Doxycycline is the test medication in this study. It is given as a pill after a possible exposure to a sexually transmitted infection. The goal is to see if taking doxycycline can lower the chance of getting infections like chlamydia, gonorrhea, or syphilis, and to examine how it might affect the bacteria living in the gut.

Juluca is a combination pill that contains two HIV medicines, rilpivirine and dolutegravir. In the trial it is used as a standard HIV treatment that some participants are already taking, allowing researchers to compare outcomes while participants also receive the doxycycline prophylaxis.

Triumeq contains three HIV drugs: abacavir, lamivudine, and dolutegravir. It serves as another background HIV therapy for participants, helping ensure that any effects seen are due to the doxycycline and not changes in HIV medication.

Dovato combines lamivudine and dolutegravir in one tablet. It is listed as a comparator HIV regimen that participants may be using during the study.

Odefsey is a fixed‑dose tablet with emtricitabine, tenofovir alafenamide, and rilpivirine. It is included as a common HIV treatment among the study population.

Biktarvy provides emtricitabine, tenofovir alafenamide, and bictegravir in a single pill. It is another standard HIV medication used by some participants in the trial.

Tenofovir disoproxil and emtricitabine is a two‑drug combination used to treat HIV. It is listed as a comparator regimen for participants who are on this established therapy.

Methicillin-resistant Staphylococcus aureus colonization – This condition refers to the presence of MRSA bacteria in the nasal passages without causing obvious illness. The bacteria can remain on the skin or in the nose for months, often spreading to other body sites. When the skin barrier is broken, the colonizing bacteria may cause localized skin infections. Repeated exposure to antibiotics can increase the proportion of resistant strains. Over time, the colonization can become more widespread within a community.

Syphilis – Syphilis is an infection caused by the bacterium Treponema pallidum. It typically begins with a painless sore at the site of entry, then may develop a rash and other systemic signs. If untreated, the infection can enter a latent phase that may last years. Later stages can involve the heart, nervous system, and other organs. The disease progresses in a stepwise manner from early to late manifestations.

Chlamydia trachomatis infection – This infection is caused by the bacterium Chlamydia trachomatis and often produces no symptoms. When symptoms appear, they may include urethral discharge or cervical irritation. The infection can ascend to the upper genital tract, leading to pelvic inflammatory disease. Repeated infections can cause scarring of reproductive tissues. Over time, chronic inflammation may develop if the organism persists.

Neisseria gonorrhoeae infection – Gonorrhea results from infection with the bacterium Neisseria gonorrhoeae. It commonly affects the urethra, cervix, rectum, or throat, causing discharge or discomfort. The bacteria can spread through the bloodstream to joints and other sites. Persistent infection may lead to chronic inflammation of affected tissues. The condition can evolve from localized symptoms to more widespread involvement.