An interventional study of Doxycycline in Sexually Transmitted Diseases, Syphilis and Gonorrhea, sponsored by Oswaldo Cruz Foundation. Recruiting at 1 site in Brazil. Open to participants aged 18 Years and older, including healthy volunteers. Per ClinicalTrials.gov, last updated 2026-03-30.
Sponsored by Oswaldo Cruz Foundation · Not applicable, Interventional, and Treatment
The goal of this clinical trial is to learn if doxycycline post-exposure prophylaxis (doxy-PEP) can help prevent bacterial sexually transmitted infections (STIs) among sexual and gender minorities in Rio de Janeiro, Brazil, including men who have sex with men, transgender women, and travestis who are living with HIV or using HIV pre-exposure prophylaxis (PrEP). Bacterial STIs such as syphilis, gonorrhea, and chlamydia remain common in these populations, even with existing prevention strategies.
This study aims to answer the following questions: whether doxy-PEP can reduce the number of new STIs, whether it is safe and well tolerated, whether participants use it as recommended, and whether its use may contribute to antibiotic resistance or changes in the body's natural bacteria. Doxy-PEP involves taking a dose of doxycycline, an antibiotic, shortly after sexual activity to reduce the risk of acquiring STIs.
Participants will first receive information and counseling about doxy-PEP, including its possible benefits and risks, and will then choose whether or not to use this prevention strategy. Those who choose to use doxy-PEP will take doxycycline after sex as instructed and will be followed for up to 48 weeks, with clinic visits approximately every three months. During these visits, participants will be tested for STIs, monitored for side effects, and asked about medication use, sexual health, and overall well-being.
Researchers will collect information on new STI diagnoses, safety, and how consistently participants use doxy-PEP. The study will also explore participants' experiences and perceptions of this strategy. The results of this study will help determine whether doxy-PEP is a practical and acceptable approach for STI prevention in Brazil and may inform future public health strategies.
523 studies on the registry are indexed under Sexually Transmitted Diseases; 77 are open to participants now.
This study's planned enrollment of 200 is below the median of 449 across 425 interventional studies indexed under Sexually Transmitted Diseases.
Browse Sexually Transmitted Diseases studies →Oswaldo Cruz Foundation is the lead sponsor of 42 studies on the registry; 7 are open to participants now.
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Exclusion Criteria:
Underlined terms in the criteria open these too. Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
An allergy, or another reaction of the immune system, to a substance. “Hypersensitivity to the study drug” means a known allergic reaction to it — not every side effect a medicine can have.
A reason a treatment, test or procedure should not be given to a particular person, such as an allergy, another illness or a medicine they take.
HIV
The human immunodeficiency virus, which weakens the immune system.
Many studies now accept people whose HIV is controlled on treatment, judged by the viral load and the CD4 count. Check the limits this study sets.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Informed consent
Before you join, the study team explains what taking part involves — its purpose, what happens, the risks and possible benefits — and answers your questions, so you can decide freely. Consent is usually recorded on a form (the informed consent form, ICF) signed by you or, where the law allows, by someone legally authorised to decide for you.
You can keep asking questions during the study, you are told of new information that matters, and you can choose to leave at any time.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Hypersensitivity
An allergy, or another reaction of the immune system, to a substance. “Hypersensitivity to the study drug” means a known allergic reaction to it — not every side effect a medicine can have.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Contraindication
A reason a treatment, test or procedure should not be given to a particular person, such as an allergy, another illness or a medicine they take.
Explained by clinicaltrials.gg, not part of the registry record. The study team decides who can take part.
Participants who choose to initiate doxycycline post-exposure prophylaxis (doxy-PEP) after receiving counseling on its risks and benefits. Participants will take doxycycline after sexual activity as instructed and will be followed for up to 48 weeks with regular clinical visits, including testing for sexually transmitted infections, safety assessments, and adherence evaluation. Participants who initially decline doxy-PEP may choose to initiate it at any time during the study.
Drug: Doxycycline
Doxycycline 200 mg taken orally within 72 hours after sexual activity, not exceeding one dose per 24 hours and a maximum of three doses per week.
Incidence of bacterial sexually transmitted infections (STIs)
Cumulative incidence of laboratory-confirmed symptomatic and asymptomatic bacterial STIs (syphilis, gonorrhea, and chlamydia).
Time frame: Up to 48 weeks
Incidence of symptomatic and asymptomatic bacterial STIs
Cumulative incidence of laboratory-confirmed symptomatic and asymptomatic bacterial STIs (syphilis, gonorrhea, and chlamydia).
Time frame: Up to 48 weeks.
Incidence of specific STIs (chlamydia, gonorrhea, and syphilis)
Cumulative incidence of each STI (chlamydia, gonorrhea, and syphilis), including symptomatic and asymptomatic cases.
Time frame: Up to 48 weeks
Antimicrobial resistance in Neisseria gonorrhoeae
Proportion of Neisseria gonorrhoeae isolates with resistance to tetracycline and other relevant antibiotics over time.
Time frame: Baseline and up to 48 weeks
Colonization with antimicrobial-resistant bacteria
Proportion of participants colonized with methicillin-resistant Staphylococcus aureus (MRSA) and extended-spectrum beta-lactamase (ESBL)-producing enterobacteria.
Time frame: Baseline and week 48
Adverse events
Occurrence of adverse events (AEs), including serious adverse events (SAEs), grade 3 or 4 AEs, and AEs leading to discontinuation of doxy-PEP.
Time frame: From enrollment to 48 weeks
Adherence to doxy-PEP
Adherence measured by self-reported coverage of sexual exposures with doxy-PEP and by doxycycline levels in hair samples.
Time frame: Up to 48 weeks
Uptake of doxy-PEP
Proportion of participants who choose to initiate doxy-PEP after counseling.
Time frame: At enrollment
Acceptability of doxy-PEP
Participant-reported perceptions, experiences, and acceptability of doxy-PEP assessed through questionnaires and qualitative interviews.
Time frame: Up to 48 weeks
Plan to share: No
No publications or documents are linked to this record.
Started Oct 2025 (actual). Primary completion is estimated for Oct 2027, 24 months after the start.
Of 172 non-industry interventional studies with no phase under Sexually Transmitted Diseases, half reached primary completion within 32 months of starting (the median), counting those still running; a quarter within 17 and three quarters within 47. This study's planned 24 months is below that.
Counting every sponsor, half of 182 studies reached it within 31 months; a quarter within 16 and three quarters within 46.
How these figures are worked out, and which studies are left out of them.
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Studies of the drug tested here: Doxycycline
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