Rates of bacterial sexually transmitted infections (STIs) are rising globally, demanding innovative interventions beyond the scope of current efforts to prevent STIs. The United States Doxycycline Post-exposure Prophylaxis (DoxyPEP) Study has demonstrated the efficacy of doxycycline post-exposure prophylaxis (PEP) among men who have sex with men and transgender women; but puzzlingly, doxycycline PEP was found ineffective in cisgender women in the Kenyan doxycycline Post-Exposure Prophylaxis (dPEP) study, with preliminary data suggesting the low medication adherence may explain the null result. By study end, the investigators will have developed adherence measurement methods for doxycycline in hair, blood, and urine, and will use these techniques to help interpret the Kenyan dPEP study, and to examine the relative performance of these methods within the United States DoxyPEP trial, establishing adherence metrics for current and future rollout studies of doxycycline post-exposure prophylaxis

Inclusion Criteria:

• not currently at risk of a sexually transmitted infection and willing to use other methods, such as condoms, for STI prevention;
• if diagnosed with an STI in the past two years and/or a partner was diagnosed with an STI in the past year, is willing to use condoms for STI prevention;
• willing to provide hair, blood, and urine samples;
• not currently enrolled in other STI prevention studies;
• able to speak English
• transgender women participants should be currently using estrogen gender affirming hormone therapy with blood estrogen levels demonstrating consistent use
• transgender men participants should be currently using testosterone gender affirming hormone therapy with blood testosterone levels demonstrating consistent use
• able to provide a hair sample of ~50-100 strands of hair that are non-gray, not bleached, and at least 1cm in length
• willing to NOT shave hair or have a very short haircut or apply bleach for the duration of the study
• has suitable venous access for consecutive blood draws

Exclusion Criteria:

• any health condition that may interfere with participation or the ability to provide informed consent, including any debilitating or life-threatening conditions;
• pregnancy or plans to become pregnant;
• liver cirrhosis or fulminant liver disease;
• known hypersensitivity reaction to doxycycline.
• detectable doxycycline in hair at enrollment.
• unable to provide a hair sample of ~50-100 strands of hair that are non-gray, not bleached, and at least 1cm in length
• does not have suitable venous access for consecutive blood draws