Randomized Directly Observed Therapy Study to Interpret Clinical Trials of Doxy-PEP

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