Taking the antibiotic doxycycline after sex can lower the risk of sexually transmitted infections (STIs), according to a recent study. Pre-exposure prophylaxis (PrEP) dramatically cuts HIV acquisition, but forgoing condoms leaves people prone to other STIs. The DoxyPEP trial recruited more than 500 men and transgender women who have sex with men in San Francisco and Seattle. They were randomly assigned to get either a single dose of oral doxycycline as post--exposure prophylaxis (PEP) within 72 hours after condomless sex or standard care (regular testing and treatment after diagnosis). The study was halted a year early after an interim analysis showed that doxycycline PEP significantly reduced STI incidence. Among participants living with HIV, doxycycline reduced the likelihood of being diagnosed with gonorrhea, chlamydia or syphilis by 62%. Among HIV-negative people on PrEP, the risk reduction was 66%. Adherence was high (87%), and doxycycline PEP was safe and well tolerated. Further analysis is underway to see whether this approach promotes drug resistance.