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 * An…