A Study of Doxycycline to Treat Chlamydial Infection

Inclusion Criteria: 1. Has untreated urogenital chlamydia (CT) (in women) or rectal CT (in men), diagnosed with a positive nucleic acid amplification test (NAAT) (point-of care or laboratory-based)\* result within 14 days \*Point-of-care (POC) test may or may not be FDA-cleared for CLIA waiver for diagnosis depending upon anatomic site of infection. 2. Must be age \>/=16 years (where the IRB permits individuals aged 16-17 years old to consent to research); otherwise age \>/= 18 years 3. Willing and able to understand and provide written informed consent before initiation 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 with a clinical diagnosis of pelvic inflammatory disease (PID) Per the CDC's 2021 STD Treatment Guidelines or refer to local guidelines. 2. Signs and symptoms that, in the judgement of a qualified clinician, warrant a prolonged course of treatment with doxycycline For example, 21 days of doxycycline for presumed lymphogranuloma venereum infection. 3. Received antimicrobial therapy active against C. trachomatis within 21 days prior to positive chlamydia (CT) test result, or between the positive chlamydia (CT) test result and study enrollment Use of the following antibiotics is exclusionary: doxycycline and related tetra- or glycylcyclines, macrolides (including azithromycin), fluoroquinolones, rifampin, quinupristin-dalfopristin, and linezolid. Note: Amoxicillin, penicillin, ceftriaxone, and other beta-lactam antibiotics are not considered exclusionary for this study. 4. Planning to take antimicrobial therapy active against chlamydia (CT) during the study period (e.g. doxycycline post-exposure prophylaxis, treatment for Mycoplasma genitalium infection, acne, or any other non-STI medical condition). 5. Currently enrolled in or plan to enroll in another study using antimicrobial therapy active 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-induced photosensitivity. 8. Plan to move or travel to another location that would preclude study follow-up appointments in clinic in the next 30 days 9. Use of a medication contraindicated to treatment with doxycycline within 7 days prior 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 participation because 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 exposure to gonorrhea in the time between CT testing and study enrollment. Gonococcus (GC) infection identified during the course of pre-screening or screening will be treated per clinic standard practice and in accordance with local guidelines without concomitant azithromycin or other treatment for Chlamydia trachomatis infection. * Clinical diagnosis of concomitant untreated primary or secondary syphilis or known exposure to syphilis * Urethritis among men * Contraception status * Diagnosis of HIV