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