Superiority of On-demand PrEP Versus PEP on Using Doxycycline for Preventing STI in MSM

The goal of this clinical trial is to compare the effectiveness of doxycycline taken for on-demand pre-exposure prophylaxis (DoxyODPrEP) and its post-exposure use (DoxyPEP) in preventing bacterial sexually transmitted infections (STI), including chlamydia, gonorrhoea, and syphilis among men who have sex with men (MSM). The main questions it aims to answer are: 1. Is DoxyODPrEP superior to DoxyPEP? 2. Are both regimens safe? 3. Does the MSM community accept the use of doxycycline to prevent bacterial STI? Participants will be asked to take doxycycline according to the study arm they are randomly assigned to, and attend regular clinical follow-ups during the 2-year observation period. Researchers will compare the bacterial STI incidences between the two groups to see if DoxyODPrEP is superior to DoxyPEP.