Doxycycline Prophylaxis for Prevention of Sexually Transmitted Infections

Inclusion Criteria:

* Age 13-29 years, inclusive;
* Self-reports GC, CT, and/or early syphilis diagnosis in the 12 months prior to screening;
* Self-reports condomless (oral, vaginal, and/or anal) intercourse at least once in the 30 days prior to screening
* Is at least 45 kilograms (99.2 pounds) in weight
* Fluent in English (able to read, speak, and understand English);
* Willing to undergo all required study procedures; and
* Willing to provide written informed consent to participate in the study.

Exclusion Criteria:

* Is male;
* Is pregnant, breastfeeding, or planning to become pregnant in the next 12 months;
* Documented history of tetracycline allergy or self-reports an allergy to tetracyclines;
* Current or anticipated use of a medication (e.g., barbiturates, phenytoin, lithium, carbamazepine, systemic retinoids, warfarin) with a contraindicated interaction to doxycycline ;
* Current or planned use of doxycycline for a prolonged period (\> 2 weeks) in the 30 days prior to enrollment, including current prescription for doxycycline for STI prophylaxis;
* Prolonged antibiotic use (\> 2 weeks) within the last 30 days prior to enrollment;
* Co-enrollment in any other concurrent interventional research or studies that may interfere with this study, such as an STI prevention trial, unless approved by Protocol Team in writing;
* Does not have consistent access (or anticipated weekly access for 52 weeks of survey completion) to smartphone that can access the HMP app; or
* Any other medical condition, medical/behavioral intervention, or other conditions that, in the opinion of the clinical site Project Lead (PL)/Investigator of Record (IoR) or designee, could interfere with adherence to study procedures or compromise interpretation of study results.