This study looks at how a medicine called doxycycline works in the body when taken every day for a week. Doxycycline is an antibiotic that can help prevent some sexually transmitted infections (STIs). The study wants to see how much of this medicine gets into certain body tissues, like the skin inside the mouth and private parts, in people who were assigned as male or female at birth. People will take the medicine for 6 days at home, then have some tests on the 7th day. These tests will involve taking small samples from their bodies. The results will help in creating better ways to prevent STIs in the future.

• **Duration**: The study lasts for 7 days, with medicine taken daily.
• **Visits**: Participants must visit the study site for sample collection.
• **Eligibility**: Participants must be generally healthy and agree to certain conditions, like using condoms and not taking certain medications.

People who have liver disease, are pregnant, or have certain medical conditions cannot join. There are no known serious risks, but participants should discuss any concerns with the study team.