A Study of Doxycycline to Treat Chlamydial Infection

Inclusion Criteria:

1. Has untreated urogenital chlamydia (CT) (in women) or rectal CT (in men), diagnosedwith a positive nucleic acid amplification test (NAAT) (point-of care orlaboratory-based)* result within 14 days

*Point-of-care (POC) test may or may not be FDA-cleared for CLIA waiver fordiagnosis depending upon anatomic site of infection.

2. Must be age >/=16 years (where the IRB permits individuals aged 16-17 years old toconsent to research); otherwise age >/= 18 years

3. Willing and able to understand and provide written informed consent beforeinitiation of any study procedures

4. Willing to complete a 7-day study drug regimen

5. Willing to abstain from condomless anal or vaginal sex during the trial

6. Willing and able to adhere to planned study procedures for all study visits

7. Has valid contact information

Exclusion Criteria:

1. For women: lower abdominal or pelvic pain or other signs or symptoms consistent witha clinical diagnosis of pelvic inflammatory disease (PID) Per the CDC's 2021 STDTreatment Guidelines or refer to local guidelines.

2. Signs and symptoms that, in the judgement of a qualified clinician, warrant aprolonged course of treatment with doxycycline For example, 21 days of doxycyclinefor presumed lymphogranuloma venereum infection.

3. Received antimicrobial therapy active against C. trachomatis within 21 days prior topositive chlamydia (CT) test result, or between the positive chlamydia (CT) testresult and study enrollment Use of the following antibiotics is exclusionary:doxycycline and related tetra- or glycylcyclines, macrolides (includingazithromycin), fluoroquinolones, rifampin, quinupristin-dalfopristin, and linezolid.Note: Amoxicillin, penicillin, ceftriaxone, and other beta-lactam antibiotics arenot considered exclusionary for this study.

4. Planning to take antimicrobial therapy active against chlamydia (CT) during thestudy period (e.g. doxycycline post-exposure prophylaxis, treatment for Mycoplasmagenitalium infection, acne, or any other non-STI medical condition).

5. Currently enrolled in or plan to enroll in another study using antimicrobial therapyactive against C. trachomatis during the study period

6. Pregnant or lactating, or plan to become pregnant within the study period

7. Known moderate to severe allergy to tetracyclines, excluding tetracycline-inducedphotosensitivity.

8. Plan to move or travel to another location that would preclude study follow-upappointments in clinic in the next 30 days

9. Use of a medication contraindicated to treatment with doxycycline within 7 daysprior to enrollment or during the study period (systemic retinoids, barbiturates,carbamazepine, phenytoin, warfarin)

10. Previous enrollment in this trial

11. Any condition that, in the judgment of the investigator, precludes participationbecause it could affect participant safety or determination of study endpoints.

Of note, the following factors will NOT exclude participants from the study:

• Concomitant untreated gonorrhea (rectal, pharyngeal, or urethral) or known exposureto gonorrhea in the time between CT testing and study enrollment. Gonococcus (GC)infection identified during the course of pre-screening or screening will be treatedper clinic standard practice and in accordance with local guidelines withoutconcomitant azithromycin or other treatment for Chlamydia trachomatis infection.

• Clinical diagnosis of concomitant untreated primary or secondary syphilis or knownexposure to syphilis

• Urethritis among men

• Contraception status

• Diagnosis of HIV