Doxycycline After Sex to Prevent Chlamydia, Gonorrhea and Syphilis

Table of Contents

1. What Doxycycline Postexposure Prophylaxis Is
2. The DoxyPEP Trial in San Francisco and Seattle
3. The IPERGAY Substudy in France
4. The DOXYVAC Trial in Paris
5. The dPEP Trial in Kenyan Women Found No Significant Reduction
6. Pooled Results and Results Outside the Trials
7. What the 2024 Centers for Disease Control and Prevention Guideline States
8. Resistance in Gonorrhea
9. Resistance in Other Bacteria
10. Side Effects the Studies Measured
11. Gut and Vaginal Microbiome Research
12. What Is Not Known
13. Key Research Papers
14. Connections
15. Featured Videos

What Doxycycline Postexposure Prophylaxis Is

"Prophylaxis" means prevention, and "postexposure" means after the possible contact. Doxycycline postexposure prophylaxis (doxy-PEP) is a dose of the antibiotic taken after condomless sex, whether or not a partner is known to be infected. It differs from treatment, which starts after an infection is found, and from a daily preventive pill, which is taken regardless of sex. A few studies on this page tested a daily schedule, and they are labelled as such.

Doxycycline belongs to the tetracycline family of antibiotics. The 2024 federal guideline from the Centers for Disease Control and Prevention (CDC) describes it as well absorbed, with a half-life of about 12 hours. Chlamydia and syphilis are caused by bacteria that the antibiotic has long been used against. The same report states that doxycycline is not a recommended treatment for gonorrhea because of elevated resistance, although it still works against many strains in the United States. That difference between the three germs runs through every result below.

The method targets bacteria only. The trials measured chlamydia, gonorrhea and syphilis. Human immunodeficiency virus (HIV), herpes simplex and human papillomavirus are viruses, which an antibiotic does not act on. Many infections also go unnoticed. In the IPERGAY substudy, 72 of 102 infections (71%) caused no symptoms, and the trials tested participants on a fixed schedule.

Three terms recur below. "Relative risk" and "hazard ratio" compare two groups: 1.0 means no difference, and 0.25 means about one quarter the rate. The "confidence interval" is the range the true value plausibly falls in, and an interval that crosses 1.0 means the difference could be chance. "Open-label" means participants knew which group they were in. All four main trials were open-label, with no placebo pill.

The DoxyPEP Trial in San Francisco and Seattle

This United States trial enrolled men who have sex with men and transgender women who had a bacterial sexually transmitted infection in the past year. Everyone was either taking preexposure prophylaxis against human immunodeficiency virus or living with that virus, and the results were reported separately for these two groups. The trial gave 200 mg of doxycycline within 72 hours after condomless sex, compared with standard care, with testing every three months.

The 2023 report covered 501 people. The share of three-month periods with an infection was:

• People on preexposure prophylaxis (327): 10.7% with doxycycline against 31.9% with standard care, relative risk 0.34 (0.24 to 0.46).
• People living with the virus (174): 11.8% against 30.5%, relative risk 0.38 (0.24 to 0.60).

By infection, the relative risks were:

• Chlamydia: 0.12 (0.05 to 0.25) on preexposure prophylaxis; 0.26 (0.12 to 0.57) living with the virus.
• Gonorrhea: 0.45 (0.32 to 0.65) and 0.43 (0.26 to 0.71).
• Syphilis: 0.13 (0.03 to 0.59) and 0.23 (0.04 to 1.29). The second interval crosses 1.0, so that one result is not statistically firm on its own.

The final report, published in 2025, covered 637 people enrolled. Infections appeared in 12.0% of three-month periods with doxycycline (129 of 1,077) against 30.5% with standard care (139 of 455), relative risk 0.39 (0.31 to 0.49). The randomized phase ended early because the method was working, and those still enrolled were offered doxycycline. In that open extension, when people already knew the result, infections appeared in 13% of periods among those continuing and 17% among those newly starting.

The IPERGAY Substudy in France

This is the earliest of the four main trials, published in 2018. It enrolled 232 men who have sex with men and who used tenofovir disoproxil fumarate plus emtricitabine to prevent human immunodeficiency virus. The trial gave a single 200 mg dose within 24 hours after sex, compared with no prophylaxis, with a median follow-up of 8.7 months. Participants took a median of 680 mg per month.

• Any first infection: 28 men with doxycycline against 45 without; estimated nine-month probability 22% against 42%; hazard ratio 0.53 (0.33 to 0.85).
• Chlamydia: hazard ratio 0.30 (0.13 to 0.70).
• Syphilis: hazard ratio 0.27 (0.07 to 0.98).
• Gonorrhea: no significant difference, hazard ratio 0.83 (0.47 to 1.47).

So the gonorrhea result was already uncertain in the earliest trial.

The DOXYVAC Trial in Paris

DOXYVAC enrolled men who have sex with men at ten Paris hospitals. None had human immunodeficiency virus; all used preexposure prophylaxis and had a bacterial infection in the prior 12 months. The trial gave 200 mg (two 100 mg pills) within 72 hours after condomless sex, with no more than three doses per week, compared with no prophylaxis. It also tested the meningococcal group B vaccine against gonorrhea, as a separate question in the same men. Of 556 randomly assigned, 545 were analyzed, with a median follow-up of 14 months.

• Chlamydia or syphilis (combined main outcome for doxycycline): 8.8 first episodes per 100 person-years against 53.2, adjusted hazard ratio 0.17 (0.12 to 0.26).
• Vaccine against gonorrhea: 58.3 against 77.1 per 100 person-years, adjusted hazard ratio 0.78 (0.60 to 1.01). The interval crosses 1.0, and the authors did not show efficacy.

The 2024 federal guideline cites earlier results from this trial by infection: an adjusted hazard ratio of 0.49 (0.32 to 0.76) for gonorrhea, 0.11 (0.04 to 0.30) for chlamydia and 0.21 (0.09 to 0.47) for syphilis. Those are initial figures from a smaller group of 502 men, not the final analysis above.

The dPEP Trial in Kenyan Women Found No Significant Reduction

This is the only randomized trial of the after-sex method in cisgender women named in the 2024 federal guideline. It enrolled 449 Kenyan women aged 18 to 30 who were using preexposure prophylaxis against human immunodeficiency virus. The trial gave 200 mg of doxycycline hyclate within 72 hours after condomless sex, compared with standard care, for 12 months.

• Any infection: 50 with doxycycline (25.1 per 100 person-years) against 59 with standard care (29.0), relative risk 0.88 (0.60 to 1.29), P = 0.51. Not significant.
• Chlamydia: 35 against 50, relative risk 0.73 (0.47 to 1.13). Not significant. Chlamydia made up 85 of the 109 infections.
• Gonorrhea: relative risk 1.64 (0.78 to 3.47), as given in the 2024 federal guideline. Not significant.
• Syphilis: could not be evaluated, because the 2024 federal guideline records only two early syphilis infections during the study.
• Trichomonas (a later secondary analysis): 27 against 29 diagnoses, relative risk 0.96 (0.54 to 1.73). No difference.

Two findings bear on how to read this null result. First, the drug was often not there to measure. Among 50 randomly selected women in the doxycycline group, it was detected in 58 of 200 hair samples (29.0%), although women reported dosing coverage in 78% of weekly surveys, according to the 2024 federal guideline. The trial's authors state that use was low. Second, every gonorrhea sample tested from the trial was resistant to doxycycline. The 2024 federal guideline says that nonadherence might explain the lack of efficacy, and that other reasons, such as biological differences, still need to be explored. The trial cannot separate these explanations.

One small observational study from Tokyo points in a different direction, but it tested something else. Forty female sex workers at a private clinic took daily doxycycline, 100 mg per day. Their overall infection rate fell from 232.3 to 79.2 per 100 person-years compared with their own earlier rate, an incidence rate ratio of 0.33. The fall in chlamydia was of marginal statistical significance, syphilis cases dropped to zero, and gonorrhea did not change significantly. It had no comparison group and no after-sex dosing, so it does not settle the question the Kenyan trial raised.

Pooled Results and Results Outside the Trials

A 2026 meta-analysis pooled three randomized trials with 1,278 participants, all men who have sex with men or transgender women at high risk. The pooled relative risks were 0.22 (0.12 to 0.39) for chlamydia, 0.24 (0.13 to 0.44) for syphilis and 0.79 (0.66 to 0.95) for gonorrhea. The authors describe the gonorrhea benefit as smaller and long-term resistance as uncertain.

Studies from clinics and cities measure what happens once people use the method in ordinary life. They are observational, so other changes over the same period may account for part of what they show.

• Northern California health system, 2,253 people dispensed doxycycline: three-monthly positivity fell from 9.6% to 2.0% for chlamydia (rate ratio 0.21), from 1.7% to 0.3% for syphilis (0.20), and from 10.2% to 9.0% for gonorrhea (0.88, a modest change). Rates stayed stable in people not dispensed it.
• San Francisco citywide, 13 months after local guidelines: chlamydia was 49.64% and early syphilis 51.39% below projected cases among men who have sex with men and transgender women. Monthly gonorrhea cases rose compared with projections, by 1.77% per month.
• King County, Washington, 2017 to mid-2025: syphilis diagnoses after rollout were 52.3% below the expected number overall, 53.1% among cisgender men and 46.9% among cisgender women, a group that seldom receives the drug. Syphilis in pregnant people rose from 13 cases in 2017 to 48 in 2024, and congenital syphilis, passed to the baby, from 0 to 23.
• Taiwan, 397 cisgender men using preexposure prophylaxis: incidence before and after starting fell for syphilis (6.4 to 2.0 per 100 person-years) and chlamydia (31.9 to 8.8). No significant difference appeared for gonorrhea or for Mycoplasma genitalium.
• Southern California health system, 26,582 people, 2,262 with a prescription fill, 2023 to mid-2025: estimated effectiveness was 66.5% against chlamydia and 60.7% against syphilis, and held for more than two years. Against gonorrhea it was −1.8% (−18.5 to 12.5), meaning no measurable protection overall. The section on gonorrhea resistance gives the time trend.

What the 2024 Centers for Disease Control and Prevention Guideline States

The Centers for Disease Control and Prevention published its clinical guideline in June 2024. The points below are what the report itself states.

• Evidence summary: in three large randomized trials, 200 mg taken within 72 hours after sex reduced syphilis and chlamydia by more than 70% and gonococcal infections by about 50%.
• Who the recommendation covers: men who have sex with men and transgender women who had syphilis, chlamydia or gonorrhea diagnosed in the past 12 months. The agency recommends that they receive counseling that the method can be used, and that providers offer a prescription after shared decision-making. The report rates this as a strong recommendation on high-quality evidence.
• Who it does not cover: the report states that no recommendation can be given at this time for cisgender women, cisgender heterosexual men, transgender men, and other queer and nonbinary persons, because evidence is insufficient to assess the balance of benefits and harms.
• Dose in the guideline: 200 mg, as soon as possible within 72 hours after oral, vaginal or anal sex, with a stated maximum of 200 mg every 24 hours.
• Follow-up in the guideline: testing for bacterial infections at the body sites exposed, at the start and every 3 to 6 months, with ongoing need reassessed on the same schedule, inside a wider sexual health approach that includes vaccination and services related to human immunodeficiency virus.
• The trade-off in numbers: the report notes that people in the DoxyPEP study took on average an additional 43 doses to prevent an average of 1.3 bacterial infections a year.
• Harms: the report calls potential harms apparently low in the short term and unknown but potentially concerning in the long term.

Other countries read the same evidence differently. The 2024 federal guideline describes position statements ranging from recommending against widespread use, because of resistance concerns, to conditional use for people at risk of syphilis, to case-by-case use. The 2025 national guideline from the British Association for Sexual Health and HIV is written for the prevention of syphilis specifically.

This guideline predates several findings on this page, including the 2025 and 2026 gonorrhea resistance data below. Decisions about prescription medicines belong with a clinician.

Resistance in Gonorrhea

Gonorrhea is the infection most studied for resistance, for two reasons: the protection against it was weakest, and the bacterium has a long record of acquiring resistance to antibiotics. Tetracycline resistance was already widespread before this method existed. A 2025 systematic review of 80,645 samples from 51 countries put median resistance at 54.2%, with a range from 4.0% to 100.0%. It was highest in East Asia and the Pacific (82.1%) and sub-Saharan Africa (81.6%), and lowest in North America (26.5%). Only 9% of the studies included women. The review's authors conclude that high and rising resistance limits the method's potential against gonorrhea.

A plasmid-borne gene called tetM is central to the newer data. A plasmid is a small piece of DNA that bacteria can pass among themselves, and this gene gives high-level resistance to tetracyclines.

• Inside the trials: final DoxyPEP data found tetracycline resistance in 8 of 29 people with a gonorrhea culture taking the drug (27%) and 5 of 21 not taking it (24%), small numbers that show little difference. In the DOXYVAC substudy, all 78 samples with measured values were resistant, and high-level resistance from tetM was more common with doxycycline (35.5% against 12.5%).
• King County, Washington: among 2,312 men who have sex with men with gonorrhea, tetracycline resistance held at a mean of 27% from 2017 to early 2023, after which local guidelines appeared, then rose to 70% by the second quarter of 2024. High-level resistance went from 2% in early 2021 to 65%. Taking more than three doses a month was associated with resistance, while any use at all was not.
• Melbourne, Australia: among 1,020 samples with known use, tetracycline resistance was 89.2% in users against 77.2% in non-users, and high-level resistance 54.2% against 29.7%.
• Milan, Italy: 19 of 77 episodes with reported use had high-level resistance, against 45 of 298 without, a difference that did not survive correction for multiple comparisons. The authors describe the drug's contribution as limited in a setting where 96.0% of samples were resistant to tetracycline.

The strongest single finding comes from southern California. Effectiveness against gonorrhea fell from 42.3% (2.7 to 65.8) before statewide rollout to −15.0% (−51.1 to 11.4) in the first half of 2025. It was 47.2% when tetM was in 20.0% to 29.9% of local samples, and −8.5% when it was in 50% or more. Effectiveness against chlamydia and syphilis was sustained. The authors state that protection against gonorrhea might not be durable, and that risk-benefit considerations "should assume limited or transient benefits" for gonorrhea prevention.

A further question is whether doxycycline use could also weaken the drugs used to treat gonorrhea, a concern known as co-selection. So far the signal is small and inconsistent. In the DOXYVAC substudy, all samples remained susceptible to ceftriaxone and cefixime, though samples with reduced cefixime susceptibility tied to one gene variant were more frequent with doxycycline (32.3% against 10.0%). In Milan, reduced ceftriaxone susceptibility appeared in 5 of 77 exposed episodes against 3 of 298 unexposed, with no fully resistant samples. In Melbourne, ceftriaxone resistance stayed rare, at 3 of 11,334 samples over ten years. These counts are small, and the DOXYVAC and Melbourne authors call for continued monitoring.

Resistance in Other Bacteria

An antibiotic dose reaches every bacterium in the body, not only the one it was taken for. Researchers call the effect on the others "off-target."

• Staphylococcus aureus in the DoxyPEP trial: the 2024 federal guideline gives nasal carriage falling from 44% to 31% with doxycycline over 12 months, while tetracycline-resistant carriage rose from 5% to 13%. In the standard-care group, carriage went from 48% to 43% and resistant carriage from 10% to 5%.
• Sexual health clinic patients in King County: among 703 men who have sex with men, Staphylococcus aureus colonization was less common in users (27% against 36%). Colonization with tetracycline-resistant Staphylococcus aureus (18% against 8%) and group A Streptococcus, the cause of strep throat (9% against 4%), was more common.
• Daily-dose pilot trial: in 52 participants, doxycycline-resistant Staphylococcus aureus emerged in six people, five of them in the arm with longer exposure (P = .077, not significant).
• Other organisms in the trials: the 2024 federal guideline states that trends in methicillin-resistant Staphylococcus aureus and in one group of resistant Escherichia coli did not differ between study arms, and that limited testing of chlamydia strains found no significant trend toward resistance.
• Mycoplasma genitalium: doxycycline did not reduce this infection. In IPERGAY, it was present in 10.2% with the drug and 9.6% without at six months. In Taiwan, incidence was similar before and after (rate ratio 0.89, 0.51 to 1.57). Tetracycline-related mutations went from 10.0% to 24.1%, an increase that was not statistically significant (P = 0.28).

Colonization with a resistant germ is not the same as illness from it. None of the abstracts read for this page report a rise in resistant infections that made people sick. The Staphylococcus and Streptococcus studies measure carriage, not disease.

Side Effects the Studies Measured

Serious harm attributed to the drug was rare in the trials. Stomach complaints were the most common problem.

• IPERGAY: gastrointestinal side effects in 53% with doxycycline and 41% without (P = 0.05), with similar rates of serious events.
• DoxyPEP, all quarters on the drug: one grade 2 laboratory abnormality and five grade 3 adverse events possibly or probably related, with no serious adverse events attributed to doxycycline by site investigators.
• DOXYVAC: one drug-related serious event, a fixed-drug eruption, which is a rash tied to a specific medicine. Six people (2%) stopped because of gastrointestinal effects. There were no deaths.
• dPEP in Kenyan women: no serious adverse events were considered related to doxycycline.

The trials ran for months, while real use may last years. To look further, a systematic review gathered 67 studies of doxycycline taken for eight weeks or more, many of them on acne, rosacea and malaria prevention. Adverse events ranged from 0% to above 50%, most often nausea, vomiting, abdominal pain, rash, headache and dizziness. In a meta-analysis of 18 placebo-controlled trials, gastrointestinal and skin effects were more likely on doxycycline. Stopping because of side effects was relatively uncommon in most studies, and the authors call for more research on metabolic effects. These were daily schedules, so they do not exactly match an after-sex dose.

The 2024 federal guideline lists the effects most associated with doxycycline as sun sensitivity and gastrointestinal symptoms, including erosion and ulceration of the esophagus, and states that most resolve when the drug is stopped. Its counseling points include taking the dose on a full stomach with a full glass of liquid and not lying down for one hour afterward, to prevent esophagitis. None of the abstracts read for this page report results in pregnancy.

Gut and Vaginal Microbiome Research

The microbiome is the community of microbes living in and on the body. When the federal guideline appeared, the report noted that no studies yet existed on long-term, intermittent doxycycline use and the microbiome. Some have appeared since.

• DoxyPEP gut substudy, 2025: rectal swabs at the start and at six months, from participants who took a median of 42 doses. The diversity of gut bacteria, the mix of species and the total bacterial mass did not differ over time. But tetracycline resistance genes, which were present in every starting sample, grew from 46% to 51% of the resistance genes found, and from 4% to 15% of those being actively expressed. No significant increases were seen in other classes of resistance genes. More doses correlated with more enrichment.
• Reanalysis of the same data, 2026: a statistical search for dose thresholds placed a rise in resistance genes at about 65 doses over six months. Of 133 bacterial groups, 4 (3%) showed significant shifts. This is a secondary analysis of trial data.
• Daily-dose pilot trial, 2025: 41 men on 100 mg per day for up to 48 weeks showed no significant change in rectal bacteria at the genus level, with a slight decrease under 10% in one measure at the phylum level. The authors note that function and resistance still need study.
• Vaginal infections: in the Tokyo daily-dose study of 40 women, bacterial vaginosis and vaginal yeast infections showed no significant change. No comparable result from the after-sex trial in women appeared in the abstracts read for this page.

Together, these studies describe a gut community that looks much the same after six to twelve months, while carrying a larger share of resistance genes. What this means for later infections is not established, and the DoxyPEP authors call for further studies and population-level surveillance.

What Is Not Known

• Why the trial in women found no significant reduction. Low use, resistance and biological differences have all been raised, and the single trial cannot distinguish among them.
• Whether the method works in other groups. The 2024 federal guideline lists cisgender women, cisgender heterosexual men, transgender men and nonbinary persons as lacking sufficient evidence. In the Taiwanese cohort, heterosexual cisgender men made up only 1.8% of participants.
• How long protection against chlamydia and syphilis lasts. Southern California data show it holding for more than two years, and the trials above followed people for 24 months or less.
• Whether gonorrhea protection returns anywhere, or holds where tetM is rare. The available data tie its loss to local resistance, which differs by country.
• Whether doxycycline use pushes resistance to the drugs that treat gonorrhea. The current signals are small and rest on few samples.
• Long-term effects on the body. The 2024 federal guideline lists long-term harms as unknown, and the safety review calls for research on metabolic effects.
• What the falls in citywide syphilis mean. Time-trend studies cannot prove cause, and the King County drop among women who seldom receive the drug is reported without an established explanation. It did not extend to pregnant people or to congenital syphilis.

Key Research Papers

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PubMed Topic Searches

1. PubMed: doxycycline postexposure prophylaxis randomized trials
2. PubMed: doxycycline prophylaxis and gonorrhea resistance
3. PubMed: doxycycline prophylaxis, microbiome and resistance genes
4. PubMed: doxycycline prophylaxis in cisgender women

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Connections

• Chlamydia — the main page on the infection, its treatment and complications
• Syphilis — stages, testing, congenital syphilis and penicillin treatment
• Gonorrhea — the infection where protection and resistance diverge most
• Neisseria gonorrhoeae — the organism behind gonorrhea
• Syphilis Resurgence and Challenges — the rise in cases that prompted this research
• Pelvic Inflammatory Disease — a complication of untreated chlamydia and gonorrhea
• Tick Bite Prophylaxis — another single-dose preventive use of doxycycline
• Antibiotic Resistance Animation — how resistance spreads through a population of bacteria

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