This study is about taking doxycycline, an antibiotic, daily for 7 days to see how it gets into body tissues. The focus is on people who were assigned male or female sex at birth. This helps to understand how doxycycline might protect against sexually transmitted infections (STIs), like bacteria that cause infections. Participants will take one pill at the study start and then one pill daily for 5 more days at home. On the 7th day, some body samples will be collected to check how much doxycycline is in the body tissues. People need to be healthy, not on certain medications, and can't be pregnant to join. They also need to agree to use condoms and have some small medical procedures done.

• This study lasts for 7 days, with samples taken on the last day.
• Participants will take 7 doses of doxycycline, with the first dose at the study site.
• Some samples and small procedures will be done to check doxycycline levels.