This study tests the effects of doxycycline—a drug approved by the FDA for bacterial infections—on inflammation (body's response to injury or infection) and bacteria in people with HIV or those using HIV prevention methods like pre-exposure prophylaxis (PrEP). The study aims to see if doxycycline can prevent sexually transmitted infections (STIs) by reducing inflammation or affecting the microbiome (good and bad bacteria living in our bodies).

Participants will be healthy males, 18 or older, with or without HIV, and on HIV medication. The study will last about 12 weeks, with five visits, including some blood and tissue sampling. Some people can't join if they have certain health conditions, allergies to doxycycline, or are on specific medications.

• This study lasts 12 weeks and includes 5 visits.
• Participants must be healthy males over 18, with or without HIV.
• Some health conditions and medications might exclude you from participating.