achieved with on-demand doxyPEP

If you have very frequent exposures, talk to your provider about whether on-demand is sustainable for you.

What to do alongside doxyPEP

DoxyPEP is one layer of an STI prevention strategy:

  • HIV PrEP (if HIV-negative and at risk) — see PrEP vs PEP guide
  • Condoms when appropriate
  • HPV vaccine — see HPV vaccine for adults over 26
  • Hepatitis A and B vaccines
  • Routine STI screening every 3-6 months for sexually active MSM
  • Open partner conversations about sexual health

The future of doxyPEP

Active research questions:

  • Does doxyPEP work for cisgender women? (Why didn't the Kenya trial show benefit?)
  • Long-term resistance impact?
  • Different antibiotics for non-doxy-susceptible gonorrhea?
  • Combination approaches (vaccine + doxyPEP)?

Updates from CDC, WHO, and academic groups are ongoing.

Bottom line

DoxyPEP is:

  • One 200 mg doxycycline pill within 72 hours of condomless sex
  • Reduces chlamydia/syphilis ~85%, gonorrhea ~55%
  • Recommended for MSM and transgender women with recent STI history
  • Not effective against HIV, HPV, herpes, hepatitis, or many other infections
  • Not currently recommended for cisgender women (under study)
  • An addition to — not a replacement for — PrEP, condoms, vaccination, and screening

If you qualify, doxyPEP is one of the most impactful prevention tools we have. Worth talking to your provider about — especially if you've had a recent bacterial STI or are on HIV PrEP.

For more on prevention: PrEP vs PEP — HIV prevention, PrEP side effects, do condoms prevent STIs?. For testing: STI testing window periods.