The objective of this research is to evaluate the efficacy and safety of doxycycline in MSM for preventing sexually transmitted infections (gonorrhea, chlamydia, and syphilis) in Thailand

Participation in this study will last approximately 12 weeks and will include 2-3 clinic visits, along with 10-11 follow-up phone contacts over a period of 11 weeks.

Inclusion Criteria:

Biological male, aged not over 60 years

Thai nationality

Able to read, communicate, and understand the Thai language

Men who have sex with men (MSM), aged 18 years or older and not over 60 years

Having at least one of the following risk factors:

A history of condomless sex, or condom failure (breakage, leakage, or slippage), via any sexual route with at least two sexual partners within the past 3 months

A diagnosis of gonorrhea, chlamydia, or syphilis within the past 6 months

Current use of HIV pre-exposure prophylaxis (PrEP)

Willing and able to participate in study procedures (e.g., able to return to the clinic for scheduled testing for gonorrhea, chlamydia, and syphilis; able to complete doxycycline use records; and able to receive follow-up phone calls from the research team)

Willing and able to provide written informed consent to participate in the study

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Exclusion Criteria:

• History of allergy to tetracycline-class antibiotics

Diagnosed with syphilis and not yet treated, or currently receiving treatment with benzathine penicillin G or doxycycline

Receipt of benzathine penicillin G within the past 12 weeks

Prior receipt of a gonorrhea vaccine

Current use of medications or products that may interact with doxycycline, including:

Vitamin A derivatives (e.g., isotretinoin, acitretin)

Antiepileptic drugs (e.g., phenytoin, carbamazepine, phenobarbital)

Immunosuppressive agents (e.g., methotrexate)

Medications for heart failure or cardiac arrhythmias (e.g., digoxin)

Anticoagulants (e.g., warfarin)

Use of oral or injectable medications that affect blood cells, such as anticancer drugs, chemotherapy, systemic corticosteroids, or targeted therapies

History of renal impairment (eGFR < 60 mL/min/1.73 m²) or hepatic impairment (AST or ALT > 3 times the upper limit of normal)

Uncontrolled or unstable underlying diseases, such as hematologic disorders, cancer, autoimmune diseases, or neurological disorders

Severe medical or psychiatric conditions that, in the investigator's judgment, may increase study-related risk or impair the participant's ability to provide informed consent

Alcohol dependence (inability to control alcohol consumption, withdrawal symptoms upon reduction or cessation, or continued use despite harm) or illicit drug dependence (regular, uncontrolled use) within the past 12 months prior to enrollment, which may affect doxycycline use for STI prevention

Previous participation in this study with prior randomization

Men who have sex with men

Individual participant data (IPD) from this study will not be publicly shared due to ethical and legal considerations. The study involves sensitive sexual health information from key populations, and even de-identified datasets may carry a risk of re-identification. Data sharing is also subject to Thai data protection regulations and the conditions approved by the institutional ethics committee and participant informed consent.

Labotatory staffs