Taking a daily dose of the antibiotic doxycycline may be an effective form of pre-exposure prophylaxis (PrEP) against various bacterial sexually transmitted infections (STIs), including gonorrhea, chlamydia and syphilis. Researchers recruited 30 HIV-positive men who have sex with men (MSM) who’d contracted syphilis at least twice since being diagnosed with HIV. The men were randomly assigned to either take doxycycline—the same drug you may have taken for teenage acne—each day for 36 weeks, or to receive financial incentives for keeping STI-free for that long. Compared with those in the financial incentive group, the study participants who took doxycycline were 73 percent less likely to contract any of the three STIs before reaching the 12-week mark after completing the antibiotics. However, since the study was so small, the researchers could not be very confident about the 73 percent figure. They concluded that their findings justify a larger study of STI PrEP.