SEEMS LIKE A GOOD WAY TO PROMOTE RESISTANCE: One dose of antibiotic after sex halved STD rates, study shows. “Just one dose of the antibiotic doxycycline taken after sex halved the number of chlamydia and syphilis cases in San Francisco, promising new research shows. In the study, gay and bisexual men and transgender women who had a history of sexually transmitted infections or multiple sex partners were given a supply of the antibiotic and asked to take two 100-milligram pills within 72 hours of unprotected sex. The strategy is known as doxy-PEP.”