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
